Sodium regulation of agonist and antagonist binding to β-adrenoceptors in intact and N_s-deficient membranes

Summary

Agonist binding to various hormone receptors mediating adenylate cyclase inhibition is decreased by sodium ions. We studied the influence of Na⁺ on agonist and antagonist binding to β-adrenoceptors in membrane preparations of guinea pig lung, S49 lymphoma wild-type cells (WT) and their N_s-deficient cyc⁻ variants by measuring binding of the antagonist, [¹²⁵I]iodocyanopindolol ([¹²⁵I]CYP). At 37° C, sodium decreased the receptor affinity for the agonist, isoproterenol, in all three membrane preparations. In lung and WT membranes, Na⁺ steepened the shallow agonist competition curves in a manner similar to and synergistic with guanine nucleotides. When binding was performend at 4° C, sodium regulation but not guanine nucleotide regulation of agonist binding was preserved. At the low temperature, [¹²⁵I]CYP affinity was reduced, and sodium increased [¹²⁵I]CYP binding in both N_s-containing and N_s-deficient membranes by increasing the antagonist affinity without significant change in total receptor number. Compared to Na⁺, Li⁺ and K⁺ were much less potent and efficient in decreasing agonist and increasing antagonist binding. Na⁺ and Mg²⁺ had opposite effects on agonist binding in the N_s-containing lung and WT membranes but not in the N_s-deficient cyc⁻ membranes. The data indicate that sodium not only regulates binding of inhibitory hormone receptors but also agonist and antagonist binding to the adenylate cyclase stimulatory β-adrenoceptor. The finding that sodium regulation of β-adrenoceptor binding is also observed in the N_s α₂ cyc⁻ membranes, furthermore, indicates that the target of sodium is not the α-subunit of N_s but possibly a component common to both types of receptor systems regulating adenylate cyclase activity.