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.
D000818:Animals; D002395:Catecholamines / Q000502:physiology; D002415:Cats; D046508:Culture Techniques; D000242:Cyclic AMP / Q000502:physiology; D006168:Guinea Pigs; D006801:Humans; D007545:Isoproterenol / Q000494:pharmacology; D008729:Methoxamine / Q000494:pharmacology; D009200:Myocardial Contraction / Q000187:drug effects; D010210:Papillary Muscles / Q000294:innervation*; D010646:Phentolamine / Q000494:pharmacology; D010869:Pindolol / Q000494:pharmacology; D011817:Rabbits; D051381:Rats; D011941:Receptors, Adrenergic / Q000502:physiology*; D011942:Receptors, Adrenergic, alpha / Q000502:physiology*; D011943:Receptors, Adrenergic, beta / Q000502:physiology*; D013268:Stimulation, Chemical
D002395:Catecholamines; D011941:Receptors, Adrenergic; D011942:Receptors, Adrenergic, alpha; D011943:Receptors, Adrenergic, beta; D010869:Pindolol; D000242:Cyclic AMP; D008729:Methoxamine; D007545:Isoproterenol; D010646:Phentolamine