active and passive arteriolar regulation in spontaneously

757

Active and Passive Arteriolar Regulation inSpontaneously Hypertensive Rats

H. Glenn Bohlen, Julia M. Lash

Abstract This study determined to what extent active andpassive wall tensions increase in in vivo intestinal arterioles of13- to 15-week-old and 25- to 27-week-old spontaneouslyhypertensive rats (SHR) to maintain normal or smaller arte-riolar diameters during microvascular hypertension. Acetyl-choline and nitroprusside were used to determine whethervascular muscle relaxation to endothelium-derived relaxingfactor or cyclic GMP is impaired. Large arterioles of hyper-tensive rats have passive tension-circumference relations thatare steeper and shifted to the left compared with those ofage-matched controls; passive resistance to distension limitsvasodilation in hypertensive rats except at their naturallyelevated arteriolar pressure. Passive tension contributes ap-proximately 30% of the total resting tension in arterioles ofhypertensive and normotensive rats because a greater passivetension occurs at the 20% to 25% constricted resting diameterin hypertensive rats. Absolute and relative changes in the

Studies of intestinal arterioles in our laboratoryindicate that in large arterioles of both normaland diabetic rats, 60% to 75% of the total vessel

wall tension at rest is derived from active muscletension, and therefore, active tension dominates theregulation of resting arteriolar diameter.1'2 A very dif-ferent situation may occur in the arterioles of thespontaneously hypertensive rat (SHR). In a series ofstudies Baumbach and colleagues3-8 have shown thatthe passive distensibility of cerebral arterioles is in-creased in the stroke-prone SHR compared with nor-motensive animals when distensibility is defined as thepercent change in diameter per millimeter of mercury.However, their data indicate that the inner diameters ofpassive arterioles in adult SHR (^6 months) are muchsmaller than in normotensive rats at equivalent distend-ing pressures. These data also suggest that at equivalentdiameters passive tension will be much higher in thearterioles of hypertensive than normotensive rats be-cause a greater arteriolar pressure is required to distendSHR arterioles. The possible effect is, that for any givenvessel diameter in the presence of active vascular con-trol, passive tension may contribute more to maintainingthe resting diameter in hypertensive than normotensiverats. However, the relative contributions of active andpassive wall tension to the regulation of resting arteri-olar diameter depend on the complex interaction of thepassive tension versus diameter (circumference) rela-

Received April 15, 1993; accepted in revised form March 23,1994.

From the Department of Physiology and Biophysics, IndianaUniversity Medical School, Indianapolis.

Correspondence to Dr H. Glenn Bohlen, Department of Phys-iology and Biophysics, Indiana University Medical School, 635Barnhill Dr, Indianapolis, IN 46202.

diameter of SHR arterioles during acetylcholine and nitro-prusside application were equal to or greater than those inWistar-Kyoto rats. However, reduction in active tension wassuppressed in older SHR and remained approximately 50%higher than that found in older Wistar-Kyoto rats during drugapplication. Vasoconstriction and increased passive resistanceto distension of the arteriolar wall diminish the active tensionrequired to maintain normal or smaller resting diametersagainst microvascular hypertension. However, the elevatedmicrovascular pressure in hypertensive rats is required to allownear-normal dilation to compensate for their increased passiveresistance to stretch and decreased ability to relax activetension through cyclic GMP mechanisms. (Hypertension.1994;23[part l]:757-764.)

Key Words • endothelium-derived factors • hypertension •arterioles

tion, the prevailing microvascular pressure, and themagnitude of active tension generation.1 At this time,the role of changes in passive distensibility in thegeneration and maintenance of increased vascular re-sistance during hypertension is not clear.

The purpose of this study was to determine to whatextent active and passive wall tensions increase in intes-tinal arterioles of SHR to maintain normal or smallerarteriolar diameters in the presence of microvascularhypertension. We chose the large arterioles of the intes-tine for study because these vessels clearly demonstrate a40% hypertrophy of the muscle layer cross-sectional areaby age 17 to 19 weeks.9 Similar hypertrophy of the musclelayer has been documented for large cerebral arteri-oles610 and small arteries of the mesentery1114 and issuggested for the largest arterioles of skeletal muscle.15

Furthermore, on theoretical grounds Borders andGranger16 have determined that the greatest power dis-sipation, which includes all forms of resistance, occurs inthose microvessels with the greatest flow per vessel, thatis, the large arterioles. For the current study, westudied normotensive and hypertensive rats at 13 to 15and 25 to 27 weeks of age because the earlier agescoincide with the time at which the elevated arterialpressure begins to plateau in SHR, and arterial pres-sure remains relatively constant to the later ages.Evaluations at these particular times provide an op-portunity for determining the active and passive vas-cular consequences of the rapid rise in pressure up toage 13 to 15 weeks and any additional vascular conse-quences or compensations that may occur over time ata relatively stable hypertensive pressure. We useddirect application of acetylcholine and nitroprussideto determine the extent to which diameter regulationis altered by both active cellular processes and passive

by guest on April 7, 2018

http://hyper.ahajournals.org/D

ownloaded from

http://hyper.ahajournals.org/

758 Hypertension Vol 23, No 6, Part 1 June 1994

mechanical properties of the vessel wall during the twophases of hypertensive life studied.

