Effect of Changes in Hydrostatic Pressure in Peritubular

Capillaries on the Permeability of the Proximal Tubule

JOHNP. HAYSLETT

From the Departments of Medicine and Pediatrics, Yale University School ofMedicine, NewHaven, Connecticut 06510

A B S T R A C T The effect of increased hydrostatic pres-sure in the peritubular vessels on net sodium reabsorptionfrom the proximal tubule was examined in the Necturus.An increase in the pressure gradient of 2.0 cm H20across the wall of the proximal tubule, produced by liga-tion of the postcaval vein was associated with a markedreduction in net reabsorption and an increased backflux of water and electrolytes. This change was accom-panied by a slight, but significant drop in the transepi-thelial electrical potential but not by an alteration in thesteady-state chemical gradient. These studies highlightthe importance of changes in the permeability charac-teristics of the proximal tubule on net sodium transport.

INTRODUCTION

Recent studies have shown that alterations in hydrostaticand oncotic pressure in peritubular capillaries (1, 2),may influence net sodium reabsorption in the proximaltubule. While it is not certain that hydrostatic pressurevaries to a great enough extent under physiological con-ditions to play a significant role in sodium reabsorption(3), studies from our laboratory indicate that changes inoncotic pressure subserve this mechanism as a result ofvariations in filtration fraction (4). The final pathway bywhich these factors alter sodium reabsorption remainsunclear. It is not known, for instance, whether an in-crease in the hydrostatic pressure gradient diminishesdiffusion from intercellular spaces into capillaries, in-creases back diffusion into the cell or tubular lumen, ordirectly influences active sodium transport.

In the present experiments the mechanism by which arise in the hydrostatic pressure of peritubular capillariesinhibits net proximal reabsorption was studied in theamphibian Necturus maculosa. Since transport processes

Dr. Hayslett is an Established Investigator of the Ameri-can Heart Association.

Received for publication 9 February 1972 and in revisedform 26 January 1973.

occur at a much slower rate in the amphibian, comparedwith the mammal, the active and passive components ofsodium transport can be more easily characterized. Inaddition, the dual renal blood supply in the lower verte-brate permitted significant alterations in hydrostaticpressure without affecting oncotic pressure or causingapparent concurrent changes in renal blood flow.

METHODSExperiments were performed on adult Necturi (LembergerCo. Ashkosh, Wisconsin) weighing 100-200 g which werestored unfed at 15-18'C. Half-time studies were performedduring July and August. Studies to determine steady-statesodium values and transepithelial potentials were performedduring November to March.

The animals were prepared for micropuncture using tech-niques similar to those described by Shipp et al. (5). Tri-caine methanesulfonate (MS-22, 660 mg/liter) was used forinduction of anesthesia. During experiments anesthesia wasmaintained with MS-22 in a dose of 60 mg/liter. All animalsreceived an infusion of amphibian phosphate buffer solutioncontaining 119 meq Nat, 2.4 meq K@, 120 meq Cl-, 1.0 meqHP04=, 0.85 meq H2PO4-, and 0.5 meq Ca++ per liter at arate of 0.04 ml/min via a constant infusion pump (HarvardApparatus Co., Inc., Millis, Mass.; model 901). Amphibianphosphate buffer was used to bathe the kidney surface duringall experiments. For measurements of steady-state concen-trations of sodium the tip of the collection pipette was sealedwith oil before it was withdrawn from the tubular lumen.

In order to increase the hydrostatic pressure within peri-tubular capillaries the postcaval vein was ligated at the levelof its entry into the liver. Approximately 20 min wereallowed to elapse after this procedure before beginningmicropuncture measurements. Experiments were performedonly in animals in which blood flow in peritubular vesselswas brisk. It should be noted that the postcaval vein hasnumerous branches between the renal veins and its entranceinto the liver. Presumably alterations in resistance in thesebranches were responsible for the maintenance of blood flowthrough the renal venous portal system into the postcavalvein following venous ligation. Ligation of the postcavalvein immediately cephalad to the kidneys resulted in com-plete secession of blood flow within the portal system. Inall cases blood flow in the renal portal system continued inthe normal direction, toward the medial border of the kidney,after ligation was applied.

1314 The Journal of Clinical Investigation Volume 52 June 1973 1314-1319

Postcard ino Ivein

Mesonephric

Renalportalsystem

Lateral Medial

FIGURE 1 This diagram illustrates the relationship of the renal portal system and branchesof the renal arteries on the ventral surface of the right kidney of Necturus maculosa.

