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

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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 (Tmax) 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