Ventilatory response to CO2 in birds. I. Measurements in the unanesthetized duck

Respiration Physiology (1978) 35, 349-359 © Elsevier/North-Holland Biomedical Press

V E N T I L A T O R Y R E S P O N S E TO CO 2 IN BIRDS. I. M E A S U R E M E N T S IN T H E U N A N E S T H E T I Z E D DUCK I

F R A N K L. POWELL, M. R O G E R F E D D E 2, R O N A L D K. G R A T Z and PETER SCHEID Department o[' Physiology Max-Planck-lnstitut fiir experimentelle Medizin, 3400 Gdttingen, FRG

Abstract. Ventilation and blood gases were measured in unanesthetized ducks at various levels of inspired CO 2 partial pressure (Pico_~). Ventilation was markedly augmented with increasing PIco,, whereas arterial and mixed venous Pco., stayed essentially constant up to a PIco: of about 20 torr and changed only slightly between that and the highest level tested (35 torr). -After carbonic anhydrase had been blocked, blood Pco_, was elevated at all levels of Plco_, but the ventilatory response to increases in PIco_, were attenuated. The response to CO 2 in the normal bird (before administration of acetazolamide) shows similarities to that in mammals. Qualitative differences between both classes of vertebrates after blockade of carbonic anhydrase may, however, suggest differences in their systems that control ventilation. Acetazolamide Avian respiration Blood gases

Cairina moschata Carbon dioxide response curve Control of respiration Respiration at rest

Distinct differences exist between the structural arrangements of avian and mammalian lungs, and resulting differences in the gas exchange properties of both systems appear to be well described (cf. Piiper and Scheid, 1977). Bouverot (1978) recently reviewed the dissimilarities in the ventilatory control between birds and mammals. Although the functional properties of avian intrapulmonary receptors, which are sensitive to CO 2 (ef. Burger et al., 1974; Fedde et al., 1974) and appear to have no counterpart in mammals (cf Kunz et al., 1976), have been described, their role in the control of ventilation remains unknown (cf. Fedde, 1970; Bouverot, Accepted for publication 24 July 1978 I Supported in part, from a grant-in-aid from the American Heart Association, Kansas Affiliate, Inc., Contribution No. 78-124j, Department of Anatomy and Physiology, Kansas Agricultural Experiment Station, Manhattan 66506, U.S.A. 2 Present address: Dept. of Anatomy and ~hysiology, College of Veterinary Medicine, Kansas State University, Manhattan, Kansas 66506, U.S.A. 349

350

F.L. POWELL et al.

1978). Because CO 2 receptors influence respiration, they possibly play some role in adjusting ventilation when the animal is given CO, to breathe. In this paper, we describe changes in ventilation and blood gases in unanesthetized ducks upon changes in inspired CO~ concentration. In addition, those variables were measured when the carbonic anhydrase was blocked. Similar experiments in mammals indicate distinct readjustments in ventilation and blood gaseswhen that enzyme is blocked (cf Maren, 1967). In a subsequent communication (Scheid et al., 1978), we will report changes in the CO~ receptor activity in response to similar experimental test conditions and attempt to identify the contribution of these receptors to the adjustments observed in the intact animal.