Renal vasculature, measurements of hydrostatic pressureand rate of blood flow in peritubular vessels. The intra-renal branches of the renal venous portal system could beeasily distinguished visually via a binocular microscopefrom branches of the renal arteries by the direction of bloodflow. Flow was toward the medial border of the kidney invenous portal vessels and toward the lateral surface in thearterial system. The hydrostatic pressure was measured invessels using the method of Landis as modified by Flaniganand Oken (6). Since the diameter of all vessels exceeded theoutside tip diameter (8-10 tm) of the glass pipette by severalfold, occlusion of the vessel did not occur.

Additional studies were performed in another group ofanimals to verify the origin of vessels selected for pressuremeasurements. Microfil (Canton Biomedical Co., Boulder,Colo.) was used to perfuse the renal venous system via thecaudal vein and the renal arterial system via the abdominalaorta. By using different colors in perfusing each system theentire vascular could be examined after dehydration andclearing with methylsalicylate. The relationship of the twovascular systems is shown in Fig. 1. These studies confirmedthe visual observations that tributaries of the renal portalsystem constituted the largest proportion of the renal vas-cular and formed the peritubular vessels.

Studies were performed in five animals to determine theeffect of ligation of the postcaval vein on the rate of bloodflow through peritubular vessels before and after ligation.The same criteria was used in selecting these animals forstudy as was used in the other physiological experiments.Since the urine flow rate was insufficient to permit measure-ments of para-aminohippuric acid clearance in the non-di-uretic state, the velocity of blood flow was estimated visu-ally. After introduction of a micropipette into a straightsegment of a peritubular vessel a bolus of 5% lissaminegreen was injected and the velocity of the day front over a

distance of approximately 1.0 mmwas measured with theaid of a filar eye piece micrometer. The edge of the dyefront was easily distinguishable in these experiments. Theaverage value of three to five measurements from the con-trol and experimental period was determined for each ani-mal. In order to determine whether the diameter of the peri-tubular vessels changed as a result of venous ligation andincrease in hydrostatic pressure photomicrographs, usingEthachrome E daylight film, were taken of the same surfaceareas before and after ligation under the same magnifica-tion. Measurement of vessel diameters were made fromprojections of the transparencies onto a glass screen. Thevalues for the diameter before and after ligation at thesame loci are expressed as a ratio. Estimates of volumewere not made because of the apparent change in vesseldiameter that is normally observed in surface vessels.

Reabsorptive half-time. The rate of net proximal reab-sorption was measured using the split droplet technique ofGertz (7). All studies were performed on the early por-tion of the proximal tubule and only straight segmentswere used. Phosphate buffer amphibian Ringer's solutionwas used as perfusate. The length of the oil column onboth sides of the aqueous droplet averaged about 10 timesthe diameter of the tubule. Measurements of tubular di-ameter and droplet length were made using a filar eye piecemicrometer calibrated with a stage micrometer. The diameterof the droplet was measured as the diameter of the oilcolumn and droplet length as the distance between menisciof the oil droplets. Approximately 60 s elapsed after com-pletion of perfusion before the initial length of the dropletwas measured. The change in droplet length was expressedas a ratio of the initial length and the slope of each dropletseries was calculated as a regression curve. The results areexpressed as the reabsorptive half-time (to)-the time re-quired for the droplet shrink to 50% of its original length.

Proximal Tubule Permeability during Increased Hydrostatic Pressure 1315

Observations were continued until the droplet length havereached at least 50% of the initial length.

Studies of permeability characteristics. Split dropletstudies were performed using isosomotic raffinose solution(200 mosm/liter) as the perfusion solution. The change indroplet length in relationship to the initial length was esti-mated in a way similar to the reabsorptive half-time study.When isotonic raffinose is injected between oil droplets theinitial increase in volume of the raffinose solution will re-flect the passive isotonic entry of electrolytes and water andisotonic exit of raffinose down their concentration gradient.Since raffinose is a relatively impermeant non-electrolyteand the initial luminal concentration of electrolytes is zero,the early expansion of the droplet is primarily an expressionof the permeability of the tubular wall for water. In thesestudies only the initial linear portion of the ascending phaseof expansion was estimated. From this slope the doublingtime (t2ho), the time required for the initial volume todouble, was calculated. In these experiments and in studiesof isotonic reabsorption, droplet length was used as an esti-mation of volume since droplet diameter was constant.

