The sensitivity of opioid receptor types to regulation by sodium and GTP

The effects of Na+ and GTP on agonist binding to mu, delta, and kappa opioid binding sites in membranes from guinea pig cortex have been studied. All incubations were in a modified Krebs-Hepes buffer at 37 degrees; effects of Na+ were evaluated by replacement with an equal concentration of K+. mu binding sites appeared more sensitive than delta sites, while kappa sites were the least sensitive to both regulators. However, even at kappa sites, Na+ induced a significant reduction in agonist affinity and GTP caused a significant reduction in maximum binding.