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Chemical control of breathing in the lizard, Varanus exanthematicus
C,, ,,,, ,, U,,~~,wr,, Pl,,w,l 1 Pcrgmnn Prcr* Lid
0100-9619
Vol. b?A. ,‘p 999 ,o 1003 1979. Prmtcd I” Gvxt Bnlam
79 0401.W99SO2.00
0
CHEMICAL CONTROL OF BREATHING IN THE LIZARD, VARANUS EXANTHEMATICUS M~GENS L. GLASS. STEPHEN C. Woon, REED W. HOYT and KJELL JOHANSEN* Department of Physiology, University of New Mexico, School of Medicine. Albuquerque. NM 87131, U.S.A (Received 31 May 1978) Abstract-l. In Varanus exanthemaricus. alterations of inspired and end-tidal gas pressures resulted in rapid ventilatory responses. This, as well as other physiological characteristics, contributes to a homeostatic ability unusual for a reptile. 2. In contrast to the lizard Lncerta. the varanid showed a positive ventilatory response to hypoxia. 3. Whereas COz breathing depressed breathing frequency in other lizards. both tidal volume and frequency increased in Varanus. 4. End-tidal Pro, increased significantly after feeding. presumably compensating for an alkaline “tide” of digestion. The increased end-tidal Pco, was not due to a decrease in ventilation.
temperature of ca. 2040°C. Radiotelemetric studies of body temperature (Tb) indicate a preferred Tb of 36.5”C (Sanchez et al., 1978). Ventilation was measured on unrestrained animals using a pneumotachographic method (Glass et al., 1978). Expired CO2 was analyzed on a breath-to-breath basis using an LB-1 CO,-analyzer (Beckman Inst.). Hypercapnic or hypoxic gas mixtures were prepared by flushing pure gases from stock cylinders through Fisher & Porter flowmeters into a mixing chamber connected to an animal container. The partial pressure of O2 (PO,) was measured using an oxygen electrode (Instrumentation Laboratory, Lexington, MA, models 113.127). The experiments were conducted at an elevation of 1700m (Albuquerque, NM) corresponding to an inspired PO, of 124 torr. This elevation is also experienced in some of the natural habitats of the Savannah monitors. e.g. Kenya. The experimental chamber was large enough to allow the lizards free movement, and was equipped with a window for observation of the animal. A heat pad covered the bottom of the container and maintained Tb at 35°C. After a lizard was placed in the animal container. it was left undisturbed for several hours until a stable minimum ventilation was recorded, at which time a test gas was introduced. The gas mixing arrangement made it possible to make stepwise changes of the inspired Pco, or PO, of 2 or 3 torr. Generally, a fast run was made to get an overall impression of the response characteristics of the animal. On the basis of this information Pco, or PO, values were chosen for longer exposures which formed the basis for plotting response curves (20 min for each gas).
INTRODUCTION
The genus Varanus includes the living representatives of an ancient and anatomically distinct family of lizards (Romer. 1966). Varanids are agile, predatory animals. including both terrestrial and aquatic species. Physiologically, varanids are characterized by high aerobic capacity (Bartholomew & Tucker, 1964; Bennett, 1972), and capability for sustained activity. Unique among reptiles are temperature-independent blood pH (Bennett. 1973; Wood et al.. 1977a) and absence of net intracardiac shunting (Millard & Johansen, 1974; Wood et al.. 1977b; Berger & Heisler. 1977). Weight-specific ventilation in reptiles is generally lower than in mammals or birds, as is the weight-specific resting oxygen consumption (cf. Dejours, 1975). The difference in minute ventilation is primarily due to a lower breathing rate and intermittent periods of breath-holding. Such prolonged breath-holding causes pronounced fluctuations of blood gases in many reptiles (cf. Wood & Lenfant, 1976). However, in Varanus exanthematicus breathing is regular (Wood et al.. 1977a) which tends to stabilize blood and endtidal gas pressures. Perfusion of the lungs is also periodic in many reptiles, as blood is shunted away from the lungs during prolonged breath-holding (cf. White, 1976). As stated above, varanids also differ in this respect. In view of the tendency of varanids to involve ventilation in homeostasis of pH and gas pressures, the present study was designed to assess the ventilatory responses to alterations of alveolar gas pressures.
RESULTS
METHODS
Eleven Savannah monitors. Varanus exanthematicus. obtained from Africa, were studied. Weights ranged from 240 to 124Og (mean 570g). The lizards were fed mice. gained weight, and were in excellent condition during the period of experimentation. The animal cage was equipped with heat lamps which provided a range of ambient air * Present address: Department of Aarhus University, Aarhus. Denmark.
