THE VENTILATORY RESPONSE OF RODENTS TO CHANGES IN ARTERIAL OXYGEN-CONTENT

A strong correlation between the arterial oxygen partial pressure (Pa-O2) values for the threshold of the hypoxic ventilatory response (HVR) and the shoulder of the oxyhemoglobin equilibrium curve (OEC) is retained in heterothermic rodents as body temperature changes despite changes in hemoglobin-oxygen affinity. It has been suggested that this may reflect either temperature-induced changes in the response characteristics of arterial chemoreceptors or an ability to sense changes in arterial O-2 content (Ca-O2). This study examined the extent to which changing Ca-O2 independent of Pa-O2 with carbon monoxide could contribute to the HVR in heterothermic (golden-mantled ground squirrels) and non-heterothermic rodents (rats). The HVR of isocapnic, anaesthetized rodents was assessed during both hypoxic hypoxia, which alters Pa-O2 and Ca-O2 simultaneously, and carbon monoxide hypoxia, which alters Ca-O2 independent of Pa-O2. While both species exhibited ventilatory responses to hypoxic hypoxia and carbon monoxide hypoxia, the HVR of the squirrel was consistently stronger than that of the rat. Reductions in Ca-O2 independent of Pa-O2 could still produce 60% of the full HVR seen with hypoxic hypoxia in both species. Simultaneous changes in Pa-O2, however, were necessary to produce the full response. While it seems likely that the results can be explained by the changes in tissue P-O2 which would occur at receptor sites under the various conditions, such an explanation is not totally supported by other studies.