Methods ANIMAL PREPARATION

Five adult muscovy ducks (Cairina moschata) (average body weight 1.7 kg) were used in these experiments. A long-lasting, local anesthetic(Depot-NovanestR, containing Procaine and Tetracaine) was applied for the surgical interventions, but for the measurements the animals were awake. Catheters, introduced into the right ventricle and brachial artery allowed recording of blood pressure and withdrawal of arterial and mixed venous blood samples for analysis of blood gases and pH by electrodes maintained at the animal's body temperature, as described by Meyer et al. (1976). A thermistor probe was inserted about 10 cm into the colon for continuous monitoring of the animal's body temperature (Tb). The trachea was transected in the mid-cervical region, and a short silastic tube was secured over the exterior of the peripheral end to minimize irritation of the tracheal mucosa and ensuing mucous production. A pneumotachograph (Godart-Statham, type 17212, with a size O head) was attached to the tracheal cannula to record ventilatory flow. A non-rebreathing valve allowed collection of the expirate in a bag, the volume of which was measured in a spirometer. Fractional concentrations of 02 and CO 2 in inspired (FIo_~ and FIcQ) and mean expired (= bag) gas were measured with a respiratory mass spectrometer (Model Q806R, Centronic, Croydon, England) and converted to partial pressures at the animal's body temperature. The total dead space of the apparatus was 7 ml; the apparatus offered a gas flow resistance of 36 cm H,O •1-~. sec during inspiration and 24 cm H20.1 -~ • sec during expiration, independent of the flow in the experimental range. Each duck was blind-folded with small, black discs taped over the eyes. It was secured upright by placing loose-fitting metal rings around its neck and cranial thoracic region, and around its tail and abdominal region to prevent forward or backward movements. The bird rested quietly on its feet, and there was no apparent restriction of ventilatory movements.

CO 2 RESPONSE IN BIRDS

351

EXPERIMENTAL PROTOCOL AND CALCULATIONS

Ventilatory response to inhaled CO2 was studied at various Flco" between 0 and 0.07; Flo, in all trials was kept at 0.30. After each change to a new level of FIcQ, a period of at least 20 minutes was allowed for attainment of new steady-state conditions, as judged by the stability of blood pressure and respiration. Gas was then collected into a bag for about 4 minutes while arterial and mixed-venous blood samples were withdrawn. Mean respiratory frequency, fresp, was calculated for that period; the minute ventilation, VE, and mean tidal volume, VT, were calculated from the volume collected in the bag. 02 uptake, IVlo:, and CO, output, lqco:, were calculated from the inspired and mean expired (= bag) 02 and CO2 concentrations and from VE. Inspiratory and expiratory periods, T1 and TE, were obtained from the pneumotachograph record, and mean inspiratory and expiratory flow rates calculated as Vin = VT/TI and Vex = VT/TE, respectively. After the foregoing measurements had been completed, acetazolamide (Diamox, 50 mg per kg body weight) was injected intravenously. During the next 90 minutes, the ventilatory parameters were measured while a new steady-state was approached. The entire protocol then was repeated as before Diamox (the period before administration of Diamox shall be called the Control period). Differences were tested for significance using the Student 't' test and a level of 5%.

Results N O R M A L VALUES

The respiratory variables measured in the awake ducks under Control conditions (before Diamox) at Flco~= 0 are probably similar to those that would be found in these animals during air breathing, although 30% oxygen was given to the inspired mixture (see Discussion). Hence, we report these data (in table 1) as representative for an unanesthetized, resting duck.

R E S P I R A T O R Y RESPONSE TO CO 2

Ventilation In the Control conditions, total ventilation increased more than linearly with increments in Plco_~(fig. 1). The animals tolerated a PIco_~of about 35 torr without exhibiting signs of discomfort, but became restless when Plco_~was further elevated. In the range ofPIco~ from 0 to 35 torr, "¢E increased almost four times. This increase was due mainly to an increase in VT, while fresp increased only about 50%. After Diamox was given and respiration had stabilized, ~/E w a s lower than before (Control) at any level of Plco~ tested. In particular, when the animal inspired no CO2 resting ventilation was about 25°4, below the Control level, the difference

352

F.L. P O W E L L et al.

TABLE 1 Average values (mean + SD) of respiratory variables in unanesthetized, resting ducks breathing a normocapnic hyperoxic mixture (PIo_, = 205 torr) Weight Ta Tb VF~ VI fresp Mo2 l'Vlco2 RE Pao2 Paco~ PVo2 Pvco 2 PE'co 2 pHa pHv

1.6 24 40.1 270 30 9.4 0.75 0.61 0.81 138 35.2 51.5 44.4 39.9 7.42 7.38

+ 0.2 __ 0.5 +_ 0.7 _+ 30 + 3 _+ 1.7 _+ 0.08 + 0.06 + 0.06 + 9 + 1.4 + 6.3 + 1.8 + 3.3 _+ 0.03 + 0.04

kg C C ml BTPS . r a i n - I . kg l ml BTPS • k g - l min -I mmol .min I mmol .min -t

(5)

(5) (5) (5) (5) (5) (3) (5) (3) (5) (5) (5)

torr torr torr torr torr

Data obtained at the beginning of the experiment, before the first administration of CO> In brackets, number of birds. Ta, Tb, ambient and body temperature; VE, Vr, expired ventilation and tidal volume; IVlo2, IVlco . 02 uptake and CO 2 output; RE, respiratory quotient in mean expired air.