MethodsMale Wistar-Kyoto (WKY) rats and SHR were purchased at

age 12 to 13 weeks (Taconic Farms) and used for experimentsat either age 13 to 15 or 25 to 27 weeks. The experimentalprotocol was approved by the Animal Resource Committee ofthe Indiana University Medical School. All animals wereanesthetized with sodium thiopental (initial dose, 100 mg/kgIP) (Abbott Laboratories) and were usually given a supple-mental dose equal to one fourth the initial dose after surgerywas completed; additional doses were rarely needed. Thetrachea and right femoral artery were cannulated for measure-ment of arterial blood pressure and replacement of fluids anddrugs. Body fluid replacement was performed to maintain aconstant arterial pressure. Fluids (Normosol, Abbott) wereadministered by continuous infusion (0.5 mL/100 g per hour)through the arterial cannula. The maximum flow of approxi-mately 0.04 mL/min through the arterial cannula had nodiscernible effect on arterial pressure measurements (model23 Db Statham transducer, 0 to 200 mm Hg calibration, GouldBrush 2400 chart recorder).

The intestine was prepared for observation using an estab-lished technique17 modified to facilitate micropipette penetra-tion studies.18 Intestinal motility was suppressed in all animalswith a bathing medium concentration of less than or equal to5xl0"7 mol/L isoproterenol, which is below the thresholdconcentration that causes blood flow to change.18 The tissueswere bathed in a bicarbonate-based physiological solution18

equilibrated with 90% N2, 5% O2, and 5% CO2. The fluid wasprotected from the atmosphere en route to the tissue bath andheated to 37°C before entering the heated (37°C) tissuesupport device. The animal's core temperature was monitoredby a thermistor in the esophagus, and the body received heatfrom both the tissue support device and a water jacket placedunder the body.

The tissue and vessels were observed with an OlympusBMHJ microscope using a Nikon x20 water immersion lens(numerical aperture, 0.33). The image was recorded with aHamamatsu Charge Coupled Device camera (model XC-77)and digitized with an Image 1 image-analysis system (UniversalImage Corp). The image-analysis system magnification factorswere calibrated in the horizontal and vertical planes using astage micrometer marked in 10- and 100-/un units. Averagevessel inner diameters were determined from measurementsobtained from digitized images taken at 10-second intervalsduring the last 30 seconds of the control and drug releaseperiods.

Microvascular pressures were measured with a servo-nullsystem (model 4A, Instrumentation for Physiology & Medi-cine) using micropipettes filled with 2 mol/L NaCl and sharp-ened to an outer diameter of 4 to 6 jim. The system wascalibrated at both 0 to 50 and 0 to 100 mm Hg. Drugs wereapplied to the wall of the selected vessel using an iontophore-sis programmer (model 160, World Precision Instruments)(sodium nitroprusside, 10~3 mol/L; acetylcholine chloride, 10~3

mol/L; Sigma Chemical Co). A retaining current of 20 to 25 nAwas used to prevent inadvertent diffusion of drugs from themicropipette. Drugs were released at currents of 25, 50, 100,and 200 nA. Each drug was released for 2 minutes at eachcurrent, and the current was then increased to the next highersetting. In most cases, steady-state diameter responses werereached during the first minute of drug release, and diametermeasurements were obtained from 90 to 120 seconds after aconstant drug release at a given setting was initiated. In eachexperiment, the ionotophoresis micropipette was placed asneeded, and then the vessel was penetrated approximately 200fim downstream with the servo-null micropipette. After dualmicropipette placement, the vessel was allowed to stabilize forapproximately 5 minutes. If clots formed on the tip of theservo-null micropipette, the micropipette was withdrawn and

reinserted once clot formation at the vessel penetration sitehad stopped.

To obtain the passive pressure-inner diameter relation foreach vessel, nitroprusside was locally released at a supramax-imal dose of 500 nA. This nitroprusside dose made the vesselrefractory to intentional micropipette irritation, which nor-mally is capable of causing vessel constriction to closure. Afterdata for resting conditions were obtained, the microvascularpressure was lowered with a bolus injection of 0.1 mL IA of10"3 mol/L sodium nitroprusside. As the arterial pressure felland then gradually recovered over a 5- to 7-minute period,diameter and microvascular pressure data were obtained atapproximately 8 to 12 mm Hg steps in arteriolar pressure.During this period, the arteriole was continuously exposed tothe 500 -nA current of nitroprusside to ensure that the vesselremained completely passive. Typically, the microvascularpressure fell to 10 to 15 mm Hg, which corresponds to anarterial pressure of 30 to 40 mm Hg in normotensive rats and50 to 60 mm Hg in SHR. This manipulation does not injure theanimal or vessel, as arterial pressure was virtually alwaysrestored to that before drug administration, and the restingvessel diameter was regained within approximately 8 to 10minutes. The resting vessel diameter after the passive phaserecovery was 98.4±1.37% (mean±SEM) of the initial controldiameter in all WKY rats and 104.1 ±3.95% of the initialcontrol diameter in all SHR. If topical sodium nitroprussideand local occlusion of the inflow artery are used to reducemicrovascular pressure, as previously described,1'2 a similarrecovery time is required. Also, there is the additional sideeffect of edema rapidly occurring in hypertensive rats becauseof the high microvascular pressures that occur when vasculartone is abolished. With the current approach, microvascularpressures in exchange vessels never exceed those observedwhen resting vascular tone is present. In a group of fournormotensive rats, arterial pressure was lowered or raisedfrom a low pressure by sustained infusion rates of sodiumnitroprusside into the systemic vasculature, and 10"4 mol/Lsodium nitroprusside was simultaneously suffused over thetissue. The slope of the log10 passive tension versus circumfer-ence relation was determined for one of the largest arteriolesfor the conditions in which arterial pressure was maintained ata specific pressure for 3 to 5 minutes and then for the samevessel using the standard passive-state conditions describedabove. With the use of sustained drug infusions, the slope ofthe passive tension versus circumference relation was0.00973±0.00079 N/m2 (mean±SEM) compared with0.01027±0.001 N/m2 when blood pressure was allowed torecover naturally after an intravascular bolus injection ofsodium nitroprusside while nitroprusside was simultaneouslyapplied to the vessel wall with a micropipette, illustrating thevalidity of the technique used to determine the vessel passivetension versus circumference relation.

