What role do alpha- and beta-adrenoceptors play in the regulation of the heart?

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.