2 5 -

(L" rain "1}

80~

OI

20

15 ,

i

0

J

i

I0

f

i

20

i

i

,

30

i

i

1.0

PIc02

(tort)

i

SO

0

r

0

J

i

i

20

i

~

/~0

PIC02

(torr)

Fig. 1. Ventilatory responses to inspired C O 2 before (Control) and after administration of Diamox.

353

CO 2 RESPONSE IN BIRDS

120"

A.

zo

1oo-

,,//

I

Vm \ #

,

A

'mr BTPS) 80-I

,o

607o ,

iS

°o,.

TI20

~/0

~0

÷ {mrews)/,, /

°\ lco~ro~ *SO

_/

'

/

z0

",s

~ contro,

i

o,

PIc°~t°"l ~

'

B.

/

,o

0

2

4

Tresp (see)

6

2 TI

0 I$e¢)

2

/, TE

Fig. 2. Changes in respiratory pattern when inspired CO~ is elevated. ( A ) P | o t of VT against total respiratory period. Lines represent constant levels of total ventilation. The Plco ~ is indicated at each data point. (B) Differentiation between inspiratory (TO and expiratory time (TE) in the ventilatory response to CO 2. In this case, the lines represent values of constant mean inspiratory ('v'in) and expiratory flows (£ex). For details see text.

being statistically significant. For any given increment in Picot, VE increased less after Diamox than before. The highest level of PIco_~ that was quietly tolerated by the animal was about 50 torr, and thus was higher than in the Control. The mean changes in the respiratory pattern with increasing Plco, during the Control period and after Diamox are shown in fig. 2. Figure 2A is a plot of VT against the mean total respiratory period, Tresp (= 1/fresp). It is evident that with increasing Plco:, VT increased and respiratory period decreased. After Diamox, VT did not change much with Plco_~, and the increase in "V'Ewith PIco~ was mainly due to a decrease in Tresp (increase in fresp). In fig. 2B, VT is plotted separately against the inspiratory, TI, and expiratory time, TE. Changes in TE were mainly responsible for the changes in the total respiratory time upon changing inspired CO2 concentration, both before and after Diamox; Tl was virtually constant under all test conditions. The mean inspiratory flow (£in) increased nearly proportionally with VT. However, the mean expiratory flow (Vex) increased more than proportionally with VT but did not reach as high a value. Blood gases and metabolism

Up to Plco~ of 20 torr, there was no significant change in arterial, mixed venous, or end-expired Pco~ (PE'co~) either in the Control period or after Diamox (fig. 3A); the values after Diamox, however, were approximately 35 torr above the Control values. PE', Pa, and PV were significantly elevated when PIco_,was raised above 20 torr both before and after Diamox (except for Pa at PIco2 = 35 torr after Diamox, which was not significantly different from the eucapnic level). In four measurements before Diamox, PE'cQ exceeded Pvco.~ (on the average by 3 torr), and this occurred at all levels of Plco.,.

354

F.L. POWELL et al.

A°

100

750

B.

Diamox

pH

Control

80

730 Diamox 6O

Pco2 (torr)

710

6

o.8

z.O

'

10

C.

'

T

¢4co 1

co2

0.90

-RE

20

-0.80 0

0

20 40 P I c o 2 (tort)

,

,

i

,

0

20

z,0

PI C02 (torr)

,

Fig. 3. Effects of Plco_~ on blood and end-expired Pco_, (A), on blood pH (B), and on CO a output, Mco,. and respiratory quotient in mean expired air, RE (C). Mean values before (closed circles) and after Diamox (open circles) (_+ SE).

