The ventilatory response of rodents to changes in arterial oxygen content

A strong correlation between the arterial oxygen partial pressure (PaO2) 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 O2 content (CaO2). This study examined the extent to which changing CaO2 independent of PaO2 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 PaO2 and CaO2 simultaneously, and carbon monoxide hypoxia, which alters CaO2 independent of PaO2. 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 CaO2 independent of PaO2 could still produce 60% of the full HVR seen with hypoxic hypoxia in both species. Simultaneous changes in PaO2, however, were necessary to produce the full response. While it seems likely that the results can be explained by the changes in tissue PO2 which would occur at receptor sites under the various conditions, such an explanation is not totally supported by other studies.