The following terms and formulas were used to define themechanical status of each vessel. Passive wall tension is theproduct of radius and microvascular pressure (newtons permeter), as defined by the Laplace relation, and refers toconditions in which vascular tone was fully abolished. Totalwall tension is the product of microvascular pressure andradius (newtons per meter) and for our purposes is measuredwhen the arteriolar wall is capable of generating active tensionsuch that total tension is the sum of active plus passive tension.Active wall tension (newtons per meter) is calculated forspecific vessel diameters by subtracting the passive tension atthat diameter from the total wall tension; we assume thatactive and passive tensions are predominantly arranged inparallel.

The pressure and vessel inner radius data for passiveconditions were used to calculate the passive tension innewtons per meter using the Laplace relation, assuming a near0 mm Hg external pressure. The passive tension and circum-ference data were used to calculate a linear regression fit ofthe logio °f passive tension versus circumference. We have



ownloaded from


Bohlen and Lash Arteriolar Wall Forces in Hypertension 759

TABLE 1. Body and Cardiovascular Characteristics of Young and Adult Wlstar-Kyoto and SpontaneouslyHypertensive Rats

Variable Y-WKY Y-SHR A-WKY A-SHR

Weight, g 309.6±10.3 298±10.8 560.3±17 356±6.9*

Mean arterial pressure, mm Hg 102.2±3.5 167.6±4.3* 106.2±2.4 174.2±6.8*

Inner resting diameter, ^m 71.7±3.0 59.2+3.0* 62±3.8 45.4±3.4*

Arteriolar pressure, mm Hg 60.4+3.4 93.9±6* 58.2±2.8 81.9±5.2*

Total wall tension, N/m 0.288±0.021 0.382±0.042* 0.243±0.02 0.224±0.038

Active wall tension, N/m 0.184±0.018 0.276±0.029* 0.174±0.021 0.195±0.019

Passive wall tension, N/m 0.105±0.023 0.104±0.024 0.079±0.019 0.057±0.014

Active/total tension, % 65.6±6.4 74.0±6.6 72.8+8.6 70.1 ±3.2

Passive slope tension/circumference, (N/m)^m 0.0072±0.0007 0.0084±0.0008* 0.0087±0.0008 0.0096±0.0012*

Intercept, N/m -2.69±0.19 -2.66+0.15 -2.98±0.23 -2.63±0.15

Y indicates young (13 to 15 weeks); A, adult (25 to 27 weeks); WKY,*Ps.O5, equal-aged WKY rats and SHR.

Wistar-Kyoto rats; and SHR, spontaneously hypertensive rats.

previously shown that a linear fit of the log-transformed dataaccurately represents the passive tension versus circumferencerelation (r2a.9) for arterioles of normal and diabetic rats.1-2

Equivalent results were obtained for all groups of SHR andWKY rats in that the minimum r2 value for any animal was .89.The slope (M) and intercept (B) of the relation allowed us tocalculate the passive tension (Tp) at any given circumference(C) as Tp=10<|MxC1+B). This is essential for interpolation be-tween passive data points because many different diameterswere encountered during application of various amounts ofboth vasoconstrictor and dilator drugs, which altered activevessel wall tension generation directly and passive tensionindirectly as the vessel changed diameter.

All calculations were performed with LOTUS 1-2-3 forWindows and CSS:STATISTICA (StatSoft, Inc) on a Gateway486 33-mHz computer. Three-way ANOVA (strain, age,dose) was used to find significant effects, and a least signif-icant difference analysis was used for individual group com-parisons. A probability value less than or equal to .05 wasconsidered to indicate a significant difference. All data arepresented as mean and 1 SEM.

ResultsThe data in Table 1 indicate that the 13- to 15-week-

old WKY rats and SHR (Y-WKY and Y-SHR) hadequivalent body weights, but by age 25 to 27 weeks,adult WKY (A-WKY) rats were much larger than adultSHR (A-SHR). SHR were very hypertensive comparedwith WKY rats at each age; however, during the intervalfrom 13 to 27 weeks, mean arterial pressure changedlittle. At rest, SHR arterioles were constricted com-pared with their normotensive age-matched counter-parts, and arteriolar pressure was much higher thannormal. In Y-SHR the arteriolar hypertension wassufficient to significantly (P<.05) increase total andactive tension approximately 30% and 50% despite anapproximate 17% vasoconstriction relative to the nor-mal resting diameter. However, in A-SHR the approx-imately 25% constriction substantially offset the approx-imately 30% increase in arteriolar pressure such thattotal and active tensions were normal. Active tensionwas dominant and accounted for approximately 70% ofthe total tension in all groups.

Although passive tension at rest was comparable for agiven age group of WKY rats and SHR (Table 1), thedata in Fig 1 illustrate that the arterioles of both SHRgroups had passive tension-circumference relations

which were shifted to the left of that for their age-matched controls. In effect, the passive elements re-sisted vessel expansion much more in SHR than inWKY rats, even at their constricted resting diameter. Asan additional consequence, a higher-than-normal mi-crovascular pressure was required to maintain a givenpassive diameter. Fig 1 shows the mean resting circum-ference and total tension for each animal group. Thesedata emphasize that when active tension is present, it isthe dominant contributor to total tension. However,should the arterioles dilate, passive tension increasesexponentially as the circumference enlarges, particu-larly in SHR, which have a steeper tension-circumfer-ence relation.

