Non-competitive interaction between normorphine and calcium on the release of noradrenaline

The interaction between calcium and magnesium or normorphine was studied on the electrically evoked outflow of previously incorporated [3H]noradrenaline in the isolated mouse vas deferens. The stimulation-evoked outflow of [3H]noradrenaline increased with increasing concentrations of Ca2+ ions in the medium. Mg2+ counteracted the effect of Ca2+ in a manner compatible with competitive antagonism. In contrast, normorphine interfered with the effect of Ca2+ non-competitively, thus suggesting that its mode of action differed from that of Mg2+.