Inhibition by 5-hydroxytryptamine of the beta adrenoceptor-mediated positive inotropic responses to catecholamines in rabbit papillary muscles: direct interaction with beta adrenoceptors.

The effects of 5-hydroxytryptamine (5-HT) on the positive inotropic responses to catecholamines were investigated in isolated rabbit papillary muscles. 5-HT produced a concentration-dependent positive inotropic effect, an effect which was antagonized by prazosin, but not by propranolol. The positive inotropic effect of 5-HT diminished greatly in muscles from rabbits pretreated with 6-hydroxydopamine. Thus, it is likely that 5-HT causes a release of norepinephrine and increases force of contraction indirectly through alpha-1 adrenoceptors. In the presence of prazosin, 5-HT exerted a concentration-dependent inhibition of the positive inotropic response to isoproterenol. The positive inotropic responses to tyramine and a beta-1 adrenoceptor agonist T-1583 were also inhibited by the addition of 5-HT. The inhibitory effect of 5-HT on the beta adrenoceptor-mediated responses was unaffected by methysergide, ketanserin, ICS 205-930 or atropine. Pretreatment with pertussis toxin did not block the inhibitory effect of 5-HT on the inotropic response to isoproterenol, while abolishing the cholinergic interaction against the isoproterenol response. In contrast to its antagonizing effect on the inotropic response to isoproterenol, 5-HT produced an additive effect on the positive inotropic response to norepinephrine. However, when neuronal amine uptake was blocked by cocaine, the positive intropic response to norepinephrine was suppressed by the addition of 5-HT. 5-HT inhibited (-)-[125I]iodocyanopindolol binding to the membranes from rabbit ventricles with a monophasic displacement curve.(ABSTRACT TRUNCATED AT 250 WORDS)