CHARACTERIZATION OF OPIOID RECEPTORS ON SMOOTH-MUSCLE CELLS FROM GUINEA-PIG STOMACH

On the basis of opioid-stimulated contraction of dispersed gastric smooth muscle cells it has been suggested that these cells possess opioid receptors of three subtypes: kappa (kappa), mu (mu), and delta (delta). We have used selective peptidase-resistant radioligands, agonists and antagonists, to examine receptor subtypes on dispersed gastric smooth muscle cells from guinea pigs prepared by collagenase digestion. The kappa-agonist U-50488H, the mu-agonist [D-Ala2,N-Me-Phe4,Gly5-ol]enkephalin (DAGO), and the delta-agonist [D-Pen2,Pen5]enkephalin (DPDPE) each caused muscle contraction. The concentrations required to caused half-maximal contraction were U50488H (6 pM) > DAGO (13 pM) > DPDPE (6 nM). The abilities of these agonists to inhibit binding of [H-3]U-69593 (kappa-preferring) by 50% were U50488H (43 nM) > DAGO (43-mu-M) > DPDPE (200-mu-M). Their abilities to inhibit binding of [H-3]naloxone (mu-preferring) by 50% were DAGO (0.2-mu-M) > U50488H (10-mu-M) > DPDPE (> 100-mu-M). No binding could be detected with the delta-selective ligand [H-3]DPDPE. The kappa-preferring antagonist Mr2266 (10 nM) preferentially inhibited contraction stimulated by the kappa-agonist U50488H, and naltrexone (10 nM) (mu-selective antagonist) preferentially inhibited contraction stimulated by the mu-agonist DAGO. ICI 174864 (200-mu-M; delta-selective antagonist) had no effect on contraction stimulated by mu-, kappa-, or delta-agonists. Contraction stimulated by the delta-agonist DPDPE was inhibited by both kappa- and mu-receptor antagonists. Studies on the effect of the antagonists on binding of [H-3]naloxone and [H-3]-U69593 also provided evidence for kappa- and mu-sites but not for delta-sites. These data demonstrate that gastric smooth muscle cells possess kappa- and mu-receptors, occupation of which causes muscle contraction, but no delta-receptors could be detected by these methods.