PHARMACOLOGICAL ANALYSIS OF THE CARDIAC ACTIONS OF XAMOTEROL, A BETA-ADRENOCEPTOR ANTAGONIST WITH PARTIAL AGONISTIC ACTIVITY, IN GUINEA-PIG HEART - EVIDENCE FOR INVOLVEMENT OF ADENYLATE-CYCLASE SYSTEM IN ITS CARDIAC STIMULANT ACTIONS

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  • 1 September 1987
    • journal article
    • research article
    • Vol. 242  (3) , 1077-1085
Abstract
The pharmacological effects of xamoterol, a beta adrenoceptor antagonist with partial agonist activity, were examined in guinea pig cardiac preparations and compared with those of isoproterenol to assess possible mechanisms of its cardiac stimulant actions. Xamoterol produced a positive inotropic effect in the papillary muscles and a positive chronotropic effect in the spontaneously beating right atria in a concentration-dependent manner. The maximum inotropic and chronotropic effects of xamoterol were about 33 and 35% of those of isoproterenol, respectively. Although xamoterol failed to produce a consistent increase in contractile force in the left atria, the positive inotropic effect of the agent was observed clearly in preparations obtained from reserpine-pretreated animals. The positive inotropic and chronotropic effects of xamoterol were antagonized by atenolol, but not by ICI 118,551. On the other hand, xamoterol antagonized competitively the positive inotropic and chronotropic responses to isoproterenol. In papillary muscles the increases in contractile force induced by xamotherol and isoproterenol were depressed markedly in the presence of carbachol or adenosine. In all of left atria, right atria and papillary muscles obtained from reserpine-pretreated animals, xamoterol caused a significant elevation in cyclic AMP levels, while inhibiting the isoproterenol-induced increase in cyclic AMP levels. Computer-assisted analysis of concentration-response curves for the inhibition by xamoterol of the binding of [125I]iodocyanopindolol in the membranes from guinea pig ventricles showed the existence of the 5''-guanylimidodiphosphate sensitive, highly affinity site of beta adrenoceptors for xamoterol, suggesting that xamoterol may induce the formation of a ternary complex with a beta adrenoceptor and a stimulatory guanine nucleotide regulatory protein. The percentage of total receptors in the high affinity site for xamotherol was less strong evidence supporting the promise that the cardiac stimulant actions of xamoterol may involve activation of adenylate cyclase similar to full beta adrenoceptor agonists.