pulmonary ventilation and cardiac activity in hibernating and arousing golden-mantled ground ...

7/29/2019 Pulmonary Ventilation and Cardiac Activity in Hibernating and Arousing Golden-Mantled Ground Squirrels (Spermop

1/9

Steffen, J. M. , and M. L. Riedesel " L982. Pulmonary ventilationand cardiac activity in hibernating and arousing golden-mantledground squirrels (Spermophilus lateralis). Cryobiology 19: 83-91.cRYoBroLocY 19,83 91 (1982)

Pulmonary Ventilation and Cardiac Activity in Hibernating andArousing Golden-Mantled Ground Squirrels (Spermophilus lateralis)J. M. STEFFEN' aNo M. L. RIEDESELDeparttnent oJ' Biologl', University of Net, Mexico, Albuquerque, Nev' Mexico 87131

Mammalian hibernation is characterizedby a pronounced depression of body tem-perature. a marked reduction of whole-body metabolism, and the capacity tospontaneously arouse and rewarm to nor-mothermia within several hours without re-course to an exogenous source of heat. Theprocess of arousal is associated with ele-vated rates of oxygen consumption (1'7,34)and heat production (14). Thus the hiber-nating, arousing. and normothermic states,all of which may be encountered withinseveral hours, are characterized by widelydiffering metabolic demands. Metabolicintegrity is insured by: (i) biochemicaladaptations that permit continued operationand regulation of key enzymatic reactionsover a wide range of body temperatures,and (ii) individual and concerted actions ofthe respiratory and cardiovascular systemswhich insure that oxygen and substrate de-livery, as well as carbon dioxide elimina-tion, are matched to the specific metabolicrequirements.Cardiovascular parameters such as heartrate (3), blood pressure (7), cardiac output(28), and regional blood flow (1) have beenexamined in both hibernating and arousinganimals. Quantitative evaluation of pulmo-nary ventilation has been hampered, how-ever, by the sensitivity of the hibernatinganimal to handling. This has limited the ap-plication of precise methods of investiga-tion in this area (24.35). In addition, few

Received May 19, 1981; accepted July 31, 1981.I Present address: Dept. Physiology and Biophysics,University of Louisville School of Medicine, HealthSciences Center. Louisville. KY. 40292.

studies have attempted to correlate ventila-tion with cardiac activity during hibernationor arousal (21,22). The purpose of the pres-ent study was twofold: (i) to monitor pul-monary ventilation with a modified ple-thysmographic technique which allowsthe collection and analysis of end-tidalgases, and (ii) to simultaneously recordventilatory and cardiac activity in hiber-nating and arousing ground squirrels.METHODS

Golden-mantled ground squirrels (Sper-mophilus lateralis) of both sexes weretrapped in the Jemez mountains of northernNew Mexico. Following capture animalswere caged singly and provided with foodand water ad libitum. The diet consisted ofcommercial rat chow supplemented withapples, lettuce, carrots, and peanuts. FromOctober through April squirrels werehoused in darkness in an environmentalchamber al'| -1- 2"C. with food and wateravailable. Body weights at the time of studyranged from 154 to 254 g.Oxygen uptake (Vor) was measured in a1.0-liter closed vessel maintained at 8 +- 1"Cin a walk-in cold room. Animals were al-lowed a 30-min acclimation period in thevessel and thereafter gas samples weretaken at approximately 1-hr intervals. Fol-lowing sampling the chamber was flushedwith room air by a small pump attached tothe vessel. Two or three measurementswere made and averaged for each squirrel.Gas samples were introduced into an oxy-gen analyzer (Model S-3A, Applied Elec-lrochemistry. Sunnyvale. Calii.) and Vo,calculated (STPD) from the gas concentra-

o0 1 1 -2240 821 0 10083-09$02. 00/0Copyright @ 1982 by Academic Press, lnc.All rights of reproduction in any form reserved

