NICOTONIC AND MUSCARINIC COMPONENTS IN ACETYLCHOLINE STIMULATION OF PORCINE ADRENAL-MEDULLARY CELLS

Adrenal medullary chromaffin cells secrete catecholamines (CA) in resp onse to cholinergic receptor activation by acetylcholine (ACh) release d from splacnic nerve terminals. in cultured bovine chromaffin cells n icotinic receptors play a preponderant (> 90%) role in the control of CA release. By contrast, we found and report here that up to 40% of th e ACh-evoked CA secretion from cultured porcine chromaffin cells can b e associated with muscarinic receptor activation. The following result s support our belief that in porcine adrenal medullary cells ACh (100 mu M) evoked CA secretion is mediated by both nicotinic and muscarinic cholinergic receptors. 1) Hexamethonium (100 mu M), a nicotinic recep tor antagonist, inhibited ACh-induced CA secretion to ca. 40% of the c ontrol release and atropine (1 mu M), a muscarinic receptor antagonist , inhibited to ca. 60% of the control value. 2) We also found that ACh (100 mu M) evoked intracellular Ca2+ concentration ([Ca2+](i)) rise wa s inhibited by these receptor antagonists to a different extent, and r eversibly reduced by lowering the concentration of Ca2+ in the externa l medium ([Ca2+](0)). This last maneuver ([Ca2+](0) < 0.1 mu M) per se caused a marked reduction in the peak phase of the [Ca2+](i) rise evo ked by ACh (40% of the control response). Switching the external mediu m back to physiologic [Ca2+](0) in the continued presence of ACh cause d a partial recovery of the elevated [Ca2+](i). This [Ca2+](0)-depende nt [Ca2+](i) rise was blocked by hexamethonium (100 mu M) but not by a tropine (1 mu M). Conversely, the ACh-evoked [Ca2+](i) rise in low ext ernal [Ca2+](0) was blocked by atropine but not by hexamethonium. From these data we conclude that in porcine adrenal medullary cells an imp ortant fraction (ca. 0.4) of both ACh-induced CA secretion and peak [C a2+](i) rise is due to muscarinic receptor activation.