In Fig 2, the possible active tensions for each circum-ference at the prevailing resting arteriolar pressure(Table 1) have been calculated using methods previ-ously developed,1 and the resting active tension at thecontrol circumference is plotted for each group. For aconstant microvascular pressure over a range of circum-ferences, one can calculate the total tension at anycircumference and then subtract the passive tension at

0 50 100 150 200 250 300 330

ARTERIOLAR INNER CIRCUMFERENCE IN UICRONS

FIG 1. Plot shows total tension vs vessel inner circumferencerelations for passive conditions and resting values when activetension is present. Coefficients for each passive curve arepresented in Table 1. Overall data indicate that for a given age ofyoung (13 to 15 weeks) or adult (25 to 27 weeks) spontaneouslyhypertensive rats (SHR) the passive curve shifts left such that ahigher tension occurs at smaller-tnan-normal diameters. Nor-motensive Wlstar-Kyoto (WKY) rats show a slight tendency forthe same type of shift with advancing age. Data were obtainedfrom the following numbers of vessels with the number ofanimals in parentheses: young WKY, 11 (7); young SHR, 12 (7);aduft WKY, 12 (11); and adult SHR, 10 (8).



ownloaded from



O.4

0 3

0.2

0.1

0 0

o

A

YOUNG•-•• YOUNG

- ADULT• - • ADULT

WKYSHR

WKYSHR

.•'fF"'\ACETYLCHOLINE

0 90 100 ISO 200 290 300 190

ARTERI01AR INNER CIRCUMFERENCE IN MICRONS

FK3 2. Plot shows active tension vs vessel inner circumferencerelations in young and adutt Wistar-Kyoto (WKY) rats and spon-taneously hypertensive rats (SHR). Each curve represents theactive status for a specific circumference at resting arteriolarpressure (listed in Table 1). Symbols with error bars representresting active state and circumference as listed in Table 1. Notethat arterioles of both normotensive and hypertensive rats havecontrol active tension near peak tension. Numbers of vesselsand animals are as in Fig 1.

that circumference (determined from the passive ten-sion-circumference relation). The result is the esti-mated active tension for a constant microvascular pres-sure at any given circumference. In Y-SHR activetension was approximately 50% higher than normal, butactive tension was not significantly elevated in A-SHR(Table 1, Fig 2). For each animal group, the restingactive tension calculated from radius and microvascularpressure measurements was at or near the peak possibletension for the prevailing arteriolar pressure (Y-WKY,93.3±2.1%; Y-SHR, 93.1±4.2%; A-WKY, 96.4±9.3%;A-WKY, 92 ±1.6%). The relatively flattened top of theparabolic active tension versus circumference relationallowed the active tension to be near maximal, but forSHR the resting diameter was substantially below thediameter at which maximum tension occurred. ForY-SHR the resting diameter (circumference) was90.8±3.7% of that for maximal tension developmentcompared with 104.4+5.3% (/><.O3) for Y-WKY; val-ues for A-SHR and A-WKY were 83.3 ±4.5% and103.3±8.3% (P<.03), respectively.

The top panel of Fig 3 presents the actual diameters ofarterioles in all groups as the dose (current) of acetyl-choline was increased, and the bottom panel presents thepercentage of control responses to acetylcholine. For agiven age group during release of a given acetylcholinedose, there were no significant (/><.O5) differences be-tween arteriolar inner diameters of age-matched WKYrats and SHR. However, arterioles of Y-SHR andA-SHR were constricted significantly (P<.05) relative tothose of WKY rats at rest. As a consequence, the relativedilation of SHR arterioles was greater than that of WKYarterioles at all doses, from threshold to maximal re-sponse. During these measurements, arteriolar pressurechanged little because only a single vessel dilated. As anexample, which is representative for all groups, theresting arteriolar pressure in A-SHR before applicationof acetylcholine was 87.8±6.7 mm Hg, and during re-lease of the 25, 50, 100, and 200 nA acetylcholine dosesit was 89.4±7.8, 89.7±7.8, 91.1±7.5, and 92.1±6.9mm Hg, respectively. The overall data for Y-SHR andA-SHR during acetylcholine application indicate thatendothelium-mediated dilation by acetylcholine is not

50 100 200

CURRENT (NA)

FIG 3. Bar graphs show actual diameters and percentage ofcontrol responses during iontophoresis of acetylcholine ontoarterioles of young (Y) and adult (A) Wistar-Kyoto (WKY) rats andspontaneously hypertensive rats (SHR). On an absolute basis,SHR vessels dilate as much or more than normal and reachnormal diameters at all doses at and above 25 nA. On apercentage of control basis, Y-SHR vessels demonstrate equalresponses to normotensive age-matched WKY vessels, andarterioles of A-SHR exceed responses by vessels of A-WKY.Numbers of vessels and animals are as in Fig 1.

impaired relative to that in WKY rats if judged on thebasis of vessel enlargement.

The diameter responses during microiontophoreticapplication of nitroprusside, in terms of both absolutediameter and percentage of control diameter, are shownin Fig 4. On a relative basis, shown in the bottom panelof Fig 4, the data indicate that cyclic GMP-mediatedrelaxation allowed equivalent dilation of arterioles inage-matched normotensive and hypertensive rats. How-ever, in absolute terms, as shown in the top panel of Fig4, SHR arterioles were initially constricted relative totheir normotensive counterparts, and vasodilation withnitroprusside did not fully overcome this constrictiondespite comparable absolute changes in diameter. Aswas found during acetylcholine application, microvas-cular pressure was essentially constant as the nitroprus-side dose was increased for all groups. As an example,the resting arteriolar pressure in A-SHR before appli-cation of acetylcholine was 81.8±5.2 mm Hg, and dur-ing release of the 25, 50, 100, and 200 nA nitroprussidedoses it was 81.3±5.4, 80.8±5.5, 83±5.9, and 84.3±6.2mm Hg, respectively.