Zoophysiology, 999
Varanus showed a marked ventilatory.response to hypoxic gas mixtures below an inspired PO, of 70 torr (Fig. 1). Whereas respiratory frequency was less than normal, tidal volume was greatly increased. and the product, ventilation, increased. When a hypoxic gas mixture was suddenly replaced with pure oxygen, B transient, but pronounced frequency depression was recorded (Table 1). Ventilation also decreased although large tidal volumes, characteristic of the preceding hypoxic response, were recorded during the initial, phase of the O,-breathing. With prolonged O,-breathing ventiltition increased as CO2 accumu-
ratio of maximum ventilation to baseline ventilation for room air breathing ranged from 7 to 37. Both tidal volumes and breathing rates increased during relationship the Interestingly, CO,-breathing. between ventilation and end-tidal Pco2 was highly reproducible in contrast to the relationship between the sub-parameters (frequency and tidal volumes) and end-tidal Pco2. Even for individual animals. it w&s unpredictable whether the ventilatory response to CO2 would result from higher breathing rates or larger tidal volumes. DISCUSSION
Eflects of hypoxia
I JC
Fig, 1. The effects of hypoxta on tidal volume. breathmg rate and ventilation. X 2 S.D.. N = 5. lated due to hypoventilation, but stayed below values for room air breathing. This suggested removal of hypoxic drive to ventilation present at an inspired PO, of 124 torr. A very interesting finding is that end-tidal PcoZ depended on the feeding status of the animal. During the first 24 hr after feeding end-tidal Pm, reached a peak value. then decreased to minimum values 2 or 3 days later, when the food was digested. The increase of end-tidal PcoZ during digestion had no or little effect on ventilation (Fig. 2). Paradoxically. an increased end-tidal Pep, caused by CO*-breathing would immediately ehctt a strong ventilatory response. Figures 3(A). 3(B) and 4 show the effects of increased end-tidal Pco, on breathing rate, tidal volumes and venti~tion for nine indiv.idua~ animals. Pronounced variation was present among the individual response curves, but the variations did not show a consistent relationship to the feeding status of the animal. Maximum values for ventilation were recorded when end-tidal P,,, increased by 8-19 torr. Further increase caused depression of ventilation. The
In a pioneering but largely qualitative study, Randall et al. (1944) reported positive ventilatory responses to hypoxia in snakes, lizards. chelonians and crocodilians. This has later been confirmed for chelonian reptiles (cf. Wood & Lenfant. 1976). and also for snakes and crocodilians (Glass & Johansen. 1976. 1979). In contrast to Varanus, ventilation in the lizard Lacerta was depressed by low inspired PO>. Although tidal volumes increased in Lacerta, a pronounced reduction in breathing rates caused the subnormal ventilation (Nielsen. 1962). It remains to be established whether a positive ventilatory response is typical of lizards. However. both Lacerta and Varanus showed depression of breathing rate during hypoxia. In Vuranrfs the highest breathing rates were recorded during room air breathing. as breathing of pure OZ also caused a decrease of beathing rate. This depression of breathing during hyperoxia is common in reptiles (cf. Wood & Lenfant, 1976). The time course of the depression is known in the tortoise Testudo horsfiefdi (Benchetrit et al.. 1977). In Testudo the depression of breathing results from elevated PO, at the site of peripheral chemoreceptors. The depression causes CO1 retention, which stimulates breathing. Sign&cantly. ventilation in Varanus did not reach the baseline values for room air-breathing. which suggests the presence of a P,,. mediated drive at an Inspired P,,,
Table 1. In t’aranus erunthrrnutirus:(A) normal values for frequency (f), tidal volume (VT). and ventilation (6); iIS) values during hypoxia (P,,? = 50 torr): (C) the effects of increasing P,,, from 50 to 588 torr.
(A) Room air* (B) Hypoxia (C) Hyperoxia O-5 min 1520 mm
1.4 0.2 0.8 a.1
25.6 2.4 72 II
33 3 58 8
0.2 0.1 0.8 0.3
73 17 26 1
12
All values x + SD. Experiments shown in Fig. 1. * PI,: 124 torr. See Methods.
I 22 9
HRS AFTER FEEDING
separate from those Fig. 2. Effects of feeding on end-tidal Pro, and ventilation. x + SD.. iv = 4.
