AN OVERVIEW OF THE PHARMACODYNAMIC PROPERTIES AND THERAPEUTIC POTENTIAL OF COMBINED ALPHA-ADRENOCEPTOR AND BETA-ADRENOCEPTOR ANTAGONISTS

Haemodynamically, the combination of alpha1- and beta-adrenoceptor antagonists is a logical one. alpha1-Adrenoceptor blockade causes vasodilatation and hence counteracts elevated peripheral vascular resistance, the most consistent haemodynamic derangement in established essential hypertension. Beta-Blockers, which lower elevated blood pressure by a different (not yet clearly understood) mechanism, suppress the reflex tachycardia triggered by vasodilatation. Combined alpha/beta-adrenoceptor blockade can be obtained by the simultaneous administration of both types of adrenoceptor antagonists, but also by giving drugs that possess alpha- and beta-adrenoceptor antagonistic activity in the same molecule. Carvedilol and labetalol are the best known examples of such combined alpha/beta-adrenoceptor antagonists, although their pharmacodynamic profile is a result of different receptor selectivity of their component stereoisomers, rather than combined alpha/beta-blocking activity in a single chemical entity. Both compounds have been investigated clinically in the treatment of essential hypertension in moderate-to-large scale trials. A few newer combined alpha/beta-adrenoceptor antagonists, such as amosulalol, arotinolol and medroxalol have been developed, but clinical data on these compounds are relatively scarce.