5-Hydroxytryptamine and β-adrenoceptors in rat isolated atria

Abstract.

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 β-adrenoceptors. Using this preparation, we have compared the action of tyramine and 5-HT. The tyramine (0.15 μM)-induced increase in atrial rate was suppressed by atenolol (a β₁ -blocking drug) and by nadolol (a β₁  β 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 β₂ -adrenoceptors in the effects of 5-HT could result from competition between 5-HT and noradrenaline at the β₁ -adrenoceptors that results in a fixation of noradrenaline on β₂ -adrenoceptors.