Transformation of Adrenergic Receptors in the Myocardium

Abstract

SEVERAL studies suggest that the commonly accepted distinction between α- and β-adrenergic receptors is not immutable. The α-adrenergic blocking agent dibenamine effectively inhibited responses of winter but not those of summer frog hearts to adrenaline1, and adrenergic receptors of isolated amphibian and mammalian hearts appear to be qualitatively altered by ambient temperature2. The inotropic and chronotropic responses to adrenaline and noradrenaline were blocked by α-adrenergic blocking agents at lower and by β-adrenergic blocking agents at higher temperatures. The reciprocal effectiveness of the blocking drugs suggested a single type of adrenergic receptor whose characteristics were altered by temperature. An alternative interpretation was separate “pools” of α- and β-adrenergic receptors with different sensitivities to, or activation by, temperature3. To distinguish between these we made a quantitative estimate of the receptor sites by using 3H-phenoxybenza-mine (3H-POB) to label α-adrenergic receptors in the amphibian myocardium during exposure at different temperatures.