Hypoxia and Hypercapnia

The renin response to hypoxia in late gestation fetal sheep has been well characterized. However, the renin response to asphyxia-the combination of hypoxia and hypercapnia-has not been extensively studied. The purpose of this study was to determine 1) the interaction of hypoxia and hypercapnia in the control of renin secretion in late gestation fetal sheep and 2) the role of peripheral arterial chemoreceptors therein. Chronically catheterized fetal sheep (intact or sinoaortic denervated) were exposed to hypoxia and/or hypercapnia for 30 minutes. Hypercapnia alone had no effect on plasma renin activity or aldosterone but did result in a significant increase in angiotensin II. Hypercapnia combined with hypoxia resulted in a significant increase in renin activity, angiotensin II, and aldosterone. Sinoaortic denervation attenuated the renin and angiotensin II responses to hypercapnia plus hypoxia. The increase in renin and angiotensin II in response to hypercapnia with or without concomitant hypoxia strongly correlated with the magnitude of the decrease in arterial pH in intact fetuses only. Hypoxia alone and in concert with hypercapnia increased mean arterial pressure and decreased heart rate in intact but not sinoaortic denervated fetuses. We conclude that 1) hypercapnia more potently increases plasma renin activity than does hypoxia in late gestation fetal sheep, 2) arterial pH may be the relevant signal perceived by the peripheral arterial chemoreceptors for the control of the renin-angiotensin system during asphyxia, and 3) the cardiovascular response to hypoxia is mediated, in part, by peripheral arterial chemoreceptors. (Circulation Research 1990;67:722-732)