Effects of pindolol on vascular smooth muscle

Pindolol is a non-selective beta-adrenoceptor blocking agent with relatively marked intrinsic sympathomimetic activity. Stimulant effects in isolated, perfused mesenteric vessels (beta 2) equal those of isoprenaline. Effects on guinea-pig atria (beta 1) are negligible and those on guinea-pig trachea (beta 1 and beta 2) and cat atria (beta 1 and beta 2) are equivalent to 40-50% of the maximum effects of isoprenaline. It is concluded that the intrinsic sympathomimetic activity of pindolol is exerted principally on the beta 2-adrenoceptor. The beta-adrenoceptor blocking effect of pindolol is largely confined to the (-)enantiomer, whereas its stimulant effects are shared equally by the (+) and (-)enantiomers. Removal of the side chain hydroxyl group of pindolol does not diminish stimulant activity, but abolishes blocking activity. Pindolol-induced stimulation and blockade of beta-adrenoceptors may be brought about by different configurations of the molecule. Vasodilator effects observed in vitro and in vivo suggest that beta-adrenoceptor stimulation may make an important contribution to the antihypertensive effect of pindolol in man.