An estimate of the equilibrium dissociation constant for calcium as an antagonist of evoked acetylcholine release: implications for excitation-secretion coupling.

The equilibrium dissociation constant (K(d)) for Ca(2+) as an antagonist of evoked acetylcholine (ACh) release was determined in the hope of distinguishing whether divalent cations control excitation-secretion coupling selectively (by binding with high affinity to an external membrane site) or non-selectively (by screening fixed negative charges on the external surface of the nerve terminal). ACh release was detected electrophysiologically by means of conventional intracellular recording techniques at frog motor endplates. Ba(2+) was used as the agonist to support the asynchronous release of ACh by repetitive motor nerve impulses. Despite its dispersed nature, release mediated by Ba(2+) occurs through the same conductance pathway as synchronous release mediated by Ca(2+). Ca(2+) was found to be a potent antagonist of Ba(2+)-dependent release with a K(d)=0.12+/-0.02 mM (mean+/-s.e. mean, n=5). This value is 30-50 times lower than the K(d) for Mg(2+) as an antagonist of the same release process. It is suggested that antagonism of release by Ca(2+) is likely to be exerted at the same external site that binds other divalent cation antagonists, a site that appears essential for the agonist behaviour of Ca(2+). The high affinity (low K(d)) of Ca(2+) as an antagonist of ACh release suggests that a selective, binding model appears to be the most appropriate single description of the action of divalent cations at the external surface of the motor nerve ending.