The effect of calcium ions on the secretion of quanta evoked by an impulse at nerve terminal release sites.

Abstract

The effects of Ca2+ on the number of quanta secreted from all the release sites at an amphibian [Bufo marinus] motor nerve terminal recorded with an intracellular microelectrode (.hivin.m) compared with the number secreted simultaneously from a small number of release sites recorded with an extracellular microelectrode (.hivin.me) was studied. If the endplate potential was made subthreshold by lowering the external Ca concentration ([Ca]o .ltoreq. 0.4 mM), it was possible to find small groups of release sites for which .hivin.me was comparable to .hivin.m, indicating considerable nonuniformity in theprobability of release of a quantum at different groups of release sites (pe) in a given [Ca]o. Increasing [Ca]o in the range from 0.25-0.4 mM increased the probability of release of a quantum at groups of release sites (pe), independent of the initial value of pe and the dependence of pe on [Ca]o followed a 4th power relationship. A conditioning impulse enhanced the probability of release of a guantum by a subsequent test impulse at release sites, if pe was < 1.0 during the conditioning impulse. The present observations regarding the dependence of pe on [Ca]o and on conditioning impulses can be quantitatively predicted from previous observations regarding the dependence of the binomial parameters .hivin.m, p and n on [Ca]o and on conditioning impulses determined with intracellular electrodes, if the probability of secretion of a quantum at a release site (pj) is different for different release sites and pj is distributed as a .beta. random variable.