[Importance of beta 2-adrenergic receptors in heart failure]

Substantial evidence has accumulated that in the human heart both beta 1- and beta 2-adrenoceptors coexist. As a rule, the amount of beta 2-adrenoceptors is higher in the atria (about 30% of the total beta-adrenoceptor population) than in the ventricular myocardium (about 20%). Both beta 1- and beta 2-adrenoceptors couple to adenylate cyclase and mediate positive inotropic effects of isoprenaline and adrenaline on isolated, electrically driven cardiac preparations. In the atria, stimulation of both beta 1- and beta 2-adrenoceptors causes maximal increases in contractile force; in the ventricular myocardium, however, only beta 1-adrenoceptor stimulation maximally increases contractile force, whereas beta 2-adrenoceptor stimulation evokes only submaximal increases. On the other hand, noradrenaline induces its positive inotropic effect on atrial and ventricular preparations solely via beta 1-adrenoceptor stimulation. Because nordadrenaline is the main transmitter of the human sympathetic nervous system, this indicates that under normal physiological conditions, the heart rate and contractility are under the control of cardiac beta 1-adrenoceptors, whereas cardiac beta 2-adrenoceptors play only a minor role, if at all. However, in situations of stress, when large amounts of adrenaline (acting at both beta 1- and beta 2-adrenoceptors with the same affinity) are released from the adrenal medulla, activation of cardiac beta 2-adrenoceptors may contribute to an additional increase in heart rate and/or contractility. In chronic heart failure, cardiac beta-adrenoceptor function decreases (presumably due to endogenous "downregulation" by the elevated catecholamines) and this decrease is related to the severity of the disease (judged clinically by NYHA functional class).(ABSTRACT TRUNCATED AT 250 WORDS)