Both α1- and β-Adrenoceptor Stimulation Determine the Time Course of the Inotropic Effect of Noradrenaline in Rabbit Heart

It has been a matter of controversy whether α₁-adrenoceptor stimulation contributes to the final inotropic and lusitropic responses in mammalian myocardium to noradrenaline during concomitant and unopposed β-adrenoceptor stimulation. In the present paper we report studies that compare time courses of the inotropic and lusitropic responses to separate and combined α- and β-adrenoceptor stimulation, respectively, in electrically driven rabbit papillary muscles by a submaximal concentration of noradrenaline. Separate α₁- or β-adrenoceptor stimulation (presence of appropriate receptor blocker) showed the characteristic slow and fast development, respectively, of the inotropic responses. Qualitatively, the respective characteristic changes were also observed: α₁-adrenoceptor stimulation caused a negative lusitropic effect giving a prolongation of the time to peak tension (TPT), while β-adrenoceptor stimulation caused a pronounced positive lusitropic effect giving a shortening of TPT. The time course of the inotropic response to combined adrenoceptor stimulation had characteristics that deviated from the respective time courses to separate α₁- or β-adrenoceptor stimulation thus indicating a contribution from both adrenoceptor populations to the final inotropic response. Combined α₁- and β-adrenoceptor stimulation gave a pronounced positive lusitropic response as might be expected due to the obviously dominating role of the β-adrenergic component. However, the maximal lusitropic effect and the shortening of TPT were both slightly less during combined adrenoceptor stimulation compared to separate β-stimulation thus indicating an influence of the α₁ -adrenoceptor mediated negative lusitropic effect. Quantitatively, the separate α₁ - and the separate β-adrenoceptor mediated inotropic effects were not additive. In accordance with other recent studies, this indicated an inhibitory interaction between the two adrenergic receptor populations in myocardium.