Abstract

The alpha adrenergic receptors of rabbit platelets can be identified by using [³H]-dihydroergocryptine. The binding of [³H]dihydroergocryptine to platelet lysates reaches equilibrium in 30 min and is slowly reversible. The maximal number of binding sites is 280 ± 25 fmoles/mg, and the dissociation constant was determined to be 2.26 ± 0.48 nM. The specificity of the binding sites for receptor agonists and antagonists is consistent with that of an alpha adrenergic response in inhibiting prostaglandin E₁-stimulated adenylate cyclase in the same lysate preparation and is thus in agreement with the classical definition of alpha adrenergic receptors in mediating physiological responses. Divalent cations such as magnesium and manganese and monovalent cations such as sodium markedly influence the binding affinity of agonists but have little or no effect on the binding of antagonists. Calcium produces no change in binding. Neither magnesium nor sodium alters the number of receptor sites. Magnesium (1.25 mM) increases the binding affinity of (-)-epinephrine by a factor of 4. In contrast, sodium chloride (100 mM) strikingly decreases the binding affinity of agonists (more than 10-fold). The effect of sodium chloride on agonists is apparent with concentrations as low as 10 mM, and shows a dose-dependent response. LiCl and KCl are only 44% and 19% as effective, respectively, as sodium chloride. The magnitude of the shift in binding affinity of alpha adrenergic agents produced by sodium chloride is directly related to the intrinsic activity of those agents for inhibition of prostaglandin E₁-stimulated adenylate cyclase. The results suggest that physiological concentrations of magnesium chloride and sodium chloride may selectively regulate the binding affinity of agonists.