1001
Control of breathing in Varanus )f 124 torr. Shortterm experiments exposing animals o simulated sea-level oxygen pressures indicated no Inference in ventillation between Pjo2 = 124 torr, and , = 150 torr. IO? $ects
of feeding
The effects of feeding on end-tidal Pco2 and ventilaion in Vat-anusseem paradoxical. Initially, after feed-
(A)
i
ing, end-tidal Pm, rises but ventilation increased only slightly. However, acute inspiration of CO2 resulting in the same increase in end-tidal Pm, elicited a much greater increase in ventilation. The relationships between end-tidal Pco2 and ventilation are difficult to explain as little information is available on pH and PCO, receptors in lizards. Recent studies show that Pc,, receptors exist in the lizard lungs (Gatz et al., 1975; Scheid et al., 1977; Fedde et al., 1977), but the functional role of these receptors is not clear. Carnivorous reptiles normally swallow large chunks or whole animals with large amounts of acid needed for digestion. In alligators, the acid-base balance is dramatically altered during digestion. Coulson & Hernandez (1964) found that plasma pH increased from ca. 7.33 to ca. 7.63 within 9 hr after feeding. This was due to an increase in plasma bicarbonate (“alkaline tide”). Calculation of Pco2 from their data on pH and HCO; reveals no significant changes in arterial Pco2. However, in Varanus. endtidal Pco2 increased stgnificantly during digestion which could, in effect, provide respiratory compensation for metabolic alkalosis. The effect of feeding on CO1 elimination, oxygen uptake and ventilation in Vurarnrs exanthenlaticus will be discussed in detail in a subsequent paper (Wood et al.. 1979). Effects of elevated inspired CO2
O~“‘I”,‘~,,“~‘~~‘~~~~~~ 0
10
5
INCREASE
OF
a
20
15
END- TIDAL
PC02,
25
tar
To obtain meaningful data on ventilatory responses of animals to CO2 breathing it is necessary to measure either end-tidal or arterial Pm,. If only the inspired Pco, is measured, as in most previous studies of lizards, conclusions as to sensitivity are impossible. However. the immediate, pronounced increase of both tidal volume and breathing frequency clearly separates Varanus exanthematicus from previously studied
U-J) f
000 INCREASE
OF
END-TIDAL
PCO,,
tar
ig. 3 (A) Effect of increased end-tidal Pco2 on breathing ite. Individual response curves for nine animals. (B) Effect of increased end-tidal P,,. on tidal volumes.
INCREASE
OF
END-TIDAL
PCD,.
lcw
Fig. 4. EtTect of increased end-tidal Ppfi. on ventilation.
MIXENS L. GLASS et II/.
Table 2. Representative
ventilatory
responses
of various
reptiles
to hypoxia
and hypercapnla
Reference
1‘T
1‘
Chranus Lacerra
+ +
~ -
+
This stud) Nielsen (1963)
.Acrochorduh
0
+
+
Glass
Testudo Pseudemys Pelomedusa
+ + +
+ + +
+ + +
Jackson (1973) Glass et al. (1978)
Crocodilus
0
+
+
Glas\
I,‘aranus Lacerta Crotaphyrus I/ma Dipsosaurus Tupinamhis
+ + + + or f ~ Not stated + Not stated + Not stated + + Slight increase +
This stud) Nielsen (1961) Templeton & Dawson Pough (1969) Pough (1969) Crawford it a/. (1977)
Acrochordus
0
Glass
Tesrudo Pseudemys Pelomrdusa
+ + +
HYPOXIA Ltzards
Snakes 6i Johansen
(1976)
Chelonians Benchetrit
c’f al. (1977)
Crocodilians & Johanscn
(1979)
HYPERCAPNIA
Lizards
(1963)
Snakes & Johansen
(I 976)
Chelonians + + +
+ + +
Glass et ul. (197X) Jackson t’l al. (1974) Glass et al. (1978)
+. increase; -? decrease. 0. no significant change. lizards where tidal volumes increase but frequency is decreased (Nielsen. 1961; Templeton & Dawson. 1963; Pough. 1969). or only changes slightly (Crawford et ul., 1977). The decrease of breathing frequency during breathing of CO, is also found on the aquatic snake. Acrochordus jatwkus (Glass & Johansen, 1976). whereas both aquatic and terrestrial chelonian reptiles increase both tidal volume and breathing frequency (Jackson et al.. 1974; Glass et al.. 1978). The diversity of species responses to hypoxia and hypercapnia is summarized in Table 2. As in other areas. it is impossible to generalize about chemical control of breathing in reptiles. Also in Vuranus the homeostasis of COZ clearance appears weak as the baseline end-tidal Pco, undergoes large fluctuations. However, these fluctuations are not random, but reflect the metabolic acid-base changes accompanying digestion. Chemical control of ventilation is efficient and immediate, and correlates well with other evidence of highly evolved cardiopulmonary responses in varanids. .-l~~~t1~~~~~/~~~/~c~i~~,~~~,\~~This research was supported by a grunt from Aarhus University (M.L.G.) and the National Sclencc Foundation PCM 77-24246 (S.C.W.). REFERENCES BARTHOLOMEW G. A. & TUCKER V. A. (1964) Size, body
temperature. thermal conductance. oxygen consumption. and heart rate in Australian varanid lizards. Physiol. Zool. 37, 341-354. BENCHETRIT G.. ARMAND F. & DEIOURS P. (1977) Ventilatory chemoreflex drive in the tortoise, Testudo horsfeldi. Resp. Physiol.
31. 183-191.
BENNETT A. F. (1972) The effect of activity on oxygen consumption. oxygen depth and heart rate in the lizards. Varanus gouldii and Sauromalus hispidus. J. camp. P/I!,siol. 79, 259-280.
BENNETT A. F. (1973) Blood physiology and oxygen transport during activity in two lizards, Varanus youldii and Sauromalus hispidus. Comp. Biochrm. Phy.siol. 46A. 673-690. BE~GER P. J. & HEISLER N. (1978) Estimation of shunting. systemic and pulmonary output of the heart. and regional blood flow distribution in unanesthetized lizards (Varanus exanrhematicus) by injection of radioactively labelled microspheres. J. exp. Biol. 71. I I l-112. COULSON R. A. & HERNANDEZ T. (1964) Biochemrsrr~ of the Alligator.
