The physiological relevance of low agonist affinity binding at opioid μ-receptors

Abstract

1 Inhibition constants (K_i) were determined for a range of opioid standards using two binding assays; [³H]-[d-Ala², MePhe⁴, Gly-ol⁵]enkephalin ([³H]-GLYOL) binding to guinea-pig brain membranes in HEPES buffer and [³H]-naloxone binding to rat whole brain membranes in Krebs/HEPES buffer. 2 These values were compared with affinity measurements determined by antagonism of GLYOL on the rat isolated vas deferens preparation and by the receptor occlusion technique of Furchgott on the guinea-pig ileum longitudinal muscle, myenteric plexus preparation. 3 Agonists demonstrated markedly reduced binding affinity in the [³H]-naloxone binding assay where binding was conducted in the presence of sodium. 4 A strong correlation was obtained between values from the [³H]-naloxone binding assay and affinity values determined in both isolated tissue preparations. K_i values obtained from [³H]-GLYOL binding did not correlate well with affinity data determined by isolated tissue techniques. 5 These findings suggest that functionally relevant receptors exhibit low agonist affinity.