Relationship between haemoglobin O2 affinity and the ventilatory response to hypoxia in the Rhea and pheasant.

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 Po2 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 PQ3 (Pao2) 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 (Sao,), the bar-headed goose and Pekin duck exhibit a similar threshold Sao2 • 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O2 saturation directly and experimental reductions in O2 content do not stimulate ventilation in the duck (D. F. Boggs, unpublished observation).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 Niceet al. (1980) have pointed out. If threshold arterial Po2 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; SchmidtNielsen & Larimer, 1958).