During the Control period at the highest level of Pco~ tested, blood pH.definitely decreased (fig. 3B). After Diamox, pH was decreased by about 0.2 unit at all inspired Pco~, and the decline with increasing PIco, was slight. 02 uptake, 1Vlo,, and CO_, output, 191co~ (fig. 3C) increased:~ignificantly with increasing PIco~ during the control period while the respiratory quotient in mean expired air, RE, remained unaltered. After Diamox, the metabolic activity was somewhat lowered and CO 2 output by the lung showed no dependence on PIco:; RE declined slightly as PIco, was elevated.

T R A N S I E N T C H A N G E S U P O N A D M I N I S T R A T I O N OF D I A M O X

Figure 4 displays the respiratory responses to injection of Diamox at PIco~ = 0 in two animals (A and B) considered typical in these experiments. In both: animals; ventilation was substantially diminished during the first 2 to 3 minutes after initiation of Diamox injection. In bird A (fig. 4A), this apneic period was followed by rapid, shallow breathing so that total ventilation returned to its control level. That this ventilation was, however, less effective for gas exchange than before Diamox (because of the large dead space ventilation) was particularly evident from die end-

CO~ RESPONSE IN BIRDS

®

355

200 120 (torr}

*i|

PCO2 {to~r)

o :llMql ,~,~ I i

VT

I,I, ,q L,i,[

.

11

I

so ,, ..........

i

400 -

.

~E (ml. rni~ -1 )

I

.

. \

:oo

-1

.

......

I

*

"

.

.

.

.

,

"

.

/\ . . . .

5

10

.~'-"-. "

15

Piarnox

20 time {rain)

Poz (tort)

[

120 ~

PC02 (torr)

400 ~ "--i ME

200

Iml. rnin -I)

-1 0 Dlamox

5

1'0'

'

15

20 time (ndn)

Fig. 4. Transient changes in respired gases and ventilation following Diamox injection. Typical examples are displayed in (A) and (B). Tracings in each are: Po:, Pco_, in gas at the tracheal cannula; tidal volume; £E, calculated from VT and fresp. In (A), the large deflections in VT (marked by *) after Diamox injection coincided with defecation. expired Po,, which remained very low. Pco_, in expired air at the same time markedly increased to a new, stable level. The response of bird B (fig. 4B) was more regular. After the apneic period, ventilation soon stabilized at a low level with decreased fresp. The patterns of end-expired Po, and Pco~ were similar to those of fig. 4A, and suggested that the effective ventilation was similarly diminished in both animals. Two of the 5 birds studied responded with markedly depressed VT and increased fresp (fig. 4A), whereas the 3 other birds exhibited decreased fresp at more or less unaltered VT (fig. 4B). These differing patterns, maintained qualitatively by the birds after attainment of steady-state, accounted for the larger variability in VT and fresp after D i a m o x than before (fig. 1). The steady-state ventilation however, was similar in all birds, as can be seen from the small scatter of 'VE in fig. 1. On

356

F.L. P O W E L L et al.

the average, a steady value of all measured variables was not attained until 90 min after injection of Diamox.

