Possible mechanisms underlying the pressure responses evoked in conscious cats by emotional stress.

The character of baroreceptor reflexes was studied in conscious cats both under resting conditions and during emotional stress. During naturally evoked emotional tension 32 cats showed a two-phase change in arterial pressure: an initial hypotension was followed by a rise in arterial pressure. The pressor component was accompanied by a suppression of the baroreceptor reflex evoked by phenylephrine (i.v.). Electrical stimulation of the hypothalamic "defence area" also resulted in a raised arterial pressure and a suppression of the baroreceptor reflex. 3-7 days after bilateral section of the carotid sinus and aortic nerves 5 cats showed a reduction in arterial pressure only when confronted by a dog. When a reflex response to phenylephrine began to return, the depressor responses seemed to diminish and pressor responses reappeared. "Mild" (50-100 microA) stimulation of the hypothalamic "defence area" also caused a reduction of arterial pressure. However, a rise in arterial pressure occurred when the electrical stimulation of the hypothalamic "defence area" was increased. In 2 cats the pressor response caused by confrontation with a dog was not accompanied by any noticeable inhibition of the baroreceptor reflex. After cutting the sino-carotid and aortic nerves in these animals, emotional stress produced accentuated pressor responses. The results suggest that the presence of baroreceptor reflexes is a prerequisite for the development of hypertensive responses during emotional stress.