Sodium regulation of agonist binding at opioid receptors. I. Effects of sodium replacement on binding at mu- and delta-type receptors in 7315c and NG108-15 cells and cell membranes.

The effects of varying the sodium concentration (at constant ionic strength) on opioid binding at mu- and delta-opioid receptors in 7315c and NG108-15 cells has been examined. The binding of [3H]etorphine to mu-receptors on 7315c cells was increased by replacing the sodium in the incubation medium with potassium or N-methyl-D-glucamine. This effect was shown to be attributable to an increase in affinity, with no change in the maximum number of binding sites, both in cell membrane suspensions and in intact 7315c cells. Replacement of sodium with potassium or N-methyl-D-glucamine in NG108-15 membrane or intact cell suspensions also resulted in an increase in [3H]etorphine binding, but in these cells the effect was associated with an increase in the number of binding sites measurable under these experimental conditions. The effects of sodium on opioid inhibition of adenylate cyclase in membrane preparations from 7315c and NG108-15 cells also differed. Sodium reduced apparent agonist affinity in 7315c membranes. In NG108-15 cell membranes, sodium was essential for the demonstration of opioid inhibition of cyclase activity. Increasing the sodium concentration above 0.5 mM resulted in an increase in the fraction of total enzyme activity inhibited by opioid, but the opioid IC50 did not change. In the companion paper, it is shown that the effects of sodium removal on mu- and delta-receptor binding in guinea pig brain neural membranes were similar to those observed in the cell preparations. An increase in intracellular sodium concentration without change in extracellular concentration was effected by incubation of 7315c and NG108-15 cells with the sodium-selective ionophore, monensin. When sodium was present in the extracellular medium, monensin reduced [3H]etorphine binding by 50% or more, both at mu-receptors in 7315c cells and at delta-receptors in NG108-15 cells. In the absence of sodium, however, monensin treatment produced only a small inhibition of binding. These results suggest that sodium acts at an intracellular site to regulate opioid agonist binding at both mu- and delta-receptors, but that the mode of regulation is not identical at each site. Since a reduction in intracellular sodium concentration by removal of extracellular sodium increases agonist binding, and an increase in intracellular sodium following monensin treatment reduces agonist binding, it is probable that the intracellular sodium concentration is a critical regulator of opioid agonist binding in intact cells.