In a situation in which microvascular pressure isrelatively constant as vasodilation is induced by acetyl-choline and nitroprusside, the active tension will followthe active tension versus circumference curve, as shown



ownloaded from



11010090

80

70

60

50

40

30

20

10

NITROPRUSSIDEFT] I

L;

CONTROL 25 50 100

CURRENT (NA)

200

180

170

160

150

140

130

120

110

100

• p<0.05 YWKY VS YSHR I+p<0.03 AWKY VS ASHR X

50 100

CURRENT (NA)

200

FIQ 4. Bar graphs show actual diameters and percentage ofcontrol responses during iontophoresis of nitroprusside ontoarterioles of young (Y) and adult (A) Wistar-Kyoto (WKY) rats andspontaneously hypertensive rats (SHR). On an absolute basis,SHR vessels change diameters as much as or more than normalbut do not fully reach normal diameters for most nitroprussidedoses. On a percentage of control basis, vessels of YSHR andASHR demonstrate responses equivalent to normotensive age-matched WKY vessels. Numbers of vessels and animals are asin Fig 1.

in Fig 5 for young WKY rats and SHR; similar eventsoccurred for the adult groups. For the depression ofactive tension during acetylcholine and nitroprussideapplication to be appreciated, the active tensions areshown in Fig 6. In Y-SHR, both acetylcholine andnitroprusside caused a suppression of active tensionequal to or greater than that in normotensive rats.However, in A-SHR, active tension did not decreaseproportionately as much as in normotensive animals,except at near-maximal dilation. Arterioles of adultSHR and WKY rats have similar active tensions at rest;therefore, during various degrees of vasodilation up tonear-maximal dilation, absolute tensions were approxi-mately 50% higher than normal in A-SHR. This wasconfirmed by the calculation of active tensions shown inFig 6.

Application of norepinephrine to the arteriolar wallcaused equivalent constriction in A-SHR and A-WKY,but Y-SHR demonstrated greater absolute and relativeconstrictions than occurred in Y-WKY (Fig 7). Micro-vascular pressure decreased by small amounts in nor-motensive rat arterioles as the norepinephrine dose wasincreased, whereas an approximate 30% decrease inarteriolar pressure occurred in the hypertensive rats.

-f- CONTROL YOUNG WKY

- y - CONTROL YOUNG SHR

CLOSED - NITROPRUSSIDE

OPEN - ACCTYLCHOUNE

SO 100 130 300 230 300

AATERIOLAR INNER CIRCUMFERENCE IN U1CRON3

FIG 5. Plot shows active tension vs circumference of arteriolesfrom young spontaneously hypertensive rats (SHR) and Wistar-Kyoto (WKY) rats beginning at rest and as the acetylcholine ornitropnjsside dose was increased to cause progressively greaterdilation. As stated in "Methods," acetylcholine- and nitroprus-side-induced dilation had virtually no effect on microvascularpressure, and as one would expect the tension-circumferencepoint for each dose falls along the control isobar as shown.Similar results were obtained for adult animals but are not shownto avoid clutter.

These data are shown in Table 2. We believe that theminimal change in arteriolar pressure despite a 70% orgreater constriction in normotensive arterioles can oc-cur because constriction is restricted to only severalhundred microns on either side of the site of drugrelease. In comparison, arterioles of hypertensive ratsconstricted for at least 1 mm in both the upstream anddownstream directions, and side branch vessels con-stricted as much or more than the parent vessel. Asidentical pipette and drug conditions were used in allgroups, the longitudinally extended constriction in SHRprobably reflects either enhanced cell-to-cell communi-cation in the vessel wall or unusually great drug diffu-sion, which is highly unlikely with the microiontophore-sis technique used. The overall consequence is thatconstriction of SHR arterioles is enhanced by thedecreased distending pressure secondary to the effectsof vasoconstriction over an extended length of thevessel.

The specific large arterioles studied are known9 tohave approximately a 40% hypertrophy of the cross-sectional area of the muscle cell layer by age 19 to 21weeks in SHR, in the absence of hyperplasia andincreased cell length. These arterioles did not exhibithypertrophy of the muscle cells in SHR at age 4 to 6weeks.9 For purposes of mathematical modeling of theeffect of muscle layer hypertrophy on the wall stressversus circumference relation, the vascular smooth mus-cle area of the normal arterioles was assumed to be 500/im2 and 700 /xm2 for both groups of hypertensiveanimals, which represents a 40% increase in musclelayer area, as found in the earlier morphometric study.9

Active wall stress was calculated as the active tensionfor a specific circumference divided by the estimatedmuscular wall thickness at that circumference.1 Theresults of the mathematical model are shown in Fig 8 forthe entire range of circumferences at the resting arteri-olar pressure in each group. The active wall stress at restfor each individual vessel was determined, and thegroup averages are shown. The implications of themathematical model will be considered below.



ownloaded from



= 0.35

z 0.30

d, 0.25

0.20

* 0.15

K 0-1°

0.05

^ 0.00

ACETYLCHOLINE

•

1

j !