83


2/9

84 STEFFEN AND RIEDESEL

Frc. 1. Diagram of the plethysmograph and instrumentation for monitoring ventilation and heartrate-

tion, the chamber volume, and the timeelapsed between closure of the chamberand sampling.The plethysmographic setup for mea-surement of ventilatory parameters is illus-trated in Fig. 1. Squirrels wore a close-fitting hard plastic face mask (0.5 to 1.5-mldead space) secured to the neck by a rubberdiaphragm to create a tight seal. To reducethe incidence of arousal caused by place-ment of the mask on the animal, hibernatingsquirrels were handled at least three timesper week for a period of several monthsprior to experimental use. The mask com-municated with the exterior through a shortlength of plastic tubing. Changes in bodyvolume resulting from inspiration and expi-ration resulted in intrachamber pressurefluctuations which were monitored by adifferential pressure transducer (ModelMP45-1, Validyne, Northridge, Calif.) at-tached to a Validyne transducer indicatorand recorded. Calibration of the system was

achieved by injecting and withdrawingknown volumes of air from the chamber atintervals between breaths. A uniform calib-ration signal throughout the experiment in-sured that no leaks were present in thesystem. Catheter tubing, attached to a smallpump and placed within several millimetersof the nares, served a dual function: (i) topull air continuously past the nares and en-sure that the inspired gas was effectivelyroom air, and (ii) to pull expired gasesthrough an S-3A oxygen analyzer or LB-lcarbon dioxide ar'alyzer (Beckman, PaloAlto, Calif.). To insure the accurate mea-surement of end-tidal gas pressures lowsample flow rates (30 to 60 ml ' min-1) weremaintained during experiments on hiber-nating animals with flow rates increased forarousing animals. With low-sample flowrates end-tidal plateaus were recorded forboth oxygen and carbon dioxide. There-fore, end-tidal gas pressures were taken asrepresentative of alveolar values. Gas


3/9

VENTILATION INanalyzers were calibrated with dry room airor commercial gases.Breathing frequency (f), tid,al volume(Vr) and minute ventilation (VB) were ob-tained directly from the record of chamberpressure changes. Ventilatory parameterswere based on expired volumes. pulmonarygas exchange ratio, R", was calculated fromthe equation (11):{', : Q""qr) (l - Frc)l(Prn, * Poor)- (P^co, 'Fro"), - - (l)where A : alveolar (end-tidal), I : in-spired, and F,o, is the fractional concen-tration of inspired oxygen. Alveolar venti-lation (Vo) was calculated from the equation

Va : Rn' V y2(RTIP ^co). Q)where R : the gas constant (2.785 mlBTPS'Torr'oK.ml STPD) and 7: the ab-solute temperature in'K. Dead space (Vp)was calculated from the equation:Vo : (Va - Vo)/.f. (3)

Calculation of the dead space allowed com-parison of measured minute ventilation andcalculated alveolar ventilation, and ensuredthat they were in acceptable agreement. Inaddition, because Vo was calculated fromend-tidal gas pressures. comparison of Vuand Vo provided a test for the reliability ofend-tidal gas pressure measurements.Skin temperature (I"u) over the scapulaewas measured with a copper constantanthermocouple attached to a potentiometer.Ambient temperature inside the plethys-mograph was monitored with a telether-mometer (Model 46, Yellow Springs Inst.Co.. Yellow Springs, Ohio). At the termi-nation of each experiment both rectal (I...)and cheek pouch (2"1) temperatures weremeasured with a Yellow Springs telether-mometer. For measurement of Zr", thetelethermometer probe was inserted aminimum of 3 cm.Several weeks prior to each experiment,two 3-cm pieces of silver wire were insertedsubcutaneously over the scapular region ofhibernating squirrels. The inserted end was

Spermophilus lateralis 85exteriorized and the ends were fastened to-gether in a safety pin type arrangement. Atthe time of experimentation the silver wireswere connected to a Model MK IV cardiacpreamplifier (E and M Inst. Co., Houston,Tex.). This system served to monitor theheart rate and the electrocardiogram.Data were collected on eight hibernatingground squirrels for periods ranging from 4to 30 hr. Data on the arousal process wereobtained from 5 to 16 squirrels for each10-min period during the first hour ofarousal.

