Cation dependence of the noradrenaline-releasing action of tyramine

Abstract

Irides were incubated for one hour in Krebs-Henseleit solution containing 10 μC/ml of dl- 3H-noradrenaline. The radioactivity spontaneously released after this period into the incubation medium consisted chiefly of acidic metabolites of noradrenaline and small amounts of normetanephrine. The increase in radioactivity during incubation with tyramine was exclusively due to noradrenaline. Omission of Ca ++ from the incubation medium caused a small but statistically significant increase both in the spontaneous and in the tyramine-induced release of radioactivity. Omission of Mg ++ had no effect. Absence of both Ca ++ and Mg ++ markedly increased the spontaneous release but reduced tyramine-induced liberation of noradrenaline. When Ca ++, Mg ++ and K + were omitted, the spontaneous release was further augmented and the effect of tyramine abolished; ouabain (10 −5M) had a very similar effect. It is concluded that in contrast to the adrenal medulla (Rubin and Jaanus, 1967) the liberation of noradrenaline by tyramine from the adrenergic nerve endings of the cat iris is not dependent on the presence of Ca ++. The releasing action of tyramine seems to be linked to a cation-dependent mechanism of tyramine transport through the membrane of the adrenergic neuron.