Sympathetic nervous response to induced fall and rise of arterial blood pressure in anesthetized rabbits

Summary The baroreceptor reflex is thought to counteract both a rise and a fall in arterial blood pressure, but the evidence for this is incomplete. In 19 anesthetized rabbits, the relationship between blood pressure/aortic nerve activity and efferent sympathetic activity in renal and splanchnic nerves was therefore investigated during induced rise and fall of arterial pressure. Baroreceptor activity increased and sympathetic activity decreased with raised pressure. In contrast, reduction of baroreceptor activity during acute hemorrhagic hypotension had little or no effect on activity in the two sympathetic nerves. Following complete barodenervation, however, sympathetic activity rose on average to 238% of control values. We therefore conclude that the baroreceptor reflex mainly counteracts a rise in arterial blood pressure. Reduction of baroreceptor activity in the first few minutes of acute hemorrhagic hypotension does not lead to increased sympathetic activity. The small activity remaining in baroreceptor nerves at low pressures had a very potent sympathetic inhibitory effect.