RESULTSWhile observations on hibernating groundsquirrels generally lasted from 4 to l0 hr,the ability to continuously record ventila-tion and heart rate for periods of up to

30 hr emphasizes that the techniques em-ployed in the present study did not con-stitute an unduly irritating stimulus to thehibernating animal.A periodic breathing pattern was ob-served in all hibernating ground squirrels(Figs. 2 and Fig. 3). Nonventilatory periodsextending as long as 40 min were occasion-ally noted, though apneic intervals aver-aged 8 to 9 min. Ventilation was always ini-tiated by an inspiratory movement and wasgenerally completed with an expiration(Fig. 3). Breathing frequencies during ven-tilatory periods ranged from as low as 10 toas high as 40 breaths min-r. Due to theextended length of apneic intervals theoverall breathing frequency was low duringhibernation (Table 1). In the hibernatinganimal V.. fluctuated markedly during indi-vidual breathing periods (Fig. 3). The largertidal volumes were frequently noted at theonset of ventilation (Fig. 3.4,). Values for Voand Vo represented 60 and 407o, respec-tively, of the tidal volume. Hibernatingground squirrels also exhibited Cheyne-Stokes respiration, with a waxing and wan-ing of the tidal volume during periods ofventilation (Fig. 38).In any given period of ventilation the ini-


4/9

86 STEFFEN AND RIEDESELHIBERNATION

ARLY AROUSAL

Ftc. 2. Representative breathing and heart rate patterns during hibernation, early arousal, and at Ihr into the arousal process. The upper recording in each section is ventilation while the lower recordingis heart rate. lnspiration is represented by an upward pen deflection.

{93

tialP"",,, was low whilePu",.o, was high. Asa series of breaths continued Pu.,..,,,, de-creased (COz washout) while Po,ro, in-creased. The initial Pu,rco,, was in the rangeof 26 to 32 Torr and the initial Pn".,, was 90to 95 Torr. As end-tidal plateaus were ob-tained for both gases, the end expiratoryvalues (Table 1) were taken as representa-tive of alveolar gas pressures and were usedto calculate other parameters.Individual measurements of V,-,, rangedfrom 0.358 to 0.862 ml STPD'Kg'min 1.The V6o, (Table 1) was calculated from theV6, utilizing the mean Ru value obtainedfrom the end-tidal gas pressures.

The heart rate of hibernating squirrelsaveraged 10.3 beats min I (Fig. 4). Car-diac rhythms which were typically seenduring hibernation are illustrated in Fig. 3C.Prior to ventilation, pairing of heart beats isevident. This is followed by a marked in-crease in heart rate prior to and during ven-tilation. Cardiac activity wanes to belowthe preventilatory rate as breathing ceases.In addition to variable cardiac rhythms, theelectrocardiogram was also altered byhibernation, with a lengthening of the P-Rand abbreviation of the R-T intervals(Table 2).During the monitored period of arousal


5/9

VENTILATION IN Spermoplrilus lateralis

r1ml

t-.]"lminIt*l

Frc.3. Recordings ofrespiratory and cardiac activity in hibernating S. lateralis. Inspiration isrepresented by an upward pen deflection. The lower recording in C is heart rate. Note the preventila-tory pairing of heart beats, increase in heart rate prior to and during ventilation, and waning of heartrate as ventilation ceases.

87

B

r.r rose from 9.6 to 12.5"C. Final 7"r, and 7."averaged 15.8 and 12.8'C, respectively. Theonset of the arousal process was charac-terized by several respiratory adjustments.Apneas quickly disappeared and respirationbecame continuous. Additionally, Vt be-came quite uniform in contrast to the vari-ability of this parameter during the periodicventilatory bursts in hibernation (Fig. 2).By the end of the first hour of arousal yFhad increased 25-fold. Adjustments in heartrate qualitatively paralleled those of breath-ing frequency during arousal (Fig. a).Quantitatively. however, heart rate in-creased only 7-fold during the monitoredarousal period compared with a 26-foldincrease in breathing frequency. The in-crease in V" during the first hour of arousalis therefore completely accounted for by anincrease in breathing frequency with V1remaining unchanged. The disparate changes

in heart rate and breathing frequency duringarousal resulted in a marked change in theratio of heart beats per breath (Fig. 5).DISCUSSION

