Differentiation of cardiac chronotropic and inotropic effects of ?-adrenoceptor agonists

Abstract

The relative inotropic and chronotropic activity of β-adrenoceptor agonists was studied in the noradrenaline-depleted, anaesthetized cat. Terbutaline, a selective β₂-adrenoceptor agonist, gave at a certain dose a more pronounced chronotropic than inotropic response, while a new β₁-selective adrenoceptor agonist (−)-H 80/62 produced the same degree of chronotropic and inotropic stimulation. The results indicate that there is some difference between the β-adrenoceptors in the sinus node mediating chronotropic stimulation and β-adrenoceptors in the ventricular myocardium mediating stimulation of the contractile force. It has been shown that there are both β₁- and β₂-adrenoceptors in the heart (Carlsson et al., 1972). In the light of this finding it is hypothetized that there are differences in the relative distribution of β₁- and β₂-adrenoceptors in the sinus node and in the myocardium. Although β₁ is the predominant type of β-adrenoceptor in both regions, the β₁:β₂ concentration ratio seems to be higher in the myo-cardium, than in the sinus node.