Selective potentiation by ouabain of naloxone‐induced withdrawal contractions of isolated guinea‐pig ileum following acute exposure to morphine

Ouabain, an inhibitor of Na+/K+ ATPase induces the release of acetylcholine from central and myenteric cholinergic neurones principally due to partial depolarization of the cell membrane. The effect of ouabain has been examined on neurogenic contractions in the guinea-pig ileum arising from either electrical field stimulation or from naloxone in morphine-exposed preparations. Guinea-pig isolated ileum preparations were stimulated transmurally (0.1 Hz, 0.3 ms, 200 mA) to elicit contractions of the myenteric plexus-longitudinal smooth muscle. Incubation with morphine (0.3 μM, 60 min) was followed by naloxone (1 μM) which produced withdrawal contractions in 16/26 preparations (median of 10.7 [2.2–40.0]% of a maximal contracture to KCl (60 mM)). In parallel experiments, ouabain (1 μM) was added to the tissue before exposure to morphine (0.3 μM, 60 min). Naloxone (1 μM) subsequently displayed a withdrawal contraction in all 26/26 tissues (57.9 [30.5–151.7]% of a maximal contracture to KCl (60 mM). Ouabain neither affected the concentration-dependent contractions of guinea-pig ileum produced by carbachol nor the inhibition of electrically-evoked contraction produced by morphine (0.3 μM). The muscarinic antagonist atropine (0.1 μM) antagonized control naloxone withdrawal responses. The atropine resistant component, evident in ouabain-treated tissues, was blocked by SR140333((S)1-{2-[3-(3,4-dichlorophenyl)-1-(3-isopropoxyphenylacetyl)piperidin-3-yl]ethyl}-4-phenyl-1-azoniabicyclo[2.2.2]octane, chloride), a substance P antagonist. Clonidine (α2-adrenoceptor agonist) inhibited electrically-evoked contractions. Exposure to the α2-adrenoceptor antagonist RX811059 (2-(2-ethoxy-1,4-benzodioxan-2-yl)-2-imidazoline), resulted in a contracture which was not significantly enhanced by ouabain (1 μM). Ouabain selectively potentiates the naloxone-induced withdrawal contraction following acute exposure to morphine the major components of which are mediated by both acetylcholine and substance P.