Sodium inhibits both adenylate cyclase and high-affinity 3H-labeled p-aminoclonidine binding to alpha 2-adrenergic receptors in purified human platelet membranes
Mooney, J.J.; Horne, W.C.; Handin, R.I.; Schildkraut, J.J.; Alexander, R.W.
Molecular Pharmacology 21(3): 600-608
1982
ISSN/ISBN: 0026-895X PMID: 6287197 Document Number: 183410
The effects of Na on both the binding characteristics of the .alpha.-adrenergic receptor and the activity of .alpha.-receptor-coupled adenylate cyclase of human platelets were studied. By using a newly developed, partially purified platelet plasma membrane preparation, it was demonstrated that the partial agonist ligand 3H-labeled p-aminoclonidine ([3H]PAC) binds to the .alpha.-receptor in a manner which suggests the existence of 2 types of sites: a high-affinity, low-capacity site (Kd = 0.87 nM; Bmax = 0.294 pmol/mg protein) and a low-affinity site, the properties of which were not precisely defined. Physiological concentrations of Na ion decreased the Bmax for the high-affinity component of [3H]PAC binding while increasing the Kd of the remaining high-affinity sites. Other monovalent cations also decreased [3H]PAC binding, with the order of potency of Na+ > Li+ > K+. Guanine nucleotides markedly diminished the number of high-affinity [3H]PAC binding sites while increasing the Kd of the remaining sites. In the presence of Na ion and guanine nucleotides, a distinct high-affinity site was not identified. Na ion significantly decreased the activity of both basal and prostaglandin D2-stimulated adenylate cyclase activity in the absence of exogenous guanine nucleotide. In the presence of guanine nucleotide (100 .mu.M GTP) there was no inhibitory effect of NaCl on unstimulated enzyme activity, but NaCl inhibited prostaglandin-stimulated adenylate cyclase. Epinephrine (10 .mu.M) inhibited adenylate cyclase activity only when the enzyme was stimulated by prostaglandin in the presence of 100 .mu.M GTP. The inhibitory effects of epinephrine and NaCl on the prostaglandin-stimulated enzyme were additive. Na ion may be acting on the adenylate cyclase complex to modulate the interaction of a guanine nucleotide regulatory protein with both the .alpha.2-adrenergic receptor and the enzyme catalytic subunit.