Steady-state sodium concentration. A solution containingan isosmotic raffinose solution was perfused into the proxi-mal tubule previously blocked with oil, and 20-Al samplesof fluid were collected. All samples were collected when theraffinose droplet was decreasing in size. Under these con-ditions a state of zero net flux is approached and electrolyteconcentration reaches a steady-state equilibrium (8). Tubularfluid samples were analyzed for sodium using an ultramicroflame photometer. Serum from the abdominal aorta wasanalyzed for sodium using an IL integrating flame pho-tometer (Instrumentation Laboratory, Inc., Lexington,Mass.).

Transepithelial potentials. Transepithelial electrical po-tentials were measured using Ling-Gerard electrodes coupledto a high impedance millivoltmeter (Medistor InstrumentCo., Seattle, Wash.) through calomel half cells. The glassmicroelectrodes had resistances of 10-20 megohms and tippotentials of less than 5 mV in solutions containing raffinoseand salts in equilibrium concentration. Only potentials whichwere stable for at least 1 min and fell to no greater than1 mV upon withdrawal were accepted as adequate.

Isosmotic raffinose solution was used as the perfusate andall measurements were made during the descending expo-nential phase of the raffinose curve as determined visually.

Studies of peritubular capillary protein concentration.During control and experimental periods samples of approxi-mately 30 nl of peritubular blood were taken by direct micro-puncture in four animals for measurement of plasma proteinconcentration. The method used in these determinations inthis laboratory has been described (4). Preliminary studiesshowed that measurement of plasma protein values in Nec-turi by the Lowry method, utilized in these experiments,gave approximately the same value as determined by thebiuret technique. The ratio of Lowry to biuret values deter-mined on the same samples from six animals was 0.92.

All data is expressed as mean ±4SE and comparison be-tween groups was made using Fishers or Student's t test.

RESULTSLigation of the postcaval vein at its entry into the liverresulted in a prompt and sustained increase in the hydro-static pressure in the peritubular branches of the renalportal system. Since blood flow either continued to bebrisk or was reconstituted within minutes after constric-

tion it was assumed that sufficient collateral circulation waspresent cephalad to the kidneys. As shown in Table I thehydrostatic pressure was 2.64±0.12 cm H20 in controlanimals and rose to 5.22±0.26 in experimental animals(P < 0.001). In a group of five animals the hydrostaticpressure was simultaneously measured in the peritubularvessel and intratubular lumen within a droplet of iso-tonic Ringer's solution before and after venous ligationto determine whether the experimental procedure causeda measurable gradient across the wall of the proximaltubule. Three to five observations were made during thecontrol and experimental period in each animal. Duringthe control period the hydrostatic pressure in peritubularvessels was 3.18±0.73 cm H20 and 3.38+0.73 in thelumen (P= NS). After ligation of the abdominal cavathe pressure in the vessel rose to 6.18±0.91 cm H20 butremained unchanged in the intratubular lumen, 3.82±0.91H20. Under the experimental condition, therefore, apressure gradient of 2.36±0.34 cm H20 was produced,which was highly significant (P < 0.005).

Measurement of the velocity of blood flow in peri-tubular vessels was unchanged following venous obstruc-tion. The linear flow rate was 2.21±0.36 mm/s duringthe control period and 2.01+±0.24 mm/s (P = NS) in theexperimental condition. The ratio of vessel diameterfrom 15 observations in five animals was 1.04±0.06(P = NS). Since blood flow was unchanged by ligationof the postcaval vein at its entrance into the liver, itseems likely that dilation of collateral vessels cephaladto the kidney accounted for the maintenance of blood flowthrough the renal portal system. Venous obstruction didnot apparently cause an alteration in the hydrostaticpressure of the intrarenal arterioles.

There was a marked reduction in net fluid reabsorp-tion, associated with increased hydrostatic pressure ofperitubular vessels. Reabsorption half-time (ti) in-creased from 20.7±2.2 min to 123.6±34.2 min (P <0.005). Water outflux calculated from the individual tivalues and tubular radii (r), as Jv = (0.347/ti) r mm3/mm2min, was also significantly reduced in the experi-mental animals compared with control animals. Theseobservations were not influenced by variations in thesize of the aqueous droplet between groups, since bothinitial droplet length and droplet diameter were similar.

