Plasma catecholamine and blood pressure responses to sympathetic stimulation in pithed rats.

Sympathetic stimulation in pithed rats elicits increases in blood pressure (BP) and plasma levels of catecholamines. The BP is proportional to the logarithm of the plasma norepinephrine (NE) concentration. Adrenal medullectomy diminishes slightly only the initial phase of the BP responses, whereas bretylium blocks completely the BP response and diminishes by about 70% the increase in plasma NE. Adrenal medullectomy completely prevents the increase in plasma epinephrine (Epi) and diminishes by 30% plasma NE. Plasma levels of NE appear to reflect sympathetic neuronal activity, but the NE at sympathetic nerve endings is responsible for the increase in BP. Epi released from the adrenal medulla may enhance and accelerate the initial BP response, but plays only a minor role after the first 15 s.