Oxygen consumption is markedly re-duced during hibernation (17). The y,,2measurements of hibernating S. lateralis

TABLE 1Respiratory Parameters of Hibernating S. lateralis.l (min-,) 2.1 + 0.2' (8)aVn (ml BTPS'kg ') 4.8 * 1.1 (8)v, (ml BTPS kg ') 11.6 + 1.3 (8)V_n (ml BTPS kg min 1) 14.3 + 0.8 (8)Vu (ml BTPS.kg.min 1) 24.4 * 2.7 (8)Poto" {Torrr 106.6 - 1.4 (8)P.r, f , tTorrt 23.8 1.0 (81RE 0.71 + 0.04 (8)Vu, rml STPD.kg.min r 0.541 - 0.009(ll)v,;" {ml STPD kg min 't 0.148 0.015 (ll)" Mean :! SEM.o Number of animals


6/9

88 STEFFEN AND RIEDESEL

o lo 'o *ni.,or* oo 50 60FIc. 4. Heart rate and breathing frequency duringarousal of S. Iateralis. The 0 time value representshibernating squirrels. Vertical lines represent SEMnumbers in parentheses indicate number of animals.are in accord with previously reported val-ues for a variety of ground squirrel species(10. 16. 28. 341. The V.rfalls at lhe lowerend of the range reported for the hibernat-ing golden-mantled ground squirrel byHammel ('t al . (l 4). The V.o,. eslimate


7/9

VENTILATION IN .Sperzopft ilus lateralisas the dormouse (27) and hedgehoE$2).Inman, periodic ventilation of the Cheyne-Stokes type has been associated with aprimarily hypoxic ventilatory drive (5, 30).Blood of hibernating animals is bright redand appears well oxygenated. In addition,hibernators are highly resistant to anoxia(17). In view of the duration of the apneicperiods and greatly reduced V.,, in thehibernating ground squirrel, hypoxia wouldnot be predicted to play a central rolein the regulation of ventilation duringhibernation.The effect of reduced body temperatureduring hibernation on the medullary andpontine respiratory centers has not beenwell studied. In this context, however,Cherniack et al. (4) have induced periodicrespiration (Cheyne-Stokes) in the anes-thetized cat by ventral medullary cooling.Depression of body temperature has alsobeen reported to lengthen apneic periodsin reptiles (11).The values for both breathing frequencyand tidal volume are similar to previous re-ports on other hibernating species (8, 12,22, 25, 35). Tidal volume did not changesignificantly during the process of arousal.his is in contrast to observations of thearousing pocket mouse (35). Eliassen (7)nd Bullard (1) have reported a constantstroke volume in arousing hibernators. Iniew of the relatively small absolute volumewhich could be accommodated byheart and lungs of these smaller mam-als, it is conceivable that both changes inoutput and minute ventilation areprimarily by adjustments of fre-This hypothesis is supported byhe present finding that the increase in Vuarousal can be quantitatively ac-for by an increase in breathing fre-The end-tidal gas pressures reported inpresent study correlate well with previ-measurements of blood gases in hiber-animals. TheP"r"o, of 24Torr agreeswith the value of 28 Torr reported for

arterial blood samples taken from hiber-nating S. lateralis (33). An arterial P.o, of26.5 Torr was observed by Tahti (32) in thehibernating hedgehog immediately follow-ing a period of ventilation. Other inves-tigators have likewise reported arterial P"o,values below 30 Torr in hibernating animals(18,29,31). There have been only two re-ports of arterial Po, in hibernating animals.A value of 88 Torr has been reported for thel3-lined ground squirrel (26), while a valueof 120 Torr has been observed in the hiber-nating hedgehog (32). The P"ro, of 107 Torrin the hibernating golden-mantled groundsquirrel approximates these values.The reliability of the end-tidal gas pres-sure measurements is also reflected in thevalue of Ro, 0.71, which was calculated as-suming that end-tidal samples were repre-sentative of alveolar gas. This value is con-sistent with previous determinations byother investigators (17), and points to alipid-based metabolism in the hibernatingground squirrel. Thus it appears that sam-ples of end-tidal gas can reasonably be usedto predict arterial gas pressures in thehibernating ground squirrel.In agreement with previous work (3, 19,22) the heart rate is low in the hibernatinganimal. Pairing of heart beats in the hiber-nating golden-mantled ground squirrel issimilar to previous observations of thehibernating hedgehog and arctic groundsquirrel (6). The increased cardiac activityobserved during ventilatory periods has notpreviously been reported. However, thisobservation is reminiscent of the "bursts ofhearl beals" reported to occur at regularintervals in the hibernating arctic groundsquirrel (6). These simultaneous changes inventilatory and cardiac activity may repre-sent yet another example of the physiologicintegrity of mammals during natural hiber-nation. Enhanced cardiac activity duringlung ventilation may illustrate a parallelismbetween mammalian hibernators and divingmammals and reptiles.The qualitative correlation between


