Adrenal Medullary Catecholamine Secretion Patterns in Rats Evoked by Reflex and Direct Neural Stimulation

Epinephrine (EPI) and norepinephrine (NE), secretion patterns evoked by reflex (to hypotension and hypoglycemia) and direct neural stimulation of the adrenal medulla were measured in pentobarbital anesthetized male Sprague-Dawley rats. Secretion rates were determined by collecting adrenal venous blood. Baseline catecholamine secretion was similar in innervated and denervated glands indicating that there was little tonic sympathetic neural drive to the medulla. Both hydralazine-induced hypotension and insulin-induced hypoglycemia significantly increased catecholamine secretion with the secretion of EPI predominating. Similarly, in response to stimulation of the splanchnic nerve, frequency-related increments in EPI and NE were elicited with EPI release being greater than NE at all frequencies. However, the magnitude of the increase in secretion during splanchnic stimulation at frequencies above 1 hz greatly exceeded the release achieved by reflex stimulation. The results indicate that despite the fact that the stimuli of hypotension and hypoglycemia are integrated by different centers in the brain, the pattern of adrenal release is similar.