A Study
of Metabolism
in .SL)w Marion.
Louisiana State University Press. Baton Rouge. CRAWFORD E. C., JR, GATZ R. N. & PIIP~R J. (I 977) Ventilatory response of the Tegu hzard to inspired CO, at different body temperatures. Physiologist u)(4). 19. DEJOURS P. (1975) Principle of Comparatiw Rrspirator~ Physiology. North Holland-American Elsevier. Neu York. FEDDE M. R.. KUHLMANN W. D. & SCHEID P. (1977) Intrapulmonary receptors in the Tegu lizard I. Sensitivity to CO,. Resp. Physiol. 29, 3549. GIORDANO R. V. & JACKSON D. C. (1973) The effect of temperature on ventilation in the green iguana (lyuuna iguana). Comp. Biochem. Physio/. 45A. 235-238. GLASS M. L. & J~HANSFN K. (1976) Control of breathing in .4crochordus jacanicus. an aquatic snake. Ph r..\w/. Zoo/. 49. 328-339. GLASS M. L.. B~RGGREN W. W. & JOHANSI:~ K. (19781
Ventilation in an aquatic and a terrestrial chelonian reptile. J. exp. Biol. 72. 165- 179. GLASS‘ M. L. & JOHANSEN K. (1979) Periodic breathing in the crocodile. Crocodilus nilorirus. Consequences for the alveolar gas exchange ratio, and control of breathing. In preparation.
Control
of breathing
GLASS M. L.. WOOD S. C. & JOHANSEN K. (1978) The appli-
cation of pneumotachography on small unrestrained animals. Cor77p.Biochem. Ph~~iol. 59A. 425-427. GATZ R. N.. FEIXX: M. R. & CRAWFORD E. C.. JR (1975) Lizard lungs: CO2 sensitive receptors in Tupinamhis nigropunctatus. Experientia 31, 455-456. JACKSON D. C. (1971) The effect of temperature on ventilation in the turtle, Pseudemys scripta elegans. Resp. Physiol. 12, 131-140. JACKSON D. C. (1973) Ventilatory response to hypoxia in turtles at various body temperatures. Resp. Physio1. 18. 178-187. JACKSON D. C., PALMER S. E. & MEADOW W. L. (1974) The effects of temperature and carbon dioxide breathing on ventilation and acid-base status in turtles. Resp. Physiol. 20. 131-146. MILLARD R. W. & JOHANSEN K. (1974) Ventricular outflow dynamics in the lizard. Varanus niloticus. Responses to hypoxia, hypercarbia and diving. J. exp. Biol. 60. 871-880. NIELSEN. B. (1961) On the regulation of respiration in reptiles 1. The effect of temperature and CO2 on the respiration of lizards. Lacerta. J. exp. Biol. 38. 301-314. NIELSEN B. (1962) On the regulation of respiration in reptiles II. The effect of hypoxia with and without moderate hypercapnia on the metabolism of lizards. J. exp. Biol. 39. 107-117. POUGH F. H. (1969) Physiological aspects of the burrowing of sand lizards. Lima, Iguanidae and other lizards. Comp. Biochem. Physiol. 31. 869-884.
in Varor7us
1003
RANDALL W. C., STULKEN & HIESTAND W. A. (1944) Respiration of reptiles as influenced by the composition of the inspired air. Copeia No. 3. 136-144. ROMER A. S. (1966) Vertebrate Paleontology, third edition. University of Chicago Press. Chicago. SANCHEZ H. (1978) Thermoregulation in a monitor lizard. In preparation. SCHEID. P., KUHLMANN W. D. & FEDDE M. R. (1977) Intrapulmonary receptors in the Tegu lizard II. Functional characteristics and location. Resp. Physiol. 29, 49-63. TEMPLETON J. R. & DAWSON W. R. (1963) Respiration in the lizard. Crotaphytus collaris. Physiol. Zoo/. 36. 104-126. WOOD S. C.. GLASS M. L. & JOHANSEN K. (1977a) Effects of temperature on respiration and acid-base balance in a monitor lizard. J. camp. Physiol. 116, 287-296. WOOD S. C., GLAS M. L., HOYT R. W. & JOHANSEN K. (I 979) Effects of feeding on ventilation and gas exchange in a monitor lizard. In preparation. WOOD S. C.. JOHANSEN K. & GATZ R. N. (1977b) Pulmonary bloodflow. ventilation/perfusion ratio. and oxygen transport in a varanid lizard. Am. J. Physiol. 233. R89-T93. WOOD S. C. & LENFANT C. (1976) Respiration: mechanics. control. and gas exchange. In Biology of the Reprilia. Vol. 5. Physiology (Edited by GANS C. and DAWSON W. R.). pp. 225-274. WHITE F. N. (1976) Circulation. In Biology qf the Rrptiha. Vol. 5. Physiology (Edited by GANS C. & DAWSON W. R.). Academic Press. New York.
