Role of calcium in muscarinic autoinhibition of 3H-acetylcholine secretion in guinea-pig ileum myenteric plexus.

The stores of acetylcholine (ACh) of the cholinergic neurons of guinea-pig myenteric plexus were labelled by preincubation with 3H-(methyl)-choline, to examine the dependence of the secretory mechanisms in these nerves, and of their muscarinic 'autoinhibition', on the extracellular calcium concentration. The curves describing the secretion of labelled transmitter evoked by electrical field stimulation at 1 Hz, as a function of external calcium, were essentially hyperbolic, except at very low calcium levels. Double reciprocal plots of the secretory responses vs. the external calcium concentration, within the 2-8 mM range, yielded straight lines, permitting estimation of apparent VmaxCa (height of asymptote parallel to abscissa = the maximal secretory response, at an infinitely high calcium level) and KmCa (the calcium concentration yielding half-maximal secretion). Neither addition of eserine, to indirectly activate muscarinic autoinhibition, nor of atropine to block it, markedly affected VmaxCa. In the absence of these drugs KmCa was about 3.5 mM. Eserine raised it about 7-fold, while further addition of atropine reversed this effect. The competitive pattern of interaction of drugs altering muscarinic autoinhibition, with the calcium dependence of the secretory mechanisms, suggest that this control is exerted by restriction of the ability of the nerve terminals to utilize external calcium.