title: Frequency dependence of the alpha-adrenoceptor-mediated positive inotropic effect in guinea pig heart.
abstract: Evidence for the role of alpha-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 alpha-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 at 2.5 Hz. Phentolamine (3 X 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 concentration above 3 X 10(-8) M without affecting the curve at lower concentrations, the beta-blocker competitively anatagonized the effect of the agonist at 2.5 Hz. It was concluded that a shortening of the interval between beats abolishes the alpha-mediated positive inotropic effect.
mesh_terms: D000818:Animals; D004558:Electric Stimulation; D004837:Epinephrine / Q000008:administration & dosage / Q000494:pharmacology*; D006168:Guinea Pigs; D066298:In Vitro Techniques; D009200:Myocardial Contraction / Q000187:drug effects*; D010646:Phentolamine / Q000494:pharmacology; D011217:Practolol / Q000494:pharmacology; D011941:Receptors, Adrenergic; D013268:Stimulation, Chemical; D013997:Time Factors
chemical_list: D011941:Receptors, Adrenergic; D011217:Practolol; D004837:Epinephrine; D010646:Phentolamine