Neural effects of systemic hypoxia and hypercapnia on hindlimb vascular resistance in acute spinal cats.

The effects of systemic hypoxia and hypercapnia on the neurogenic component of hindlimb vascular resistance were studied in 10 unanesthetized acute Cl spinal cats. Hindlimb perfusion pressure (PP) was measured under conditions of constant flow of normoxic and normocapnic blood from a donor cat. Ventilation with 5% CO2 and 10% CO2 in O2 caused increases in PP of 15 +/- 2 (mean +/- SE) mm Hg and 27 +/- 3 mm Hg from a control level of 106 +/- 6 mm Hg during ventilation with 100% O2. Changing the inspired gas mixture from 95% O2 plus 5% CO2 to 12.5%, 10%, 7.5%, or 5% O2 plus 5% CO2 in N2 caused increases in PP of 1.5 +/- 1, 14 +/- 2, 38 +/- 6, and 69 +/- 15 mm Hg respectively from a control level of 121 +/- 9 mm Hg. These vasoconstrictor effects were abolished by ganglionic blockade with hexamethonium (10 mg/kg iv). We conclude that in the acute Cl spinal cat a large part of the population of sympathetic preganglionic neurons in the lumbar spinal cord, controlling vascular smooth muscle of the hindlimb, is excited by systemic hypoxia or hypercapnia over a considerable range of PaO2 and PaCO2 values.