THE ROLE OF ALPHA-ADRENOCEPTOR AND BETA-ADRENOCEPTOR BLOCKADE IN ANTIHYPERTENSIVE TREATMENT

The alpha and beta-adrenoceptors are at present subdivided into alpha(1)/alpha(2) and beta(1)/beta(2) (probably also beta(3)) subtypes. This subdivision, based on functional pharmacological studies, has been largely confirmed by molecular biological techniques. At presynaptic sites alpha(2)- and beta(2)-adrenoceptors are known to predominate, whereas both alpha(1)/alpha(2)- and beta(1)/beta(2)-receptors may be found at postsynaptic sites. This subdivision and classification of adrenoceptors, receptor changes in disease, and the availability of agonists and antagonists for the various receptor subtypes are discussed as the basis for antihypertensive drug therapy. Alpha-Adrenoceptor antagonists are vasodilators, which owe their antihypertensive activity to arterial vasodilatation, while venous dilatation occurs simultaneously. Selective alpha(1)-antagonists (prazosin, doxazosin) are preferable to the older nonselective compounds like phentolamine. Beta-Blockers are useful antihypertensive agents used on a very large scale, but their mode of action remains unknown in detail. The combination of alpha- and beta-adrenoceptor blockade would be attractive, particularly for hemodynamic reasons. Alpha-Adrenoceptor blockade reduces peripheral vascular resistance but also counteracts the vasoconstrictor effect of beta-blockers in the extremities, underlying the well-known side-effect of cold hands and feet. Beta-Adrenoceptor blockade will not only contribute to the hypotensive effect but also suppress reflex tachycardia induced by alpha-adrenoceptor blockade.