0100-9619
Vol. b?A. ,‘p 999 ,o 1003 1979. Prmtcd I” Gvxt Bnlam
79 0401.W99SO2.00
0
CHEMICAL CONTROL OF BREATHING IN THE LIZARD, VARANUS EXANTHEMATICUS M~GENS L. GLASS. STEPHEN C. Woon, REED W. HOYT and KJELL JOHANSEN* Department of Physiology, University of New Mexico, School of Medicine. Albuquerque. NM 87131, U.S.A (Received 31 May 1978) Abstract-l. In Varanus exanthemaricus. alterations of inspired and end-tidal gas pressures resulted in rapid ventilatory responses. This, as well as other physiological characteristics, contributes to a homeostatic ability unusual for a reptile. 2. In contrast to the lizard Lncerta. the varanid showed a positive ventilatory response to hypoxia. 3. Whereas COz breathing depressed breathing frequency in other lizards. both tidal volume and frequency increased in Varanus. 4. End-tidal Pro, increased significantly after feeding. presumably compensating for an alkaline “tide” of digestion. The increased end-tidal Pco, was not due to a decrease in ventilation.
temperature of ca. 2040°C. Radiotelemetric studies of body temperature (Tb) indicate a preferred Tb of 36.5”C (Sanchez et al., 1978). Ventilation was measured on unrestrained animals using a pneumotachographic method (Glass et al., 1978). Expired CO2 was analyzed on a breath-to-breath basis using an LB-1 CO,-analyzer (Beckman Inst.). Hypercapnic or hypoxic gas mixtures were prepared by flushing pure gases from stock cylinders through Fisher & Porter flowmeters into a mixing chamber connected to an animal container. The partial pressure of O2 (PO,) was measured using an oxygen electrode (Instrumentation Laboratory, Lexington, MA, models 113.127). The experiments were conducted at an elevation of 1700m (Albuquerque, NM) corresponding to an inspired PO, of 124 torr. This elevation is also experienced in some of the natural habitats of the Savannah monitors. e.g. Kenya. The experimental chamber was large enough to allow the lizards free movement, and was equipped with a window for observation of the animal. A heat pad covered the bottom of the container and maintained Tb at 35°C. After a lizard was placed in the animal container. it was left undisturbed for several hours until a stable minimum ventilation was recorded, at which time a test gas was introduced. The gas mixing arrangement made it possible to make stepwise changes of the inspired Pco, or PO, of 2 or 3 torr. Generally, a fast run was made to get an overall impression of the response characteristics of the animal. On the basis of this information Pco, or PO, values were chosen for longer exposures which formed the basis for plotting response curves (20 min for each gas).
INTRODUCTION
The genus Varanus includes the living representatives of an ancient and anatomically distinct family of lizards (Romer. 1966). Varanids are agile, predatory animals. including both terrestrial and aquatic species. Physiologically, varanids are characterized by high aerobic capacity (Bartholomew & Tucker, 1964; Bennett, 1972), and capability for sustained activity. Unique among reptiles are temperature-independent blood pH (Bennett. 1973; Wood et al.. 1977a) and absence of net intracardiac shunting (Millard & Johansen, 1974; Wood et al.. 1977b; Berger & Heisler. 1977). Weight-specific ventilation in reptiles is generally lower than in mammals or birds, as is the weight-specific resting oxygen consumption (cf. Dejours, 1975). The difference in minute ventilation is primarily due to a lower breathing rate and intermittent periods of breath-holding. Such prolonged breath-holding causes pronounced fluctuations of blood gases in many reptiles (cf. Wood & Lenfant, 1976). However, in Varanus exanthematicus breathing is regular (Wood et al.. 1977a) which tends to stabilize blood and endtidal gas pressures. Perfusion of the lungs is also periodic in many reptiles, as blood is shunted away from the lungs during prolonged breath-holding (cf. White, 1976). As stated above, varanids also differ in this respect. In view of the tendency of varanids to involve ventilation in homeostasis of pH and gas pressures, the present study was designed to assess the ventilatory responses to alterations of alveolar gas pressures.
RESULTS
METHODS
Eleven Savannah monitors. Varanus exanthematicus. obtained from Africa, were studied. Weights ranged from 240 to 124Og (mean 570g). The lizards were fed mice. gained weight, and were in excellent condition during the period of experimentation. The animal cage was equipped with heat lamps which provided a range of ambient air * Present address: Department of Aarhus University, Aarhus. Denmark.