Studies were performed with an isosmotic raffinosesolution in order to determine whether an increase inperitubular hydrostatic pressure altered passive perme-ability of the tubule to electrolytes and water. Analysisof the initial ascending phase of expansion of the dropletdemonstrated a significant increase in passive perme-ability to electrolytes and water in experimental animals.The t2ho fell from 6.4±0.6 min in control animals to4.1±0.5 in the experimental group. During volume ex-pansion in the Necturus an increase in permeability to

1316 1. P. Hayslett

TABLE IEffect of Hydrostatic Pressure in Peritubular Vessels on Net Fluid Reabsorption

and Permeability in the Proximal Tubule*

Control Experimental P

Hydrostatic pressure, cm H20Peritibular vessels

Intrarenal arterioles

Reabsorptive half-time (ti), min

Net fluid outflow

JA = (0.347/tj)r mm3/mm2min

Droplet diameter, jum

Initial droplet length, .um

Inflow doubling time (t2ho), min

Initial fluid inflow

Jv = (0.347/t2ho)r mm3/mm2min

Droplet diameter, jm

Initial droplet length, /Am

2.64±0.12n = 28a = 15

1 1.6±0.54n =8a =8

20.7±2.2n = 21a = 8

2.17 X 10-640.28n = 21a = 8

118.5±6.2n = 21a =8

207.5434.7n = 21a =8

6.4±0.6n = 22a =5

5.85 X 10-640.67n = 27a =5

106.8±4.7n = 22a =5

100.5± 7.6n = 22a =5

5.224±0.26n = 32a = 13

10.5±0.55n= 13a =9

123.6±34.2n = 17a =6

0.58 X 10-140.10n = 17a =6

119.1±t6.1n = 17a =6

223.8±30.5n = 17a =6

4.1±0.5n = 25a =5

8.63 X 10-540.84n = 25a =5

100.842.8n = 25a =5

107.3 12.6n = 25a =5

n represents number of observations, and a represents number of animals studied.* Values are mean ±SE.

raffinose, as well as to electrolytes and water has beendemonstrated (9). If a similar increase in permeabilityoccurred in the present study, the observed changeswould have underestimated the relatively increased back-fAux of electrolytes and water during experimental con-

ditions. As in the studies on net isotonic reabsorption,there was no difference in droplet diameter or initialdroplet length between the experimental and controlgroups.

In stationary perfusions with isosmotic solutions of raffi-nose the relatively slow negative volume change, which

follows initial expansion, is due to the raffinose leak, sincethe reflection coefficient (8) does not equal 1. During thisphase the ionic concentration of sodium in the intraluminalfluid approximates the steady-state limiting conditions.As shown in Table II the steady-state luminal concen-

tration of sodium was approximately 69 meq/liter inboth control and experimental animals and the chemi-cal gradient for sodium across the tubular epithelium was

unchanged by the increase in hydrostatic pressure. Un-

der the same conditions there was a small but significantdrop in the transepithelial electrical potential from

2roximal Tubule Permeability during Increaed Hydrostatic Pressure

<0.001

NS

<0.005

<0.001

NS

NS

<0.01

<0.02

NS

NS

1317

TABLE I ISteady-state Equilibrium Concentrations and Transepithelial

Potential Measurements*

Control Experimental P

Plasma sodiumconcentration, meq/liter 90.1 413.3 95.84-3.5 NS

n =7 n =6a=7 a=6

Intraluminal sodiumconcentration, meq/liter 67.5 42.6 68.5:1:1.9 NS

n =28 n =24a =7 a =5

Concentration gradient,TF/PNa 0.75±0.14 0.74±0.13 NS

n =28 n =26a =7 a =6

Transepithelial electricalpotential, mV 14.640.6 12.940.5 <0.05

n = 45 n = 54a = 10 a =8

n represents number of observations, and a represents number of animalsstudied.* Values are means ±SE.

14.6±0.6 mV in control animals to 12.9±0.5 in animalswith increased hydrostatic pressure.

Although no change in the protein concentration ofthe venous blood in the portal system was expected tohave occurred as a result of ligation the protein concen-tration in peritubular vessels was determined. Ligationof the postcaval vein did not change this value which was4.93±0.13 g/100 ml under control and 4.88±0.23 duringexperimental conditions (P= NS). There is no ap-parent explanation for the high absolute values obtainedwhich were greater than the usually reported levels of2-3 g/100 ml in Necturus plasma.

