Time course of baroreceptor resetting in short-term hypotension in the rat

Abstract

A multifiber preparation of the aortic nerve was used to analyze hypotension-induced displacement in the rat''s baroreceptor firing range. A progressive downward shift was observed in the entire baroreceptor activation range during a 6-h period of hemorrhagic hypotension. The process of adaptation was not complete at the end of the experiment. The pressure threshold for baroreceptor activation dropped 11% after 15 min of hypotension, 16% after 1 h and 26% after 6 h. A more marked downward displacement was observed in rats treated with phenoxybenzamine and subjected to controlled bleeding, suggesting that the sympathetic system reduced the baroreceptor activity during hypotension. The 10% fall in baroreceptor firing range produced by phenoxybenzamine in the control period cannot be attributed only to a tonic inhibition of the sympathetic since the concomitant hypotension could also affect the baroreceptor function. In 10 reserpinized rats with moderate hypotension (80-103 mmHg) and 15 reserpinized rats with severe hypotension (less than 80 mmHg) the baroreceptors were studied 48 h after the reserpine injection. The downward shift of the baroreceptor firing range was proportional to the fall in pressure and the process of adaptation was complete. The sequence followed by the baroreceptor resetting in hypotension appears to be quite similar to that observed at the onset of acute hypertension.