Evidence for muscarinic receptors in the adrenal medulla of the dog.

1. The release of catecholamines from the adrenal medulla by the cholinergic drugs was monitored by the perfusion pressure rise in autoperfused and sympathetically denervated hindlimbs of the dog.2. Acetylcholine, dimethylphenylpiperazinium (DMPP) and methacholine, given in relatively small doses into the aorta proximal to the blood supply to the adrenal glands, caused a marked rise in perfusion pressure which was due to the release of catecholamines from the adrenal medulla.3. Atropine (1 mg/kg intravenously) blocked the pressor response to methacholine only. The ganglion-blocking agents (mecamylamine 4-5 mg/kg or hexamethonium 10 mg/kg) given subsequently blocked the pressor responses to the other two cholinergic drugs. The ganglion-blocking agents, when given before atropine, blocked the pressor action only of DMPP. These agents partly depressed, rather than potentiated, the pressor response to methacholine.4. The results suggest that muscarinic as well as nicotinic receptors are present in the adrenal medulla of the dog.5. Neither atropine nor ganglion-blocking drugs alone reduced the pressor response to acetylcholine. It is postulated that the blockade of one set of receptors makes more acetylcholine available for the other set of receptors and so inactivation of the former receptors are compensated by the increased release through the activation of the latter's.