Abstract
Dopamine has been found to be present at various sites in the body (1). It acts as a precursor in the biosynthesis of norepinephrine and has also been ascribed a role of being a neurotransmitter (2). The effect of dopamine on the alpha adrenergic receptors is well established (3), but there are controversial reports regarding its beta receptor activity. Katz et al. (4) found that cyclopropane-dopamine and halothane-dopamine cardiac arrhythmia is blocked by H 56/28, a beta adrenergic blocking agent. However, the specificity of beta adrenergic blockers (especially pronethalol) has been questioned since some of them have local anaesthetic and quinidine like effects, which may account for their antiarrhythmic action (5-9). Eble (3) reported, that dopamine produces a depressor response after pre-treatment with alpha blocking agent, which is not blocked by a beta adrenergic blocking agent. McNay and Goldberg (10) pointed out that some of the effects of dopamine might be exerted via “dopamine receptors” which are different from beta receptors. It was thus of interest to study the beta adrenergic activity of dopamine by using propranolol, a specific beta adrenergic blocking agent.
Sympathomimetic amines act either directly or indirectly through the release of catecholamines or by both ways. Earlier dopamine was regarded to act directly on the adrenergic receptors (11-13). Blinks (14) also regarded it as a directly acting amine since he failed to observe a decrease in potency of dopamine after pretreatment with reserpine. It is also not known whether the ability of dopamine to produce arrhythmia is a direct effect or an indirect one due to release of norepinephrine (15). Although dopamine is thought not to release norepinephrine from blood vessel walls, there is evidence that it releases norepinephrine from myocardium (16). However, Tsai et al. (17) showed that dopamine has both direct and indirect effects on isolated guinea pig atria and nictitating membrane of cat. Hence it was also of interest to study whether dopamine acts directly or through the release of catecholamines or by both mechanisms. Lastly the drug parameters like intrinsic activity and affinity of dopamine has been compared with that of norepinephrine.
