Effects of Na+, k+, mg++ and Ca++ on the saturable binding of [3H]dihydromorphine and [3H]naloxone in vitro
Lee, C.Y.; Akera, T.; Brody, T.M.
Journal of Pharmacology and Experimental Therapeutics 202(1): 166-173
1977
ISSN/ISBN: 0022-3565 PMID: 874813 Document Number: 110158
The effects of Na+, K+, Mg++ and Ca++ on the saturable binding of 3H dihydromorphine and 3H naloxone to particulate fractions obtained from rat brain tissues were studied in vitro. When assays were performed in 50 mM Tris-HCl buffer (pH 7.4), Na+ decreased the maximal binding (Bmax: the binding site concentration) and the affinity of saturable binding sites for dihydromorphine in the thalamus-hypothalamus and in whole brain minus cerebellum regions which contain both agonist binding sites and antagonist-specific binding sites. K+ decreased the affinity for dihydromorphine with a minimal effect on the Bmax. NaCl, at 50 mM, increased the binding site affinity for naloxone. With 100 and 150 mM NaCl, the increase in affinity was not apparent. When assays were performed in an isotonic sucrose solution, the Bmax for naloxone was greater than that in 50 mM Tris-HCl buffer. Such an increase in Bmax was abolished by an addition of 50 mM NaCl to the sucrose solution. K+ decreased the Bmax and the affinity of saturable binding sites for naloxone under all conditions tested. In 50 mM Tris-HCl buffer, Mg++ and Ca++ decreased the Bmax and slightly increased the affinity of binding sites for naloxone. In particulate fractions obtained from cerebellum which contain only antagonist-specific binding sites, Na+ and K+ inhibited saturable naloxone binding. Apparently Na+-induced increase in opiate antagonist binding is primarily due to an increased affinity and is not the result of the unmasking of new binding sites. Na+-induced increases in affinity of saturable binding sites for naloxone are complex and not apparent at high Na+ concentration presumably due to an ionic effect of Na+ which is shared by many cations. There are 2 qualitatively different saturable binding sites for naloxone.