Intrinsic sympathomimetic activity of beta-adrenoceptor blocking agents

The pharmacological methods used to assess the intrinsic sympathomimetic activity (ISA) of beta-blockers are discussed. The clinical relevance of ISA to respiratory function, peripheral resistance and cardiac function is reviewed. It appears doubtful whether ISA is always of predominant clinical significance and an alternative explanation is offered for many clinical effects observed with certain beta-blockers, e.g. pindolol, oxprenolol, tolamolol, metoprolol, etc. Some effects of these beta-blockers resemble those of labetalol, a new drug with both alpha and beta-blocking activity. Some clinical effects of certain beta-blockers are more likely to be due to alpha-blocking activity than to their ISA.