Muscarinic component of splanchnic-adrenal transmission in the dog.

1. The effect of atropine on catecholamine release evoked by stimulation of the splanchnic nerve was studied in the adrenal medulla of the dog. The magnitude of the catecholamine release was estimated biologically on the basis of the pressor response occurring in the perfused and acutely sympathectomized hindquarters of the same dog by comparing it with responses elicited by intravenous injection of adrenaline.2. Atropine reduced the responses to nerve stimulation, and appeared to have a more prominent effect on the responses elicited by stimulation at moderate frequency (10 pulses/sec).3. Hexamethonium or nicotine caused a more powerful, but not complete, blockade of transmission; the subsequent injection of atropine caused a further inhibition of the residual responses, leading to a complete, or near complete, abolition of the release by nerve stimulation.4. The data were taken as evidence that transmission of impulses through muscarinic receptors occurs in normal conditions in the adrenal medulla of the dog, though this type of transmission is less prominent than that through nicotinic receptors.