8/9

90 STEFFEN AND RIEDESELbreathing frequency and heart rate duringarousal is apparent from Fig. 4. Initiallyduring arousal the ratio of heart beats tobreaths is reduced markedly, but thereafterremains steady at this new level. Thissuggests that arousal from hibernation re-quires the establishment of a new functionalrelationship between the respiratory andcardiovascular systems.The lengthened P-R interval of theground squirrel electrocardiogram confirmsprevious reports of some degree of A-Vdissociation in the heart of the hibernatingmammal (6, 9). The relatively short R-7interval recorded from hibernating squir-rels. however, indicates that the ventriculartissue is still capable of rapid repolarizationeven at greatly reduced body temperatures.In conclusion. an assessment of ventila-tion in the hibernating mammal without al-tering either the form or timing of respira-tion has been made using a modifiedplethysmographic technique. This studyhas documented a periodic ventilatory pat-tern in the hibernating golden-mantledground squirrel with cardiac activity in-creased during periods of lung ventilation.It was observed that end-tidal gas composi-tion approximated the expected arteriolargas pressures in the hibernating animal. Inaddition. a new functional relationship be-tween the respiratory and cardiovascularsystems appears to be set up during theprocess of arousal from hibernation.

SUMMARYPulmonary ventilation was assessed inthe hibernating and arousing golden-mantled ground squirrel by plethysmog-raphy and end-tidal gas analysis. The heartrate and electrocardiogram were monitoredsimultaneously with ventilation. The hiber-nating squirrels displayed a periodic respi-ratory pattern characterized by a variablebreathing frequency and tidal volume andoften exhibited characteristics of Cheyne-Stokes respiration. Apneic periods averag-ing 8 to 9 min resulted in a low overallbreathing frequency. Cardiac activity in

the hibernating ground squirrel was charac-Ierized by arrhythmias which correlatedwith ventilation and by alterations in theelectrocardiogram typically seen in hiber-nating animals. Arousal from hibernationwas accompanied by: (i) a replacement ofperiodic by continuous ventilation, (ii) a25-fold increase in Vu in the first hour whichwas accounted for by the increment inbreathing frequency. and (iii) a marked de-crease in the ratio ofheart beats to breaths.The techniques developed in the presentstudy will permit further quantitative in-vestigations of pulmonary ventilation andits control in hibernating animals.

REF ERENCESl. Bullard, R. W. Changes in regional blood flow andblood volume during arousal from hibernation.Ann. Ar;ad. S


9/9

VENTILATION IN Spermophilus lateralis 9l

14

12.

l3

5.

6.

