Alpha-Adrenergic Receptors in Rat Myocardium Identification by Binding of [H]Dihydroergocryptine

H]Dihydroergocryptine ([H]DHE) binds to sites in membranes derived from rat myocardium that have the characteristics expected of a-adrenergic receptors. The binding is saturable with 41 fmol [H]DHE bound per mg of protein and of high affinity with KD = 2.9 niu. The binding is rapid and readily reversible. Adrenergic agonists compete with [H]DHE for binding in the order: epinephrine > norepinephrine » isoproterenol; and adrenergic antagonists compete for binding in the order: phentolamine » propranolol. For comparison, (—)[H]dihydroalprenolol [(—)[H]DHA] was used to bind to sites in the same membrane preparations having characteristics of /{-receptors. The number and affinity of /?-receptors were quite similar to those of the a-receptors with 46 fmol (—)[H]DHA per mg protein bound at saturation and KD = 2.5 nM. These techniques allowed identification of both ftand o-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 a-receptors by radioligand-binding techniques and complements the literature previously reporting myocardial inotopic and electrophysiological responses to a-adrenergic stimulation.