The ventilatory response to hypoxia appears to be controlled largely by carotid body chemoreceptors in birds, as it is in mammals, and the stimulus to these chemoreceptors is probably the partial pressure of oxygen in the arterial blood (Bouverot, 1978; Bouverot, Douguet & Sebert, 1979). There are, however, interspecific differences in the threshold arterial stimulating the hypoxic ventilatory response. Black & Tenney (1980) found that a high-altitude species, the bar-headed goose, initiates an increase in ventilation at a lower arterial than a lowland species, the Pekin duck. The bar-headed goose also has a higher affinity haemoglobin, presumably as an adaptation to its high altitude habitat. Van Nice, Black & Tenney (1980) noted that when the ventilatory response to hypoxia is expressed in terms of the percent saturation of haemoglobin in arterial blood , the bar-headed goose and Pekin duck exhibit a similar threshold . It seems unlikely that the receptors sense haemoglobin O2 saturation directly and experimental reductions in O2 content do not stimulate ventilation in the duck (D. F. Boggs, unpublished observation). Nonetheless a relationship between the threshold of the ventilatory response to hypoxia and the ‘knee’ of the oxyhaemoglobin dissociation curve would be a reasonable evolutionary development in both birds and mammals, as Van Nice et al. (1980) have pointed out. If threshold arterial for the hypoxic ventilatory response were an interspecific constant, then those species with relatively high-affinity haemoglobins would be responding prematurely, and, therefore, ‘wastefully’, whereas those with relatively low-affinity haemoglobins would be in danger of suffering substantial arterial unsaturation before the ventilatory response is initiated. The latter eventuality would be particularly hazardous for tissue oxygen supply in those animals with low affinity haemoglobins, since these animals are in most cases small, with high weightspecific metabolic rates (Lutz, Longmuir & Schmidt-Nielsen, 1974; Schmidt-Nielsen & Larimer, 1958).
To examine this question, we investigated the hypoxic ventilatory response in the rhea (Rhea americana), which has a relatively high-affinity haemoglobin (Lutz et al. 1974), and in the ring-necked pheasant (Phasianus colchicus), in which the affinity is substantially lower. This species of rhea does not live at particularly high altitude, in its native Argentine and Brazilian grasslands, so its ventilatory response should not, in itself, represent an aspect of hypoxic adaptation, as may be the case in the barheaded goose.
The methods employed in this study were in most respects the same as those described by Black & Tenney (1980). Steady-state values of ventilation were measured in a whole-body plethysmograph. A steady state was assumed to be established 10–15 min after the plethysmograph had equilibrated with a new gas mixture if repeated measurements at 5–10 min intervals were consistent with each other. Awake, undisturbed animals were exposed to various levels of hypoxia achieved by adding N2 to air, and the response was expressed as the percent increase in ventilation with hypoxia over the mean ventilation measured while the bird was breathing air before and after each hypoxic exposure. Arterial blood samples (0·7 ml) were drawn from catheters previously implanted either in the radial artery under local lidocaine anaesthesia in the pheasants, or in a branch of the femoral artery under halothane anaesthesia (1·5 % in O2) in the rheas. Blood gases and pH were measured with Radiometer O2 and CO2 electrodes and PHM 71 Acid Base Analyzer. Following the ventilation studies, dissociation curves were determined with a standard mixing technique (Edwards & Martin, 1966), using whole blood freshly drawn from cardiac puncture. Saturated blood samples, equilibrated for 10 min with 3% CO2:30% O2: 67 % N2 in an IL 137 tonometer at 41 °C, and deoxygenated samples equilibrated with 3 % CO2 :97 % N2 were mixed in volumetric ratios to achieve known saturations. Their and pH were then promptly measured. Equilibrations with 8·2% CO2 with and without 30 % O2 were also used to establish the Bohr factor. All values were then corrected to a pH of 7·5.
Three young pheasants between 3·5 and 5 months of age (1·36, 1·02 and 0·99 kg), in adult plumage and of adult size, were used. Two young rheas, approximately 2–3 months old, but far from adult size, were also used in this study. One weighed 1·9 kg, the other weighed 2·2 kg. Preliminary studies of the ventilatory response to hypoxia were made on a total of four rheas including two smaller birds (0·59 kg and 0·42kg), but technical difficulties precluded use of the smaller ones in the blood gas/ventilation experiments reported here. Their ventilatory thresholds were the same as in the larger ones in their relationship to (fractional concentration of inspired O2).
The 12 Torr difference in P50 between the rhea and pheasant presented in Fig. 1 is similar to the 14 Torr difference observed by Lutz et al. (1974). Our P50 values for these species are higher than those reported by Lutz et al. (1974) but the technique used by those investigators gives consistently lower values than all others (Baumann & Baumann, 1977). The pheasant P50 reported here is, however, essentially the same as the 42·2 reported by Isaacks et al. (1976) (corrected to 41 °C and pH = 7·5). The pheasant body temperature was in fact always 41 °C or higher, whereas the rhea body temperature was 39 –40 °C. However, this small in vivo temperature difference would only have the effect of reducing the reported in vitro rhea P50 by 1–2 Torr, assuming the temperature coefficient of its haemoglobin is similar to that of other birds (Δlog P50/ ΔT = 0·017°C −1, Baumann & Baumann, 1977).
Resting respiratory pattern was different in the two species (Table 1). The rhea tidal volume (VT) is larger and the pheasant VT smaller than predicted on the basis of body weight (16·93 B.W.1·05, Bech, Johansen & Maloiey, 1979), while the rhea frequency (f) is less and pheasant frequency greater than predicted (17·2B.W.− 0·31, Lasiewski & Calder, 1971). The total minute ventilations, however, are within 5 % of the predicted values based on the product of the predicted VT and f. The large tidal volume, low frequency pattern of the rhea appears to be typical of other birds with long necks, such as the mute swan (Bech & Johansen, 1980) and the flamingo (Bech et al. 1979). In both the pheasant and the rhea, the ventilatory response to hypoxia is due almost entirely to increases in respiratory frequency.
The first noticeable increase in ventilation in the rhea occurs at a approximately 20 Torr below the at which the pheasant begins to increase its ventilation (Fig. 2). There does appear to be a good correlation between that at which ventilation begins to increase and the value of at which the HbO2 dissociation curve changes from the relatively flat upper portion to the more steeply sloped portions in each species. When the change in ventilation is plotted against it can be seen that the increase in ventilation is initiated at approximately 80–85% saturation (Fig. 3) in both species. Therefore the expectation of a correspondence in between the ‘knees’ of the hypoxic ventilatory response curves and the ‘knees’ of the oxyhaemoglobin dissociation curves in two lowland avian species with differing haemoglobin O2 affinities is borne out. For reasons already mentioned, that correspondence has important functional implications for the animals, but it need not imply anything fundamental about chemoreceptor function. It is most probably a consequence of natural selection favouring a high hypoxic ventilatory threshold in those species with a low affinity haemoglobin and a low threshold in those with a high oxygen affinity.
The authors wish to thank Dr S. M. Tenney for his support of this work and his helpful suggestions in the preparation of this manuscript. This work was supported by Grant No. HLO2888-25 from the National Heart, Lung and Blood Institute. D. F. Boggs is a Research Fellow of the Parker B. Francis Foundation. G. F. Birchard was supported by Training Grant No. HLO7449-02 from the National Heart, Lung and Blood Institute.