Competitive Blockade of α-Adrenergic Receptors in Rat Heart by Prazosi

Abstract: Pheiiykphriiie (PE) in presence of propranolol evokes an α-adrenergic inotroyic response in rat heart. The time course of this response is characterizcd by a transient ducrease in maximal developed tension (T_max) subcontrol levels (negative phase of the inotropic response) followed by an increase which reaches maximum after 4–5 min. (positive phase of the inotropic response). Prazosin (PRZ), a selective α₁-receptor blocker. inhibited preferentially the positive phase of the inotropic response and displaced the dose-responsecurve of PE to the right in nanomolar concentrations, indicating a competitive mechanism of inhibition. Phentolamine, a non-selrctive α-blocker. blocked both the negative and the positive phase olthe inotropicresponse toabout the same degree. PRZ appears to be a competitive α-adrenergic antagonist with high affinity in rat heart. Two populations of α-adrenergic receptors may he present: one stirnulatory (α₁) iind one inhibitory