Discussion COMPARISON WITH LITERATURE VALUES

Normal values for respiratory variables in unanesthetized birds Ventilation in unanesthetized ducks has been measured by several investigators (Anderson and L6v6, 1964; Jones and Purves, 1970; Bretz and Schmidt-Nielsen, 1971; Bouverot et al., 1974a,b) and some have also reported values for blood gases. We obtained values for ventilation that were lower than all those reported. But, for two reasons, we do not believe the values were low because of removal of a normoxic ventilatory drive by the elevated inspired Po: in our birds. First, Jones and Purves (1970) have shown for two duck species that ventilation is not significantly diminished when Pao: is raised above normoxic values. In fact, those findings led us to perform the experiments in hyperoxia to insure that changes in ventilation would not significantly influence the 02 drive on breathing. Though Bouverot et al. (1974b) observed a decrease in ventilation when the carotid bodies were denervated, that decrease could have been due to removal of a CO: drive and thus would not be decisive for an 02 drive provided by arterial chemoreceptors at normoxic arterial Po~. Second, our mean value of arterial Pco_~ when Plco~ = 0 was near or even slightly below the normal resting value measured in ducks of the same species by a remote-control sampling technique (Kawashiro and Scheid, 1975). That observation indicated that the ducks in our study did not hypoventilate when breathing 0~o CO~. We must, therefore, conclude that differences between the values in our study and those reported in the literature reflect differences in technique or in species. Ventilation in the Pekin duck is critically dependent upon environmental temperature, even near normal room temperature values of 25 C (Bouverot et al., 1974a). Room temperature during our experiments remained within _+1 C, and for the normal values reported in table 1 there was no detectable correlation of "qE with the small variations in ambient temperature. That end-expired Pco~ exceeded arterial Pco~, as is typical in birds, may be explained on the basis of the cross-current system for parabronchial gas exchange (cf. Piiper and Scheid, 1975). On the average, PE'co: was near PVco~, and in some cases PE'co_~ even exceeded P~co:, a result previously observed in the anesthetized chicken (Davies and Dutton, 1975; Meyer et al., 1976). That observation can be explained by the peculiar role that the Haldane effect may exert in the serial-multicapillary system of the parabronchus (Meyer et al., 1976).

C O 2 RESPONSE IN BIRDS

357

Ventilatory response to inhaled CO 2

It is now generally accepted that low levels of CO2 in inhaled air increase ventilation in unanesthetized ducks (Jones and Purves, 1970; Bouverot et al., 1974b), in anesthetized chickens (Ray and Fedde, 1969; Osborne and Mitchell, 1978), and in decerebrate chickens (Johnston and Jukes, 1966). Barbiturate anesthesia seems to suppress this response as it depresses resting ventilation (Fowle and Weinstein, 1966 ; Nightingale, 1975a, 1977), while Equithesin anesthesia induces less suppression (Nightingale, 1975b). The earlier view that CO2 inhibits ventilation in birds (Orr and Watson, 1913) may have emerged as a result of the high levels of CO~ administered. Fowle and Weinstein (1966) showed that above 5~o CO, in inspired air, ventilation in birds was diminished. This result agrees qualitatively with observations by Jones and Purves (1970) and Bouverot et al. (1974b). The constancy of arterial Pco~ and pH in our ducks when PIco, was increased to about 20 torr is in agreement with results of Osborne and Mitchell (1977; 1978) in anesthetized chickens; however, Kuhlmann and Fedde (1976) observed small but significant increases in Paco~ when decerebrate chickens inhaled 12.5 torr Pco:. Thus, in the range of low Plco:, ventilation is changed without (or with very small) changes in Paco ;. Hence, it is difficult to believe that arterial chemoreceptors mediate that adjustment (Scheid et al., 1978). E[fects o f D i a m o x

Acetazolamide seems to inhibit carbonic anhydrase in birds, much as it does in mammals (Maren, 1967). The acute effect of Diamox in our study was a respiratory acidosis, an effect in agreement with observations in other bird species (Nechay et al., 1960; Andersen and Hustvedt, 1967; Davis and Dutton, 1975). These changes may be explained by the decreased effective CO2-transport properties of blood ensuing upon blockade of carbonic anhydrase (Meyer et al., 1976). We know of no report of the effects of carbonic anhydrase inhibition on resting ventilation in birds except for a remark by Andersen and Hustvedt (1967) that ventilation increased after Diamox in the duck; this increase would be at variance with our observations. The transient apnea following injection of Diamox (fig. 4) may be due to the transient increase in intrapulmonary chemoreceptor activity (Scheid et al., 1978) since those receptors exert an inhibitory effect on ventilation (Bouverot, 1978).

DIFFERENCES IN RESPONSES BETWEEN M A M M A L S A N D BIRDS

Ventilation in mammals increases in response to inhaled CO2 in a manner qualitatively similar to that in birds. However, small yet significant increases in Paco_~ have been observed even at low levels of P1co:, and it is usual to plot CO2 response curves as ventilation against Paco: (Dejours et al., 1965; Stremel et al., 1978; cf. Comroe, 1965). Such a plot bears little significance in our experiments because Paco. is virtually unaltered at low levels of PIco:.

