Sodium regulation of agonist binding at opioid receptors. II. Effects of sodium replacement on opioid binding in guinea pig cortical membranes.

We have examined the effects of sodium on the binding of opioid agonists to mu-, delta-, and kappa-receptors in guinea pig cortical membranes. Concentration curves for sodium indicated that maximal inhibition of mu binding by this cation was about 60% and maximal inhibition for delta binding was about 70%, whereas that for kappa binding was only about 20%. The concentration of sodium required for half-maximal inhibition of binding to all three sites was about 10-30 mM, corresponding to the intracellular sodium concentration. The nature of the sodium effect was further characterized by saturation analysis of binding to each of the three receptor types by comparing results obtained in the presence of 120 mM sodium with those obtained with equimolar replacement of sodium by another cation. Two radiolabeled agonists with different structural characteristics were tested for each binding site. In the presence of sodium, the affinity of the labeled agonists for mu sites was approximately 2-3-fold less than in its absence, but the density of binding sites was not changed. At kappa sites, sodium reduced agonist affinity slightly but, again, did not alter the number of binding sites. In contrast, sodium reduced the apparent density of delta-binding sites while leaving the agonist affinity unchanged. Competition against antagonist binding to delta sites indicated that, in the presence of sodium, a higher proportion of sites was in a lower affinity state, as reflected by the biphasic nature of the agonist displacement curve. In contrast, the effect of sodium on displacement of antagonist from mu sites was to of sodium on displacement of antagonist from mu sites was to lower the affinity of the agonist. Competition against antagonist binding to kappa sites also showed a reduction in agonist affinity by sodium, but no change in number of receptors. The results indicate that sodium may differentially regulate agonist binding to opioid receptor types and that this regulation may occur at an intracellular site. The kappa site appears to be less sensitive to sodium than the mu and delta sites.