beta-Adrenoceptor blocking agents as partial agonists in isolated heart muscle: dissociation of stimulation and blockade.

The cardiac stimulant actions of nine beta-adrenoceptor blocking agents were examined in kitten papillary muscles and in isolated atria of kittens and guinea pigs to determine to what extent these drugs behaved as classical partial agonists. In many ways the agents do appear to comprise a spectrum of partial agonists with widely differing efficacies. However, in one respect the actions of some of the beta-blockers did not fit into the classical mold. Several beta-blockers were found to exert stimulant effects only in concentrations appreciably higher than those required for substantial beta-adrenoceptor blockade. These observations suggest that more than one type of beta-adrenoceptor may be involved in the production of sympathomimetic effects on cardiac muscle.