358

F.L. POWELL et al.

W h e n c a r b o n i c a n h y d r a s e is a c u t e l y b l o c k e d in m a m m a l s , v e n t i l a t i o n a n d a r t e r i a l Pco~ i n c r e a s e ( T o m a s h e f s k i et al., 1954" C a i n , 1962: c;[i M a r e n , 1967). W i t h c h r o n i c e n z y m e b l o c k a d e , Paco~ m a y d e c r e a s e ( C h i e s a et al., 1969). T h e m e c h a n i s m

by

w h i c h v e n t i l a t i o n is i n c r e a s e d is n o t clear. In the b i r d s o f o u r s t u d y , v e n t i l a t i o n w a s d e p r e s s e d by t h e d r u g d e s p i t e the e l e v a t e d level o f a r t e r i a l Pco,. hi s u m m a r y , h o m e o s t a s i s o f Paco ~ at l o w levels o f PIco, a n d d e p r e s s e d v e n t i l a t i o n , d e s p i t e e l e v a t e d Paco_~ a f t e r D i a m o x , a r e f i n d i n g s t h a t c o n t r a s t w i t h o b s e r v a t i o n s in m a m m a l s . It is t e m p t i n g , t h e r e f o r e , to suggest t h a t the a v i a n s y s t e m f o r c o n t r o l o f v e n t i l a t i o n , at least in r e s p o n s e to i n h a l e d CO2, differs f r o m t h a t in m a m m a l s . In the f o l l o w i n g c o m m u n i c a t i o n ( S c h e i d et al., 1978) w e will a t t e m p t to i n v e s t i g a t e if a v i a n i n t r a p u l m o n a r y

c h e m o r e c e p t o r s , w h i c h a p p e a r to h a v e n o e q u i v a l e n c e in

mammals, can account for these differences.

References Andersen, H.T. and A. L6v6 (1964). The effect of carbon dioxide on the respiration of avian divers (ducks). Comp. Biochem. Physiol. 12:451 456. Andersen, H.T. and B.E. Hustvedt (1967). Carbon dioxide excretion and pH-variations in diving ducks after carbonic anhydrase inhibition. Acta Physiol. Scand. 69: 203-208. Bouverot, P., G. Hildwein and D. Le Goff (1974a). Evaporative water loss, respiratory pattern, gas exchange, and acid base balance during thermal panting in Peking ducks exposed to moderate heat. Respir. Physiol. 2l : 255 269. Bouverot, P.. N. Hill and Y. Jammes (1974b). Ventilatory responses to CO 2 in intact and chronically chemodenervated Peking ducks. Respir. Physiol. 22 : 137-156. Bouverot, P. (1978). Control of breathing in birds as compared with mammals. Plo,siol. Rev. (in press). Bretz, W. L. and K. Schmidt-Nielsen (1971). Bird respiration: Flow patterns in the duck lung. J. Exp. Biol. 54:103 118. Burger, R.E., J.L. Osborne and R.B. Banzett (1974). lntrapulmonary chemoreceptors in Galhts domeslicus: adequate stimulus and functional localization. Respir. Physiol. 22:87 97. Cain, S. M. (1962). Additive effects of acetazolamide and fat emulsion on alveolar-arterial Pc(): difference. J. Appl. Physiol. 17:622 624. Chiesa, A., T.B. Stretton, A. A. E. Massoud and J. B. L. Howell (1969). The effects of inhibition o1" carbonic anhydrase with dichlorphenamide on ventilatory control at rest and on exercise in normal subjects. Clhz. Sci. 37:689 706. Comroe, J. H. (1965). Physiology of Respiration. Chicago, Year Book Medical Publ. Davies, D.G. and R. E. Dutton (1975). Gas blood Pco_, gradients during avian gas exchange. J. Appl. Physiol. 39:405 410. Dejours, P., R. Puccinelli, J. Armand and M. Dicharry (1965). Concept and measurement of ventilatory sensitivity to carbon dioxide. J. Appl. Physiol. 20: 890-897. Fedde, M. R. (1970). Peripheral control of avian respiration. Fed. Proc. 29 : 1664- 1673. Fedde. M. R.. R. N. Gatz, H. Slama and P. Scheid (1974). lntrapulmonary CO, receptors in the duck: I. Stimulus specifity. Respir. Physiol. 22:99 114. Fowle, A~ S. E. and S. Weinstein (1966). Effect of cutaneous electric shock on ventilatory response of birds to carbon dioxide. Am. J. Physiol. 210:293 298. Johnston. A.M. and M.G.M. Jukes (1966). The respiratory response of the decerebrate domestic hen to inhaled carbon dioxide-air mixture. J. Physiol. (London) 184:38 39P. Jones, D.R. and M.J. Purves (1970). The effect of carotid body denervation upon the respiratory response to hypoxia and hypercapnia in the duck. J. Physiol. (London) 211 : 295 309.

