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Kromer, W; Beubler, E.
Dual Opioid Modulation of Chloride Water Secretion in the Guinea-Pig Colon in-Vitro and the Rat Jejunum in-Vivo
J AUTONOM PHARMACOL 1993 13: 315-321. Doi: 10.1111/j.1474-8673.1993.tb00279.x
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Co-authors Med Uni Graz: Beubler Eckhard
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Abstract:: 1 The short-circuit current, which indicates net chloride secretion under the present experimental condition, was stimulated by PGE1 plus theophylline in the guinea-pig colonic mucosa in vitro. 2 It was dose-dependently inhibited by up to 6 5 % at 1 mumol l-1 by the kappa-selective opioid agonist, U69593. The mu-selective opioid agonist, DAGO, was inactive, while the delta-selective opioid agonist DPDPE displayed about 20% inhibition at 1 mumol l-1. Basal secretion was not inhibited by the three agonists to any biologically relevant degree. 3 The mu-selective opioid antagonist, CTOP-NH2 (0.1 mumol 1-1), partially inhibited PGE1 plus theophylline-stimulated net chloride secretion, suggesting that endogenous opioid peptides enhance secretion. The non-selective opioid antagonist, naloxone, at the high concentration of 100 mumol 1-1 also partially inhibited net chloride secretion and prevented, upon pre-administration, any further effect of CTOP-NH2. This confirms the opioid nature of the receptor affected by CTOP-NH2 in vitro. 4 The antisecretory effect of CTOP-NH2 has been confirmed in the rat in vivo using jejunal enteropooling upon continuous stimulation by PGE2. A high dose (1 mg kg-1 s.c.) of naloxone tended to either inhibit the diarrhoeal action of a high dose of PGE2 (79.1 ng min-1 i.a.) or augment the diarrhoeal action of a lower dose of PGE2 (31.6 ng min-1 i.a.). The contrasting effects of naloxone may be due to endogenous activation of both mu and kappa Opioid receptors at high doses of PGE2 but only mu opioid receptors at low doses. In either case, no further inhibition by CTOP-NH2 was observed after pretreatment with naloxone, proving the opioid nature of the receptor affected by CTOP-NH2 in vivo.