DISCUSSIONIt seems clear from recent studies that sodium reabsorp-tion in the proximal tubule varies in a proportional wayto the oncotic pressure in peritubular capillaries (2, 4)and inversely to the hydrostatic pressure (3, 10). Sinceboth hydrostatic pressure and oncotic pressure contributein a similar way to the pressure gradients between thecapillary and intercellular spaces, it is likely that themechanism by which changes in these factors exert theireffect on net sodium reabsorption is also similar. In thepresent experiments it was possible to study the effectof increased hydrostatic pressure on proximal tubularfunction in vivo without changes in oncotic pressure orrenal blood flow. In addition, the use of stopped flowmethods provided a means for evaluating sodium trans-port independently of changes in the filtered load of so-dium, tubular geometry, and tubule fluid velocity.

When the gradient for hydrostatic pressure across thetubular wall was increased by at least 2.0 cm H20 therewas a marked reduction in net sodium reabsorption.

Studies of tubular wall permeability demonstrated thatunder these same conditions there was an enhanced back-flux of water and electrolytes into the tubular lumen.Since previous observations have demonstrated (11) thatnet sodium efflux is associated with a large sodium back-flux into the tubule, these data support the concept thatan increased pressure gradient exerts its action primarilythrough augmentation of this process. Using "C-labeledsucrose as a marker for small molecules, Bank, Yarger,and Aynedjian (12) have recently reported that duringrenal vein constriction in the rat an increase in the per-meability of the proximal tubule occurs. In this study,however, alterations in the physical characteristics of theproximal tubule could not be dissociated from markedchanges in cortical blood flow. Moreover, a change inthe pressure gradient across the tubular wall was notdemonstrated. Grandchamp, Baechtold-Fowler, and Boul-paep have reported data in abstract form (13) fromexperiments designed to study the effect of hydrostaticand oncotic pressure on sodium reabsorption in thedoubly perfused Necturus kidney. Although a decreaseof the oncotic pressure of the perfusate (from 20 g/literpolyvinylpyrrolidone to 0 g/liter) reduced net reabsorp-tion, no change in reabsorption was found after a two-fold increase of capillary venous pressure. It is difficultto compare that study with the present experiments be-cause of the differences in technique. In the doubly per-fused kidney preparation, the organ is perfused with ablood-free artificial plasma at an undetermined capillaryflow rate, in contrast to these in vivo studies where theorgan was blood perfused. An alteration in the reflectioncoefficient of the proximal tubules when perfused withartificial plasma might explain the lack of change in so-dium reabsorption after a twofold increase in hydro-static pressure while a marked reduction was found invivo after a onefold increase.

Although it seems likely that the mechanism by whichhydrostatic pressure and oncotic pressure exert theireffect on the rate of net reabsorption is similar for dif-ferent epithelial membranes, this study and previous re-ports indicate that quantitative differences exist. Forexample, relatively large changes in oncotic pressureare necessary to influence the net reabsorption of fluidand electrolytes in the proximal tubule of the rat (4),while gradients as small as 0.5 cm H20, opposite to thedirection of flow, have been reported to inhibit transportacross frog skin (14) and gallbladder (15).

Electrical measurement, summarized recently byGiebisch, Baulpaep, and Wittembury (16) have shownthat the total transepithelial conductance greatly ex-ceeds that expected on the basis of resistive contributionsof the bordering cell membranes. This observation sug-gests that the presence of a low resistance shunt pathparallel with cell membrane resistances and the inter-

1318 J. P. Hayslett

cellular space has appeared to provide the logical ana-tomical counterpart. In an elegant series of experimentsBoulpaep (9) has demonstrated that reduced net sodiumreabsorption in the volume expanded Necturus resultsprimarily from increased backflux along the intercellularpath, reducing the efficiency of the active transportmechanism. He showed a significant rise in transepi-thelial conductance, without an alteration in cell mem-brane resistance, and an increase in proximal tubularpermeability for electrolytes and water and non-electro-lytes such as raffinose.

The fall in transepithelial potential difference duringvenous ligation is compatible with an increased shuntconduction. Under these same conditions, however, thetubular epithelial cells were capable of producing a nor-mal steady-state concentration gradient during a phasewhen zero net flux was approached. Since this measure-ment in part reflects the active component of sodiumtransport, this observation suggests that alteration inthe pressure gradient does not alter the characteristicsof the transport mechanism.