Zoophysiology, 999
Varanus showed a marked ventilatory.response to hypoxic gas mixtures below an inspired PO, of 70 torr (Fig. 1). Whereas respiratory frequency was less than normal, tidal volume was greatly increased. and the product, ventilation, increased. When a hypoxic gas mixture was suddenly replaced with pure oxygen, B transient, but pronounced frequency depression was recorded (Table 1). Ventilation also decreased although large tidal volumes, characteristic of the preceding hypoxic response, were recorded during the initial, phase of the O,-breathing. With prolonged O,-breathing ventiltition increased as CO2 accumu-
ratio of maximum ventilation to baseline ventilation for room air breathing ranged from 7 to 37. Both tidal volumes and breathing rates increased during relationship the Interestingly, CO,-breathing. between ventilation and end-tidal Pco2 was highly reproducible in contrast to the relationship between the sub-parameters (frequency and tidal volumes) and end-tidal Pco2. Even for individual animals. it w&s unpredictable whether the ventilatory response to CO2 would result from higher breathing rates or larger tidal volumes. DISCUSSION
Eflects of hypoxia
I JC
Fig, 1. The effects of hypoxta on tidal volume. breathmg rate and ventilation. X 2 S.D.. N = 5. lated due to hypoventilation, but stayed below values for room air breathing. This suggested removal of hypoxic drive to ventilation present at an inspired PO, of 124 torr. A very interesting finding is that end-tidal PcoZ depended on the feeding status of the animal. During the first 24 hr after feeding end-tidal Pm, reached a peak value. then decreased to minimum values 2 or 3 days later, when the food was digested. The increase of end-tidal PcoZ during digestion had no or little effect on ventilation (Fig. 2). Paradoxically. an increased end-tidal Pep, caused by CO*-breathing would immediately ehctt a strong ventilatory response. Figures 3(A). 3(B) and 4 show the effects of increased end-tidal Pco, on breathing rate, tidal volumes and venti~tion for nine indiv.idua~ animals. Pronounced variation was present among the individual response curves, but the variations did not show a consistent relationship to the feeding status of the animal. Maximum values for ventilation were recorded when end-tidal P,,, increased by 8-19 torr. Further increase caused depression of ventilation. The
In a pioneering but largely qualitative study, Randall et al. (1944) reported positive ventilatory responses to hypoxia in snakes, lizards. chelonians and crocodilians. This has later been confirmed for chelonian reptiles (cf. Wood & Lenfant. 1976). and also for snakes and crocodilians (Glass & Johansen. 1976. 1979). In contrast to Varanus, ventilation in the lizard Lacerta was depressed by low inspired PO>. Although tidal volumes increased in Lacerta, a pronounced reduction in breathing rates caused the subnormal ventilation (Nielsen. 1962). It remains to be established whether a positive ventilatory response is typical of lizards. However. both Lacerta and Varanus showed depression of breathing rate during hypoxia. In Vuranrfs the highest breathing rates were recorded during room air breathing. as breathing of pure OZ also caused a decrease of beathing rate. This depression of breathing during hyperoxia is common in reptiles (cf. Wood & Lenfant, 1976). The time course of the depression is known in the tortoise Testudo horsfiefdi (Benchetrit et al.. 1977). In Testudo the depression of breathing results from elevated PO, at the site of peripheral chemoreceptors. The depression causes CO1 retention, which stimulates breathing. Sign&cantly. ventilation in Varanus did not reach the baseline values for room air-breathing. which suggests the presence of a P,,. mediated drive at an Inspired P,,,
Table 1. In t’aranus erunthrrnutirus:(A) normal values for frequency (f), tidal volume (VT). and ventilation (6); iIS) values during hypoxia (P,,? = 50 torr): (C) the effects of increasing P,,, from 50 to 588 torr.
(A) Room air* (B) Hypoxia (C) Hyperoxia O-5 min 1520 mm
1.4 0.2 0.8 a.1
25.6 2.4 72 II
33 3 58 8
0.2 0.1 0.8 0.3
73 17 26 1
12
All values x + SD. Experiments shown in Fig. 1. * PI,: 124 torr. See Methods.
I 22 9
HRS AFTER FEEDING
separate from those Fig. 2. Effects of feeding on end-tidal Pro, and ventilation. x + SD.. iv = 4.
