Augmentation of the indirect sympathomimetic action of tyramine by cardioactive steroids is a consequence of elevated intracellular sodium

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Abstract
The augmentation by the cardioactive steroid acetylstrophanthidin of neurotransmitter release evoked by tyramine, and the dependence of the augmentation upon Na+, has been investigated in dog saphenous vein rings in which monoamine oxidase activity and uptake2 had been inhibited with pargyline and corticosterone respectively. High extracellular Na+ (Nao; 263 mmol/1) reduced basal efflux of 3H-compounds from the rings and also reduced tyramine-evoked efflux. Low Nao (25 mmol/1) increased basal efflux of 3H but reduced tyramine-evoked efflux. The increment in basal 3H-efflux caused by low Nao was cocaine-sensitive. A presumed increase in intracellular Na+ (Nai), produced by preincubating rings with acetylstrophanthidin in normal (143 mmol/1) or high Nao, augmented 3H-efflux evoked by subsequent incubation with tyramine in normal Nao Pre-incubating rings with acetylstrophanthidin in low Nao, conditions which would not be expected to increase Nai, did not cause augmentation of the subsequent tyramine-evoked 3H-efflux. An increase in Nai, produced either as above or by pre-incubating rings in high Nao alone, reduced subsequent neuronal 14C-tyramine uptake. Low Nao present only during incubation reduced neuronal 14C-tyramine uptake, but high Nao present only during incubation did not increase neuronal 14C-tyramine uptake from that measured in normal Nao The data are consistent with the following hypotheses: that tyramine uptake is dependent upon the prevailing inwardly directed Na+-gradient, that consequent noradrenaline efflux is Na+-gradient dependent and that the enhancement by acetylstrophanthidin of tyramine-evoked 3H-efflux is a consequence of the raised Nai caused by Na+,K+-ATPase inhibition.

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