Effect of dopamine on isolated left atria of the rat

Positive inotropic effects of dopamine were investigated in isolated left atria of the rat and compared with those of tyramine and norepinephrine. The following results were obtained. Norepinephrine was the most potent of these three sympathomimetic amines, and tyramine was more potent than dopamine, however the order of tyramine and dopamine was reversed in the reserpine or 6-hydroxydopamine-pretreated atria. Positive inotropic effects of tyramine and dopamine were reduced by reserpine while those of dopamine were regained by incubation of atria with cocaine. Effects of tyramine were reduced and those of dopamine were not altered by pretreatment with 6-hydroxydopamine. Effects of norepinephrine, tyramine, and dopamine were reduced to the same extent by lithium. Thus dopamine appears to be a direct and simultaneously indirect sympathomimetic amine, while the inhibitory effects of lithium depend mainly on postsynaptic inhibition.