Alpha-adrenergic receptors in rat myocardium. Identification by binding of [3H]dihydroergocryptine.

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Abstract
[3H]Dihydroergocryptine ([3H]DHE) binds to sites in membranes derived from rat myocardium that have the characteristics expected of alpha-adrenergic receptors. The binding is saturable with 41 fmol [3H]DHE bound per mg of protein and of high affinity with KD = 2.9 nM. The binding is rapid and readily reversible. Adrenergic agonists compete with [3H]DHE for binding in the order: epinephrine greater than norepinephrine greater than isoproterenol; and adrenergic antagonists compete for binding in the order: phentolamine greater than propranolol. For comparison, (-)[3H]dihydroalprenolol [(-)[3h]dha] was used to bind to sites in the same membrane preparations having characteristics of beta-receptors. The number and affinity of beta-receptors were quite similar to those of the alpha-receptors with 46 fmol (-)[EH]DHA per mg protein bound at saturation and KD = 2.5 nM. These techniques allowed identification of both beta- and alpha-adrenergic receptors in membranes derived from isolated atria, right ventricular free walls, and left ventricles including interventricular septa. This is the first report documenting direct identification of myocardial alpha-receptors by radioligand-binding techniques and complements the literature previously reporting myocardial inotopic and electrophysiological responses to alpha-adrenergic stimulation.