Frequency dependence of the α-adrenoceptor-mediated positive inotropic effect in guinea pig heart

Abstract

Evidence for the role of α-adrenoceptors in the production of a positive inotropic effect has been obtained chiefly in heart preparations stimulated at a low rate (1 or 1.4 Hz). However, our previous results suggested that the α-receptor-mediated positive inotropic effect could be frequency dependent. In the present study cumulative concentration—inotropic effect curves were obtained in guinea pig ventricle strips treated with adrenaline and stimulated at 1 and 2.5 Hz. Phentolamine (3 × 10 −6 M) and practolol (10 −6 M) were employed as antagonists. Phentolamine antagonized the effect of low concentrations of adrenaline (10 −9–10 −8 M) in preparations driven at 1 Hz, but did not modify the curve of the agonist at the higher stimulation rate. In preparations driven at 1 Hz, practolol antagonized the effect of adrenaline at concentrations above 3 × 10 −8 M without affecting the curve at lower concentrations; the β-blocker competitively antagonized the effect of the agonist at 2.5 Hz. It was concluded that a shortening of the interval between beats abolishes the α-mediated positive inotropic effect.