%'*',*'A'A

•

;" ; !s / J

N " / '

+

1

>- !!v - / •

t ^ i YOUNGE553 YOUNGES3 ADULTCD ADULT

+

i

hi J :

4-

1 I

I I

I T~

l I

< ' « s n

WKYSHR

WKYSHR

TTVi

0.35 NITROPRUSSIDE

C0NTR0L25 50 100 200

CURRENT (NA)

C0NTR0L25 50 100 200

CURRENT (NA)

DiscussionThe primary purpose of this study was to determine

to what extent active and passive wall tensions increasein intestinal arterioles of SHR to maintain or constrictvessel diameters in the presence of microvascular hy-pertension. As shown in Fig 1, for any given circumfer-ence or diameter a greater passive tension resists vesselexpansion in SHR than in WKY rats in both young andadult animals. Had we presented the data as the passivevessel diameter versus pressure relation, SHR arterioleswould be significantly smaller in diameter than theirnormotensive counterparts at similar distending pres-sures just as Baumbach and colleagues38 have reportedfor SHR cerebral arterioles. The data in Fig 1 clearlyindicate that both the developmental and sustainedphases of hypertension in SHR are associated withstructural changes in the vessel wall that result inhigher-than-normal passive tension development atsmaller diameters when compared with the arterioles oftheir normotensive counterparts. The same conclusioncan be derived from the data published by Baumbach eta l 3 8 for SHR cerebral arterioles. This increase in pas-sive wall tension minimizes the extent to which activetension must increase to maintain the vessel at any givendiameter in the presence of an elevated microvascularpressure. The combined result of vasoconstriction andincreased passive tension allows the vascular smoothmuscle of intestinal arterioles in adult SHR to havenormal active tension development at rest (Fig 1)despite a 30% to 35% increase in microvascular pres-sure. Although in young SHR active tension develop-ment is higher than in WKY rats, an additional 25%increase in active tension would be required to maintaintheir constricted diameter if there were no increase inthe passive resistance to distension. Therefore, thedecreased ability for passive vascular expansion reducesthe load that the active components of the vessel wallmust bear, but simultaneously limits the ability of thevessel to dilate unless microvascular pressure is ele-vated. However, complete compensation by passiveelements does not occur based on the fact that thevascular smooth muscle cells of these arterioles areknown to hypertrophy by approximately 40%,9 as judged

FIG 6. Bar graphs show activetension in arterioles of normoten-sive and hypertensive rats duringcontrol and drug release condi-tions for acetylcholine and nitro-prusside. Except at the highestdoses used for both drugs, whichvirtually eliminated the ability ofvascular smooth musde to de-velop tension, arterioles in adult(A) spontaneously hypertensiverats (SHR) do not decrease theiractive tension as readily as dothose in normotensive rats; a sim-ilar situation exists in young (Y)SHR during nitroprusside applica-tion. This suggests a deficit invascular smooth muscle relax-ation to cyclic GMP-mediatedmechanisms, even though data inFigs 3 and 4 indicate that relativeand absolute diameter changesare of normal to greater-than-nor-mal magnitudes.

by an increased cross-sectional area of the total musclelayer and no evidence of cellular hyperplasia.

The preceding discussion leads to the issue ofwhether the microvasculature ultimately attempts toregain a normal active tension and stress by compensa-tory structural growth and regulatory events. Based ona mathematical model of vessel wall stress using me-chanical data obtained in this study and morphologicaldata from a previous study on similarly aged animalsand identical types of arterioles,9 we have determinedthat the active wall stress is normal at rest in young SHR(Fig 8), but active tension is elevated (Figs 1 and 2,Table 1). However, although active tension appears tobe fully normalized by age 25 to 27 weeks in adult SHR(Figs 1 and 2), the vessel wall appears to have overcom-pensated in terms of active stress, in that it is 65% to70% of normal (Fig 8). In fact, if both wall hypertrophyand vasoconstriction occur, it is mathematically impos-sible for both active tension and stress to be within anormal range in arterioles of hypertensive animals. Ourdata indicate that SHR did not maintain either of theseindexes of the active state within the normal range asboth, young and adult animals. However, for any givenage at least one of these indexes appeared to be in thenormal range.

As previously noted, it is difficult to propose that anygiven mechanical index of the active state is activelymaintained in a normal range in the presence of luminalhypertension and constriction. However, it is possiblethat SHR vessels attempt to regulate the active state asa constant fraction of some ideal status rather thanmaintain a specific tension or stress. Our data wouldsupport such regulation because active tension andstress are at about the same fraction of peak activestatus (80% to 90%) in both young and adult SHR andWKY rats (Figs 2 and 8). In view of the major differ-ences in diameter, intravascular pressure, and wallthickness between normotensive and hypertensive rats,it seems beyond coincidence that normal fractions ofmaximum would exist for both indexes of the activestatus in all animal groups, especially when the absolutevalues can be substantially different between age-matched WKY rats and SHR as well as WKY rats andSHR at different ages. However, it may be that the



ownloaded from



NOREPINEPHRINE

• p<0.05 Y-WKY VS. Y-SHR+p«J.O5 A-WKY VS. A-SHR

CONTROL 23 SO 100

CURRENT (NA)

CURRENT (NA)

FIG 7. Bar graphs show actual diameters and percentage ofcontrol responses during iontophoresis of norepinephrine ontoarterioles of young (Y) and adult (A) WIstar-Kyoto (WKY) rats andspontaneously hypertensive rats (SHR). Y-WKY arterioles con-stricted to smaller actual and relative diameters than normal, butresponses by A-SHR arterioles were similar to those in nor-motensrve rats. Data shown are based on same vessels andanimals used for previous figures.

vessels are not attempting to maintain some normalfraction of maximal active status but are trying toachieve near-maximal active status for the prevailingmicrovascular pressure, passive structural forces in thevessel wall, and muscle layer thickness. The relativelysmall deviation from this goal may be due to impair-ments induced by modification of the physiologicalstatus by surgery and anesthesia as well as small com-putational and measurement errors that influence pre-diction of the maximal active status. A similar achieve-ment of near-peak active tension and stress in resting

intestinal arterioles has also been found for rats withacute and chronic insulin-dependent diabetes,2 eventhough absolute active wall tensions and stresses wereabnormal. Therefore, regulation of near-peak activestate at rest appears to occur under a wide variety ofpathological conditions in intestinal large arterioles andin a wide variety of differently aged normotensiveanimals. As we did not find evidence that the modifica-tions of vessel behavior and structure maintained anabsolute value of tension or stress, perhaps these mod-ifications occur to resolve the even more complex re-quirement of maintaining near-maximal performancefor the evolving changes in long-term resting physicalconditions.

