5-hydroxytryptamine and beta-adrenoceptors in rat isolated atria

The positive chronotropic effect of a high concentration of 5-hydroxytryptamine (5-HT) in rat isolated atria results mainly from a tyramine-like mechanism and is linked to an increase in cAMP production by an indirect stimulation of a-adrenoceptors. Using this preparation, we have compared the action of tyramine and 5-HT. The tyramine (0.15 mu M)-induced increase in atrial rate was suppressed by atenolol (a beta(1)-blocking drug) and by nadolol (a beta(1) beta(2)-blocker), while the positive chronotropic effect of 5-HT was reduced by atenolol and suppressed by nadolol. The 5-HT-induced elevation in cAMP was unchanged in the presence of atenolol and abolished by nadolol. The involvement of beta(2)-adrenoceptors in the effects of 5-HT could result from competition between 5-HT and noradrenaline at the beta(1)-adrenoceptors that results in a fixation of noradrenaline on beta(2)-adrenoceptors.