Hypoxic responses of sympathetic preganglionic neurons in sinoaortic-denervated cats

The relation between arterial O2 tension (PaO2) and the firing rate of sympathetic preganglionic neurons (SPN) was studied in 16 strands of the cervical sympathetic trunk (CST) during graded isocapnic hypoxia in 11 sinoaortic-denervated central nervous system (CNS)-intact, anesthetized cats. SPN firing rate was independent of PaO2 from normoxia down to a PaO2 of 40 Torr. Below this PaO2 level three response patterns were observed, i.e., an excitatory response (n = 8), a depressant response (n = 3), and a mixed response consisting of a depression of firing at less severe hypoxic levels and an increase in firing rate at more extreme hypoxic levels (n = 5). Similar response patterns were also observed in four strands of the CST in three unanesthetized, sinoaortic-denervated, midcollicular decerebrate preparations. Systemic arterial pressure decreased in all cats as PaO2 decreased. Phrenic nerve activity also decreased in all cats with a course resembling that of the depression of sympathetic firing and disappeared at a PaO2 of 20 Torr. The data suggest that systemic hypoxia in the sinoaortic-denervated, CNS-intact or decerebrate animal activates both excitatory and depressant mechanisms acting on SPNs.