The consequence of an active regulatory system thatstrives to maintain near-peak performance is that whenthe vessel has a higher passive tension for any givendiameter, the peak active tension and wall stress alwayswill occur at a smaller-than-normal diameter for allmicrovascular pressures at and above normal. Notice inFigs 2 and 8 that for a given age group the isobar foractive tension or stress and the circumference (diame-ter) at the peak of the curves for young and adult SHRare shifted to the left of the normal range. This occursbecause increased passive tension limits the upperrange of the diameter for any given pressure such thatthe active tension-length curve is restricted to a lowerrange of vessel circumferences and diameters, includingthe circumference at the peak active state.

We fully expected to find a decrease in the dilatoryresponse to acetylcholine similar to that generally re-ported for in vitro studies of mesenteric arteries ofSHR.1314'19-20 On both an absolute and relative basis,the arterioles of young and adult SHR increased diam-eters as much as and in some cases more than thevessels of WKY rats at equal doses of both acetylcholineand nitroprusside (Figs 3 and 4). However, the absolutediameter of the arterioles in both SHR groups remainedsmaller than normal at each nitroprusside dose, whichmay simply reflect the vasoconstriction at rest andgreater passive forces impeding vessel dilation (Fig 1,Table 1). Perhaps the best insight into endothelial cell-and cyclic GMP-mediated relaxation is provided by thedecreases in active tension shown in Fig 6. Both acetyl-choline and nitroprusside suppressed the active tensionin young SHR as much as or more than normal, butneither mechanism was as potent in adult SHR. If theend point of relaxation is to reduce active tension, thenadult SHR have decreased cyclic GMP relaxation inresponse to nitroprusside. However, if in vivo dilation isthe frame of reference, the mechanisms function rela-tively normally to both acetylcholine and nitroprusside

TABLE 2. Microvascular Pressures at Rest Immediately Before and After 3 Minutes of lontophoreticApplication of Norepinephrine to Arteriolar Walls in Young and Old WIstar-Kyoto andSpontaneously Hypertensive Rats

Group

Y-WKY (n=9)

Y-SHR (n=12)

A-WKY (n=12)

A-SHR (n=8)

Control

56±4.4

81.0±6.1

53.9±3.3

79.5+9.5

25 nA

52.4±5.7

74.4±6.3

51.7±3.9

80.4±9.6

50 nA

53.9±6.4

60.3±8.4*

46.6±3.2*

54.9±6.8*

100 nA

51.2±7.3

51.2±5.9*

48.7±3.9

47.3±6.7*

200 nA

54.5±6.8

48.5±9.9*

46.5±6.8

49.7±8.8*

Y indicates young (13 to 15 weeks); A, adult (25 to 27 weeks); WKY, Wistar-Kyoto rats; and SHR, spontaneouslyhypertensive rats.

*P<.05, control vs response.



ownloaded from



YOUW WKYYOUNO IHRADU1.T WXYADULT SHR

150 MO 110 MO

CtRCUKFIItCNCC M UCKOHS

FIG 8. Plot shows estimated active vessel wall stress at rest andover the range of circumferences possible for resting arteriolarpressure for the four animal groups. Data are the result ofmathematical modeling of wall stress in the muscle layer; as-sumptions and parameters used in the model are given in"Results." Results of the model predict that arterioles of bothnormotenslve and hypertensive young and adult rats maintainapproximately 90% of the maximum possible stress they coulddevelop at resting arteriolar pressure. Resting active stress isestimated to be near normal in young spontaneously hyperten-sive rats (SHR) but below normal in aduit SHR. WKY indicatesWistar-Kyoto rats.

application in young and adult SHR (Figs 3 and 4).Therefore, in the sense of reducing arterial resistanceand increasing blood flow in response to an endotheli-um-mediated vasodilator or direct agonist of musclecyclic GMP, the vessels could function normally as longas the arteriolar pressure is elevated even though abso-lute decreases in active tension are reduced in adultSHR.

Beginning with the first demonstration by Folkow etal21 of increased reactivity to norepinephrine in thehindlimb vasculature of SHR, there has been a generalconsensus that norepinephrine causes greater-than-nor-mal vasoconstriction in SHR. The data from intestinalarterioles of young SHR shown in Fig 7 are certainlyconsistent with this consensus and demonstrate con-striction comparable with that observed in arterioles ofthe cremasteric muscle of similarly aged SHR22 whenexposed to identical norepinephrine release onto thevessel wall. In contrast, the same vessels in adult SHRdo not demonstrate vasoconstriction, in absolute orrelative terms, that is greater than normal (Fig 7). Themicrovascular responses in young and adult hyperten-sive rats were accompanied by a major decrease inmicrovascular pressure during point source applicationof norepinephrine. As shown in Table 2, the localizedvasoconstriction during application of norepinephrinehad a minor effect on microvascular pressure in nor-motensive rats, but as shown in Fig 7 the absolutechanges in arteriolar diameters in equivalently agedWKY rats and SHR were similar. As explained in"Results," the much more extensive distance over whichvasoconstriction occurred in both young and adult SHRcaused a major decrease in arteriolar pressure andhence the total active tension and stress that must bedeveloped to maintain constriction. This raises the issueof whether exaggerated conduction of vasoconstrictionalong the arterioles is yet another means by whichinappropriate increases in resistance occur in responseto constrictor agents in hypertensive rats.

Ack nowledgmen tsThis research was supported by grant HL-20605 from the

National Institutes of Health, Bethesda, Md. The authorsthank Mary Ann Neill for her technical assistance during thestudy.

Referencesl .

n.