CO~ RESPONSE IN BIRDS

359

Kawashiro, T. and P. Scheid (1975). Arterial blood gases in undisturbed resting birds: measurements in chicken and duck. Respir. Physiol. 23: 337-342. Kuhlmann, W.D. and M. R. Fedde (1976). Control of respiration in chicken: Response to inhalation at low CO~ concentrations. Poultry Sci. 55 : 2055-2056. Kunz, A. L., T. Kawashiro and P. Scheid (1976). Study of CO 2 sensitive vagal afferents in the cat lung. Respir. Physiol. 27:347 355. Maren, T.H. (1967). Carbonic anhydrase: Chemistry, physiology and inhibition. Physiol. Rev. 47: 595 781. Meyer, M., H. Worth and P. Scheid (1976). Gas blood CO 2 equilibration in parabronchial lungs of birds. J. Appl. Physiol. 41 : 302 309. Nechay, B. R., J. L. Larimer and T. H. Maren (1960). Effects of drugs and physiological alterations on nasal salt excretion in sea gulls. J. Pharmacol. Expl. Therap. 130:401 410. Nightingale, T. E. (1975a). Barbiturate anesthesia and CO 2 sensitivity in chickens. Physiologist 18: 333. Nightingale, T. E. (1975b). Equithesin anesthesia and respiration in chickens. Fed. Proc. 34: 430. Nightingale, T.E. (1977). Comparison of cardiopulmonary parameters in awake and anesthetized chickens. PoulnT Sci. 56:147 153. Orr. J. B. and A. Watson (1913). Study of the respiratory mechanism in the duck. J. Physiol. (London) 46:337 348. Osborne, J. L. and G. S. Mitchell (1977). Regulation of arterial Pco~ during inhalation of CO 2 in chickens. Respir. Physiol. 31 : 357-364. Osborne, J.L. and G.S. Mitchell (1978). Ventilatory responses during arterial homeostasis of Pco_~ at low levels of inspired Pco_~. In: Respiratory Function in Birds, Adult and Embryonic, edited by J. Piiper. Springer, Berlin-Heidelberg (in press). Piiper, J. and P. Scheid (1975). Gas transport efficacy of gills, lungs and skin: theory and experimental data. Respir. Physiol. 23:209 221. Piiper, J. and P. Scheid (1977). Comparative physiology of respiration: Functional analysis of gas exchange organs in vertebrates. In: International Review of Physiology, Series II, Respiratory Physiology, edited by J. G. Widdicombe. Lancaster, MTP. Ray, P. J. and M. R. Fedde (1969). Responses to alterations in respiratory Po: and Pco: in the chicken. Respir. Physiol. 6:135 143. Scheid, P., R.K. Gratz, F.L. Powell and M.R. Fedde (1978). Ventilatory response to CO 2 in birds: If. Contribution by intrapulmonary chemoreceptors. Respir. Physiol. (submitted). Stremel, R.W., D.J. Huntsman, R. Casaburi, B.J. Whipp and K. Wasserman (1978). Control of ventilation during intravenous CO z loading in the awake dog. J. Appl. Physiol. 44:311 316. Tomashefski, J. F., H. i. Chinn and R.T. Clark, Jr. (1954). Effects of carbonic anhydrase inhibition on respiration. Am. J. Physiol. 177: 451~454.