1001
Control of breathing in Varanus )f 124 torr. Shortterm experiments exposing animals o simulated sea-level oxygen pressures indicated no Inference in ventillation between Pjo2 = 124 torr, and , = 150 torr. IO? $ects
of feeding
The effects of feeding on end-tidal Pco2 and ventilaion in Vat-anusseem paradoxical. Initially, after feed-
(A)
i
ing, end-tidal Pm, rises but ventilation increased only slightly. However, acute inspiration of CO2 resulting in the same increase in end-tidal Pm, elicited a much greater increase in ventilation. The relationships between end-tidal Pco2 and ventilation are difficult to explain as little information is available on pH and PCO, receptors in lizards. Recent studies show that Pc,, receptors exist in the lizard lungs (Gatz et al., 1975; Scheid et al., 1977; Fedde et al., 1977), but the functional role of these receptors is not clear. Carnivorous reptiles normally swallow large chunks or whole animals with large amounts of acid needed for digestion. In alligators, the acid-base balance is dramatically altered during digestion. Coulson & Hernandez (1964) found that plasma pH increased from ca. 7.33 to ca. 7.63 within 9 hr after feeding. This was due to an increase in plasma bicarbonate (“alkaline tide”). Calculation of Pco2 from their data on pH and HCO; reveals no significant changes in arterial Pco2. However, in Varanus. endtidal Pco2 increased stgnificantly during digestion which could, in effect, provide respiratory compensation for metabolic alkalosis. The effect of feeding on CO1 elimination, oxygen uptake and ventilation in Vurarnrs exanthenlaticus will be discussed in detail in a subsequent paper (Wood et al.. 1979). Effects of elevated inspired CO2
O~“‘I”,‘~,,“~‘~~‘~~~~~~ 0
10
5
INCREASE
OF
a
20
15
END- TIDAL
PC02,
25
tar
To obtain meaningful data on ventilatory responses of animals to CO2 breathing it is necessary to measure either end-tidal or arterial Pm,. If only the inspired Pco, is measured, as in most previous studies of lizards, conclusions as to sensitivity are impossible. However. the immediate, pronounced increase of both tidal volume and breathing frequency clearly separates Varanus exanthematicus from previously studied
U-J) f
000 INCREASE
OF
END-TIDAL
PCO,,
tar
ig. 3 (A) Effect of increased end-tidal Pco2 on breathing ite. Individual response curves for nine animals. (B) Effect of increased end-tidal P,,. on tidal volumes.
INCREASE
OF
END-TIDAL
PCD,.
lcw
Fig. 4. EtTect of increased end-tidal Ppfi. on ventilation.
MIXENS L. GLASS et II/.
Table 2. Representative
ventilatory
responses
of various
reptiles
to hypoxia
and hypercapnla
Reference
1‘T
1‘
Chranus Lacerra
+ +
~ -
+
This stud) Nielsen (1963)
.Acrochorduh
0
+
+
Glass
Testudo Pseudemys Pelomedusa
+ + +
+ + +
+ + +
Jackson (1973) Glass et al. (1978)
Crocodilus
0
+
+
Glas\
I,‘aranus Lacerta Crotaphyrus I/ma Dipsosaurus Tupinamhis
+ + + + or f ~ Not stated + Not stated + Not stated + + Slight increase +
This stud) Nielsen (1961) Templeton & Dawson Pough (1969) Pough (1969) Crawford it a/. (1977)
Acrochordus
0
Glass
Tesrudo Pseudemys Pelomrdusa
+ + +
HYPOXIA Ltzards
Snakes 6i Johansen
(1976)
Chelonians Benchetrit
c’f al. (1977)
Crocodilians & Johanscn
(1979)
HYPERCAPNIA
Lizards
(1963)
Snakes & Johansen
(I 976)
Chelonians + + +
+ + +
Glass et ul. (197X) Jackson t’l al. (1974) Glass et al. (1978)
+. increase; -? decrease. 0. no significant change. lizards where tidal volumes increase but frequency is decreased (Nielsen. 1961; Templeton & Dawson. 1963; Pough. 1969). or only changes slightly (Crawford et ul., 1977). The decrease of breathing frequency during breathing of CO, is also found on the aquatic snake. Acrochordus jatwkus (Glass & Johansen, 1976). whereas both aquatic and terrestrial chelonian reptiles increase both tidal volume and breathing frequency (Jackson et al.. 1974; Glass et al.. 1978). The diversity of species responses to hypoxia and hypercapnia is summarized in Table 2. As in other areas. it is impossible to generalize about chemical control of breathing in reptiles. Also in Vuranus the homeostasis of COZ clearance appears weak as the baseline end-tidal Pco, undergoes large fluctuations. However, these fluctuations are not random, but reflect the metabolic acid-base changes accompanying digestion. Chemical control of ventilation is efficient and immediate, and correlates well with other evidence of highly evolved cardiopulmonary responses in varanids. .-l~~~t1~~~~~/~~~/~c~i~~,~~~,\~~This research was supported by a grunt from Aarhus University (M.L.G.) and the National Sclencc Foundation PCM 77-24246 (S.C.W.). REFERENCES BARTHOLOMEW G. A. & TUCKER V. A. (1964) Size, body
temperature. thermal conductance. oxygen consumption. and heart rate in Australian varanid lizards. Physiol. Zool. 37, 341-354. BENCHETRIT G.. ARMAND F. & DEIOURS P. (1977) Ventilatory chemoreflex drive in the tortoise, Testudo horsfeldi. Resp. Physiol.
31. 183-191.
BENNETT A. F. (1972) The effect of activity on oxygen consumption. oxygen depth and heart rate in the lizards. Varanus gouldii and Sauromalus hispidus. J. camp. P/I!,siol. 79, 259-280.
BENNETT A. F. (1973) Blood physiology and oxygen transport during activity in two lizards, Varanus youldii and Sauromalus hispidus. Comp. Biochrm. Phy.siol. 46A. 673-690. BE~GER P. J. & HEISLER N. (1978) Estimation of shunting. systemic and pulmonary output of the heart. and regional blood flow distribution in unanesthetized lizards (Varanus exanrhematicus) by injection of radioactively labelled microspheres. J. exp. Biol. 71. I I l-112. COULSON R. A. & HERNANDEZ T. (1964) Biochemrsrr~ of the Alligator.
