Regulation of myocardial and vascular alpha-adrenergic receptor affinity. Effects of guanine nucleotides, cations, estrogen, and catecholamine depletion

The threshold sensitivity of cardiovascular tissues to alpha-adrenergic stimulation is determined largely by the affinity of alpha-adrenergic receptors for agonists. To determine whether changes in alpha-adrenergic receptor affinity could contribute to the regulation of cardiac and vascular responsiveness, we used the alpha-adrenergic-selective radioligands, 3H]prazosin, 3H]-WB-4101, and 3H]rauwalscine, to study and contrast the determinants of alpha-adrenergic receptor affinity in myocardium and vascular smooth muscle from the rat. In both tissues, l-epinephrine binding to the alpha 1-adrenergic receptor describes a shallow curve suggesting more than one affinity state. Computer analysis of binding to myocardial alpha 1-receptors indicates that 15% are of high affinity (Kd = 11 nM) and 85% are of low affinity (Kd = 400 nM). Expression of high affinity sites is magnesium dependent (maximum effect, 5-10 mM), and suppressed by the guanosine 5'-triphosphate analogue Gpp(NH)p (maximum effect, 1 mM) and sodium (maximum effect, 100-200 mM). In vascular smooth muscle, agonist-binding curves are also shallow and exhibit a similar response to that of Gpp(NH)p. Basal receptor affinity in myocardium is significantly higher (5.4-fold) than in vascular smooth muscle. Unlike vascular smooth muscle, in which alpha 1-adrenergic receptor affinity is increased by estrogen or reserpine treatment of the animal, the receptor in myocardium is unaffected by these treatments. In vascular smooth muscle, following reserpine-induced increase in alpha-adrenergic receptor affinity, the Gpp(NH)p effect is still present. Thus, alpha 1-adrenergic receptors in both myocardium and vascular smooth muscle exist in two affinity states and are subject to regulation by several factors, including guanine nucleotides, mono- and divalent cations, tissue of origin, sex hormones, and the level of sympathetic stimulation. Potentially, alterations in alpha 1-adrenergic receptor affinity, independent of a change in receptor number, may play an important role in the regulation of cardiovascular tissue responsiveness to catecholamines.