Interactions among the effects of normorphine, calcium and magnesium on transmitter release in the mouse vas deferens

1 Excitatory junction potentials (e. j. ps) were recorded with intracellular electrodes from smooth muscle cells of the mouse vas deferens. 2 The dependence of the e.j.p. amplitude on the extracellular calcium ion concentration was determined in the absence or presence of normorphine (50 nM-1 microM) or magnesium (1.2-4.8 mM). 3 The interaction between normorphine and calcium was non-competitive beyond a dose-ratio of 1.5, whereas the interaction between magnesium and calcium was competitive up to the highest dose-ratio investigated (1.9). 4 It is suggested that inhibition by normorphine occurs at least partly by a mechanism different from that of magnesium.