Respiratory responses to long term temperatureexposure in the box turtle,Terrapene ornata. J.Comp. Physiol. 131, 353-359 (1979).Goodrich, C. A. Acid-base balance in euthermicand hibernating marmots. Amer. J. Physiol.224, rt85-1189 (1973).Guy, A. J., and Patrick, J. M. The breathing pat-tern after breath-holding: The influence ofchestposition and the drive to breathe. Quart. J . Exp.Physiol. 63, 341 -352 0978).Hammel, H. T., Dawson, T. J., Abrams, R. M.,and Anderson, H. T. Total calorimetric mea-surements on Citellus lateralis in hibernation.Physiol. Zool. 41,341-357 (1968).Jameson, E. W., Jr. Patterns of hibernation ofcaptive Citellus Iateralis and Eutamiasspeciosus. J. Mammal. 45,455-460 (1964).Kayser, C. Les e'changes respiratoires de la gre-nouille (R. esculenta) et du spermophile (Citel-lus citellus) entre 0" et 10"C. C.R. Soc. Biol.144,1697,1699 (1950).Kayser, C. "The Physiology of Natural Hiberna-tion." Pergamon Elsmford, New York, 1961.Kreienbuhl, G., Strittmater, J., and Ayim, E.Blood gas analyses of hibernating hamsters anddormice. Pfleugers Arch. 366, 16'7 -172 (1976).Kristoffersson, R., and Soivio, A. Hibernation inthe hedgehog (Erinaceus europaeus L.).Changes of respiratory pattern, heart rate andbody temperature in response to gradually de-creasing or increasing ambient temperature.Ann. Acad. Sci. Fenn. Ser 44 71, 187-196(1964).Kristoffersson, R., and Soivio, A. Duration ofhypothermic periods and type of respiration inthe hibernating golden hamster. Mesocricetusauratus Waterh. Ann. Zool. Fettn. 3, 66-67( 1 966).Kristoffersson, R., and Soivio, A. Studies on theperiodicity of hibernation in the hedgehog(Erinaceus europaeus L.). IL Changes ofrespi-ratory rhythm, heart rate and body temperatureat the onset of spontaneous and induced arous-als. Ann. Zool. Fenn. 4,595-597 (1967).Landau, B. R., and Dawe, A. R. Respiration inthe hibernation ofthe 13-lined ground squirrel.Amer. J. Physiol. 194,75-82 (1958).Lyman, C. P. Effect of increased CO2 on respira-tion and heart rate of hibernating hamsters and

ground squirrels. Amer. J. Physiol. 167,638-643 (1951).24. Malan, A. Ventilation measured by body plethys-mography in hibernating mammals and in poi-kilotherms. Re spir. P hys iol. 17, 32- 44 (1973).25. Malan, A., Arens, H., and Waechter, A. Pulmo-nary respiration and acid-base state in hiber-nating marmots and hamsters. Respir. Physiol.17,45-61 (1973).26. Musacchia, X. J., and Volkert, W. A. Blood gasesin hibernating and active ground squirrels:HbO, affinity at 6 and 38"C. Amer. J. Physiol.221,128-130 (1971).27. Pajunen, I. Body temperature, heart rate, brea-thing pattern, weight loss and periodicity ofhibernation in the Finnish garden dormouse,Eliomys quercinus. Ann. Zool. Fenn. 7,25t-266 (1970't.28. Popovic, V. Cardiac output in hibernating groundsquirrels. Amer. J. ?hysiol. 2O7, 1345-1348( I 964).29. Reeves, R. B., and Wimsatt, W. A. Blood pH andbody temperature in the heterothermic mam-mal, Myotis lucifugus. Physiologist 9, 274(1e66).30. Rigatto, H., and Brady, J. P. Periodic breathingand apnea in preterm infants: I. Evidence forhypoventilation possibly due to central respi-ratory depression. Pediatrics 50,202 (1972).31. Siggaard Anderson, A. O., and Egsbaek, W.Hypotermiens indvirkning pa syre-base-statushas hibernerende flagermus. Ugeskr. Laeg.t24.929-932 tt962).32. Tahti, H. "Periodicity of Hibernation in theHedgehog (Erinaceus europaeus L.). SeasonalRespiratory Variations with Special Referenceto the Regulation of Cheyne-Stokes Respira-tion." Ph.D. dissertation, University of Hel-sinki,42 p.,1978.

33. Twente, J. A., and Twente, J. W. pH and pCO, ofblood of ground squirrels after known periods inhibernation. Amer. Zool. 4,296 (1964).34. Wang, L. C. H. Time patterns and metabolic ratesof natural torpor in the Richardson's groundsquirrel. Canad. J. Zool. 57,149-155 (1979).35. Withers, P. C. Metabolic, respiratory andhaematological adjustments of the little pocketmouse to circadian torpor cycles. Respir.Phys iol. 31, 295 -307 (1977).

7

pulmonary ventilation and cardiac activity in hibernating and arousing golden-mantled ground ...

Documents