Ahlquist described the existence of beta-adrenoceptors as structures mediating the positive effects of sympathetic nerve stimulation in the heart. Recently, the role of alpha-adrenoceptors has also been investigated by both biological and radioligand binding studies. The presence of alpha-adrenoceptors has now been demonstrated in the hearts of various mammalian species including man. Stimulation of beta-adrenoceptors causes positive ino-, chrono-, dromo- and bathmotropic effects while that of alpha-adrenoceptors is restricted to positive inotropic effects. Analysis of the effects mediated by alpha-stimulation as carried out in our department showed a clear-cut difference between the mechanisms underlying stimulation of beta- and of alpha-adrenoceptors: that evoked by beta-adrenoceptors is connected to an increase in cAMP, whereas that by alpha-adrenoceptors is not. Moreover, the effect of alpha-stimulation is evidently more dependent on heart rate and on the presence of Ca2+ than that of beta-stimulation. The nature of cardiac alpha-adrenoceptors is apparently distinct from that of other organs as they are stimulated by dopamine and adrenaline but not by the transmitter noradrenaline. Since the alpha-receptor-mediated increase in force of contraction is obviously less energy consuming than that via beta-receptors the heart gains a greater adaptability in adrenergic regulation especially in emergency situations such as myocardial infarction.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.