Taken together, therefore, these observations and thestudy of Boulpaep in volume expanded animals highlightthe importance of intercellular pathways in the regula-tion of net sodium transport by the proximal tubule.

ACKNOWLEDGMENTSThe author gratefully acknowledges the technical assistanceof Mrs. Kate Jefferys, Mrs. Kathleen Renquist, and Mr.Richard Whittaker and the advice of Dr. Michael Kash-garian with the electrical measurements.

These experiments were performed in part at the MountDesert Island Biological Laboratory, Salisbury Cove, Maine.

This work was supported by U. S. Public Health ServiceGrants AM-5015 and HE 13647-OlAl, and by the AmericanHeart Association.

REFERENCES1. Lewy, J. E., and E. E. Windhager. 1968. Peritubular

control of proximal tubular fluid reabsorption in the ratkidney. Am. J. Physiol. 214: 943.

2. Brenner, B. M., K. H. Falchuk, R. I. Keimowitz, andR. W. Berliner. 1969. The relationship between peri-tubular capillary protein concentration and fluid reab-sorption by the renal proximal tubule. J. Clin. Invest.48: 1519.

3. Falchuk, K. H., B. M. Brenner, M. Tadakoro, and R.W. Berliner. 1971. Oncotic and hydrostatic pressuresin peritubular capillaries and fluid reabsorption by proxi-mal tubule. Am. J. Physiol. 220: 1427.

4. Weinman, E. J., M. Kashgarian, and J. P. Hayslett.1971. Role of peritubular protein concentration in so-dium reabsorption. Am. J. Physiol. 221: 1521.

5. Shipp, J. C., I. B. Hanenson, E. E. Windhager, H. J.Schatzmann, G. Whittembury, H. Yoshimura, and A.K. Solomon. 1958. Single proximal tubules of the Nec-turus kidney. Methods of micropuncture and microper-fusion. Am. J. Physiol. 195: 563.

6. Flanigan, W. J., and D. E. Oken. 1965. Renal micro-puncture study of the development of anuria in the ratwith mercury-induced acute renal failure. J. Clin. Invest.44: 449.

7. Gertz, K. H. 1963. Transtubulare natriumchloridflosseund permeabilitat fur nichtelekrolyte im proximalen unddistalen konvolut det rattennierre. Pfluegers Arch. Ge-samte. Physiol. 276: 336.

8. Kashgarian, M., H. Stockle, C. W. Gottschalk, and K. J.Ullrich. 1963. Transtubular electrochemical potentials ofsodium and chloride in proximal and distal renal tubulesof rats during antidiuresis and water diuresis (diabetesinsipidus). Pfluegers Arch. Gesamte Physiol. 277: 89.

9. Boulpaep, E. L. 1972. Permeability changes of the proxi-mal tubule of Necturus during saline loading. Am. J.Physiol. 222: 517.

10. Koch, K. M., H. S. Aynedjian, and N. Bank. 1968. Ef-fect of acute hypertension on sodium reabsorption bythe proximal tubule. J. Clin. Invest. 47: 1696.

11. Oken, D. E., G. Whittembury, E. E. Windhager, andA. K. Solomon. 1963. Single proximal tubules ofNecturus kidney. V. Unidirectional sodium movement.Am. J. Physiol. 204: 372.

12. Bank, N., W. E. Yarger, and H. S. Aynedjian. 1971.A microperfusion study of sucrose movement across therat proximal tubules during renal vein constriction. J.Clin. Invest. 50: 294.

13. Grandchamp, A., H. Baechtold-Fowler, and E. L. Boul-paep. 1971. Influence of peritubular physical factors onproximal sodium reabsorption in perfused Necturus kid-neys. Fifth Annual Meeting, American Society of Neph-rology. 5: 28.

14. Nutbourne, D. M. 1968. The effect of small hydrostaticpressure gradients on the rate of active sodium trans-port across isolated living frog-skin membranes. J.Physiol. 195: 1.

15. Keimowitz, R. I., and T. L Stanton. Large decrementsin gallbladder transport as a result of small serosal hy-drostatic pressure gradients. Clin. Res. 20: 598.

16. Giebisch, G., E. L. Boulpaep, and G. Wittembury. 1971.Electrolyte transport in kidney tubule cells. Philos.Trans. R. Soc. Lond. Ser. B. Biol. Sci. 262: 175.

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