MUSCARINIC TRANSMISSION OF PREGANGLIONIC IMPULSES TO THE ADRENAL MEDULLA OF THE CAT

¹ Department of Pharmacology, Harvard Medical School, Boston, Massachusetts

Successive intravenous injections of nicotine cause a biphasic block of the adrenal medulla of the cat. During an initial phase, the adrenal medulla fails to respond to any stimulant; however, during the late, non-depolarizing phase, the gland responds to non-nicotinic agents (histamine, angiotensin, McN-A-343) at a time when there is a pronounced and insurmountable antagonism to nicotinic agents. During this late phase, the effects of non-nicotinic agents are potentiated. Under normal conditions, transmission of impulses from the splanchnic nerve is mediated by nicotinic (hexamethonium-sensitive) mechanisms. However, during the late phase of the block by nicotine, transmission is muscarinic (atropine-sensitive). It is suggested that transmission via muscarinic mechanisms is possible in the adrenal medulla of the cat when a) the nicotinic receptors are blocked and b) the sensitivity of the muscarinic mechanisms is increased.