Indenolol, a beta-blocker with partial agonism at vascular beta-adrenoceptors

Intrinsic sympathomimetic activity may attenuate some effects caused by treatment with beta-adrenoceptor blockers. Indenolol is a nonselective beta-adrenoceptor antagonist whose sympathomimetic properties have been shown in vitro but not in human beings. We infused indenolol cumulatively (5, 15, and 50 micrograms/100 ml tissue per minute for 15 minutes each, preceded by an infusion of saline solution) into the brachial arteries of nine hypertensive patients. Forearm blood flow (venous plethysmography), mean arterial pressure, and heart rate were monitored. During infusion at 5 micrograms/100 ml tissue per minute, forearm blood flow did not change, but it did increase dose-dependently at the greater infusion rates. This action was determined to be mediated by beta-adrenoceptor stimulation because propranolol (10 micrograms/100 ml tissue per minute for 15 minutes), given before treatment and then concomitantly with indenolol, abolished it (n = 5). Indenolol vasodilated forearm arterioles and this effect was antagonized by beta-blockade, thus demonstrating vascular intrinsic sympathomimetic activity. This property may contribute to its therapeutic action in human beings.