To examine whether hypotension reflexly initiates arousal from sleep and the mechanisms involved, we subjected sleeping lambs to hypotensive stimuli of 1-min duration, before and after sinoaortic denervation (SAD). In intact lambs, hypotension increased the probability of arousal from both quiet sleep (QS) and rapid-eye-movement (REM) sleep. Hypotension resulted in nonarousal in 42% (QS) and 47% (REM) of tests. Arousal time was significantly longer in REM (34.9 +/- 1.8 s, means +/- SE) than in QS (26.0 +/- 1.8 s). Arterial saturation of O2 (SO2) and PO2 measured at the point of arousal were unchanged from control values in those tests in which arousal occurred. In nonarousal tests, there was a significant fall in both SO2 (4.9 +/- 1.2%) and PO2 (21.6 +/- 4.2 mmHg). After SAD, hypotension did not increase the probability of arousal. Nonarousals significantly increased to 75% (QS and REM, P less than 0.02). We conclude that acute hypotension is a potent stimulus for arousal from sleep in newborn lambs. As the arousal response is abolished by SAD and is not correlated with arterial oxygenation, hypotensive arousal appears to be mediated via arterial baroreceptors.