Lash JM, Bohlen HG, Waite L. Mechanical characteristics andactive tension generation in rat intestinal arterioles. Am J Physiol1991;260:H1561-H1574.

2. Lash JM, Bohlen HG. Structural and functional origins of sup-pressed acetylcholine vasodilation in diabetic rat intestinal arte-rioles. Ore Res. 1991;69:1259-1268.

3. Baumbach GL, Dobrin PB, Hart MN, Heistad DD. Mechanics ofcerebral arterioles in hypertensive rats. Ore Res. 1988;62:56-64.

4. Baumbach GL, Siems JE, Heistad DD. Effects of local reduction inpressure on distensibility and composition of cerebral arterioles.Ore Res. 1991;68:338-351.

5. Baumbach GL, Heistad DD. Remodeling of cerebral arterioles inchronic hypertension. Hypertension. 1989;13:968-972.

6. Hajdu MA, Heistad DD, Ghoneim S, Baumbach GL. Effects ofantihypertensive treatment on composition of cerebral arterioles.Hypertension. 1991;18(suppl II):II-15-II-21.

7. Hajdu MA, Heistad DD, Baumbach GL. Effects of antihyper-tensive therapy on mechanics of cerebral arterioles in rats. Hyper-tension. 1991;17:3O8-316.

8. Baumbach GL, Heistad DD, Siems JE. Effect of sympatheticnerves on composition and distensibility of cerebral arterioles inrats. J Physiol (Land). 1989;416:123-140.

9. Miller BG, Connors BA, Bohlen HG, Evan AP. Cell and wallmorphology in intestinal arterioles from 4- to 6- and 17- to19-week-old Wistar-Kyoto and spontaneously hypertensive rats.Hypertension. 1987;9:59-68.

10. Baumbach GL, Walmsley JG, Hart MN. Composition and mechanicsof cerebral arterioles in hypertensive rats. Am J PathoL 1988;133-464-471.Angus JA, Dyke AC, Jennings GL, Korner PI, Sudhir K, Ward JE,Wright CE. Release of endothelium-derived relaxing factor fromresistance arteries in hypertension. Kidney Int. 1992;42(suppl 37)-S-73-S-78.

12. Fukuda N, Izumi Y, Minato M, Soma M, Watanabe Y, WatanabeM, Hatano M. Cyclic GMP formation of resistance vessel in thedevelopment of hypertension in spontaneously hypertensive ratsJpn Ore J. 1991;55:721-728.

13. Kong JQ, Taylor DA, Fleming WW. Mesenteric vascular responsesof young spontaneously hypertensive rats. / Pharmacol Exp Ther1991;258:13-17.

14. Diederich D, Yang Z, Buhler FR, Luscher TF. Impaired endothe-lium-dependent relaxations in hypertensive resistance arteriesinvolve cyclooxygenase pathway. Am J Physiol. 1990;258-H445-H451.

15. Boegehold MA, Bohlen HG. Arteriolar diameter and tissueoxygen tension during muscle contraction in hypertensive rats.Hypertension. 1988;12:184-191.

16. Borders JL, Granger HJ. Power dissipation as a measure ofperipheral resistance in vascular networks. Hypertension. 1986;8-184-191.

17. Bohlen HG, Gore RW. Preparation of rat intestinal muscle andmucosa for quantitative microcirculatory studies. Microvasc Res.1976;11:103-110.

18. Bohlen HG. Determinants of resting and passive intestinalvascular pressures in rat and rabbit. Am J Physiol 1987-253-G587-G595.

19. Watt PAC, Thurston H. Endothelium-dependent relaxation inresistance vessels from spontaneously hypertensive rats. J Hypertens.1989;7:661-666.

20. Fuji K, Tominaga M, Ohmori S, Kobayashi K, Koga T, Takata Y,Fujishima M. Decreased endothelial-dependent hyperpolarizationto acetylcholine in smooth muscle of the mesenteric artery ofspontaneously hypertensive rats. Ore Res. 1992;70:660-669.Folkow B, Hallback M, Lundgren Y, Weiss L, Background ofincreased flow resistance and vascular reactivity in spontaneouslyhypertensive rats. Acta Physiol Scand. 1970;80:93-106.

22. Bohlen HG. Arteriolar closure mediated by hyperresponsivenessto norepinephrine in hypertensive rats. Am J Physiol 1979;236:H157-H164.

21



ownloaded from


H G Bohlen and J M LashActive and passive arteriolar regulation in spontaneously hypertensive rats.

Print ISSN: 0194-911X. Online ISSN: 1524-4563 Copyright © 1994 American Heart Association, Inc. All rights reserved.

is published by the American Heart Association, 7272 Greenville Avenue, Dallas, TX 75231Hypertension doi: 10.1161/01.HYP.23.6.757

1994;23:757-764Hypertension.

http://hyper.ahajournals.org/content/23/6_Pt_1/757World Wide Web at:

The online version of this article, along with updated information and services, is located on the

http://hyper.ahajournals.org//subscriptions/

is online at: Hypertension Information about subscribing to Subscriptions:

http://www.lww.com/reprints Information about reprints can be found online at: Reprints:

document. Permissions and Rights Question and Answer process is available in the

Request Permissions in the middle column of the Web page under Services. Further information about thisOffice. Once the online version of the published article for which permission is being requested is located, click

can be obtained via RightsLink, a service of the Copyright Clearance Center, not the EditorialHypertension Requests for permissions to reproduce figures, tables, or portions of articles originally published inPermissions:



ownloaded from

http://hyper.ahajournals.org/content/23/6_Pt_1/757

http://www.ahajournals.org/site/rights/

http://www.lww.com/reprints

http://hyper.ahajournals.org//subscriptions/


active and passive arteriolar regulation in spontaneously

Documents