A Study
of Metabolism
in .SL)w Marion.
Louisiana State University Press. Baton Rouge. CRAWFORD E. C., JR, GATZ R. N. & PIIP~R J. (I 977) Ventilatory response of the Tegu hzard to inspired CO, at different body temperatures. Physiologist u)(4). 19. DEJOURS P. (1975) Principle of Comparatiw Rrspirator~ Physiology. North Holland-American Elsevier. Neu York. FEDDE M. R.. KUHLMANN W. D. & SCHEID P. (1977) Intrapulmonary receptors in the Tegu lizard I. Sensitivity to CO,. Resp. Physiol. 29, 3549. GIORDANO R. V. & JACKSON D. C. (1973) The effect of temperature on ventilation in the green iguana (lyuuna iguana). Comp. Biochem. Physio/. 45A. 235-238. GLASS M. L. & J~HANSFN K. (1976) Control of breathing in .4crochordus jacanicus. an aquatic snake. Ph r..\w/. Zoo/. 49. 328-339. GLASS M. L.. B~RGGREN W. W. & JOHANSI:~ K. (19781
Ventilation in an aquatic and a terrestrial chelonian reptile. J. exp. Biol. 72. 165- 179. GLASS‘ M. L. & JOHANSEN K. (1979) Periodic breathing in the crocodile. Crocodilus nilorirus. Consequences for the alveolar gas exchange ratio, and control of breathing. In preparation.
Control
of breathing
GLASS M. L.. WOOD S. C. & JOHANSEN K. (1978) The appli-
cation of pneumotachography on small unrestrained animals. Cor77p.Biochem. Ph~~iol. 59A. 425-427. GATZ R. N.. FEIXX: M. R. & CRAWFORD E. C.. JR (1975) Lizard lungs: CO2 sensitive receptors in Tupinamhis nigropunctatus. Experientia 31, 455-456. JACKSON D. C. (1971) The effect of temperature on ventilation in the turtle, Pseudemys scripta elegans. Resp. Physiol. 12, 131-140. JACKSON D. C. (1973) Ventilatory response to hypoxia in turtles at various body temperatures. Resp. Physio1. 18. 178-187. JACKSON D. C., PALMER S. E. & MEADOW W. L. (1974) The effects of temperature and carbon dioxide breathing on ventilation and acid-base status in turtles. Resp. Physiol. 20. 131-146. MILLARD R. W. & JOHANSEN K. (1974) Ventricular outflow dynamics in the lizard. Varanus niloticus. Responses to hypoxia, hypercarbia and diving. J. exp. Biol. 60. 871-880. NIELSEN. B. (1961) On the regulation of respiration in reptiles 1. The effect of temperature and CO2 on the respiration of lizards. Lacerta. J. exp. Biol. 38. 301-314. NIELSEN B. (1962) On the regulation of respiration in reptiles II. The effect of hypoxia with and without moderate hypercapnia on the metabolism of lizards. J. exp. Biol. 39. 107-117. POUGH F. H. (1969) Physiological aspects of the burrowing of sand lizards. Lima, Iguanidae and other lizards. Comp. Biochem. Physiol. 31. 869-884.
in Varor7us
1003
RANDALL W. C., STULKEN & HIESTAND W. A. (1944) Respiration of reptiles as influenced by the composition of the inspired air. Copeia No. 3. 136-144. ROMER A. S. (1966) Vertebrate Paleontology, third edition. University of Chicago Press. Chicago. SANCHEZ H. (1978) Thermoregulation in a monitor lizard. In preparation. SCHEID. P., KUHLMANN W. D. & FEDDE M. R. (1977) Intrapulmonary receptors in the Tegu lizard II. Functional characteristics and location. Resp. Physiol. 29, 49-63. TEMPLETON J. R. & DAWSON W. R. (1963) Respiration in the lizard. Crotaphytus collaris. Physiol. Zoo/. 36. 104-126. WOOD S. C.. GLASS M. L. & JOHANSEN K. (1977a) Effects of temperature on respiration and acid-base balance in a monitor lizard. J. camp. Physiol. 116, 287-296. WOOD S. C., GLAS M. L., HOYT R. W. & JOHANSEN K. (I 979) Effects of feeding on ventilation and gas exchange in a monitor lizard. In preparation. WOOD S. C.. JOHANSEN K. & GATZ R. N. (1977b) Pulmonary bloodflow. ventilation/perfusion ratio. and oxygen transport in a varanid lizard. Am. J. Physiol. 233. R89-T93. WOOD S. C. & LENFANT C. (1976) Respiration: mechanics. control. and gas exchange. In Biology of the Reprilia. Vol. 5. Physiology (Edited by GANS C. and DAWSON W. R.). pp. 225-274. WHITE F. N. (1976) Circulation. In Biology qf the Rrptiha. Vol. 5. Physiology (Edited by GANS C. & DAWSON W. R.). Academic Press. New York.