Inhibition of intestinal secretion without reduction of cyclic AMP levels

Inhibition by loperamide of deoxycholic acid induced intestinal secretion

The effect of loperamide on deoxycholic acid (DOC)-induced secretion was studied in ligated loops of the rat jejunum and colon in vivo. In controls, loperamide slightly augmented fluid absorption. 3 mmol DOC caused net fluid secretion. Loperamide reduced this secretion in the colon and reversed it to absorption in the jejunum. Na-K-ATPase specific activity and cAMP levels were measured in the jejunum, and [14C]erythritol clearance as an index of mucosal permeability in the colon. In the jejunum this opiate analogue affected neither basal or DOC-depressed Na-K-ATPase, nor mucosal cAMP. In the colon it reduced a large increase of the erythritol clearance caused by DOC. It is suggested that loperamide interferes with DOC-induced intestinal secretion in part by lowering mucosal permeability.

Characterization of the opiate receptor population mediating inhibition of VIP-induced secretion from the small intestine of the rat

Net water transport was measured from the jejunum of anaesthetized rats by a re-circulation technique. Several narcotic analgesics were administered intravenously and assessed for activity in reversing net fluid secretion induced by intra-arterial infusion of vasoactive intestinal peptide (VIP). The mu-opiate agonists, morphine, RX 783006 and FK 33-824 produced full reversal of the secretory phase of the VIP response, but failed to restore totally fluid transport to the control level of net absorption. The kappa-agonist, ethylketocyclazocine, caused only partial reversal of VIP-induced secretion while the more selective kappa-agonist, MR 2034, produced a small though non-significant antisecretory effect at high doses. The delta-agonist, D-Ala2-D-Leu5-enkephalin also had negligible antisecretory activity. Naloxone caused a parallel displacement to the right of the antisecretory dose-response line to morphine. The 'in vivo pA2' value of naloxone was 7.14. The results are compared with previously published antinociceptive activities of the opiate agonists and in vivo pA2 values of naloxone. It is concluded that stimulation of mu-opiate receptors mediates inhibition of VIP-induced fluid secretion from the rat jejunal mucosa.

Effect of opiates on human colonic adenylate cyclase activity

To elucidate a possible mechanism whereby opiates affect human intestinal fluid secretion, opiate effects on human basal and stimulated colonic adenylate cyclase activity were determined. Morphine (10 microM) and naloxone (1 microM) did not affect basal colonic adenylate cyclase activity: 32.6 +/- 2.6 (n = 23; X +/- S.E.) pmol cAMP/mg protein per 10 min or its stimulation induced by sodium fluoride or VIP. However, morphine inhibited the stimulation induced by prostaglandin E2, an effect which was blocked by naloxone. Synthetic enkephalins and metabolically stable retro inverso enkephalins also prevented PGE2-induced stimulation of colonic adenylate cyclase activity. The addition of calcium (0.1 mM) decreased the basal and PGE2-stimulated enzyme activities by 35% and 50%, respectively, regardless of the presence or absence of morphine. These results suggest that morphine and synthetic enkephalins do not affect basal human colonic adenylate cyclase activity but inhibit its stimulation induced by PGE2. This effect may be one of the mechanisms whereby opiates affect intestinal fluid transport.

Alpha 2-adrenoceptors inhibit the cholera-toxin-induced intestinal fluid accumulation

The effects of adrenoceptor agonists and antagonists on the cholera-toxin-induced intestinal fluid accumulation and the mucosal levels of cAMP were investigated in vivo. Cholera toxin produced a marked fluid accumulation. Adrenaline inhibited the effect of the toxin in a dose-dependent manner. An alpha 2-adrenoceptor blocking agent yohimbine antagonized the effect of adrenaline. The alpha 1-adrenoceptor blocking agents prazosin and phenoxybenzamine failed to antagonize the effect of adrenaline. A high dose of a beta-adrenoceptor blocking agent pindolol did not antagonize the effect of adrenaline. Yohimbine or pindolol alone did not produce any effects on the toxin-induced fluid accumulation. However, prazosin and phenoxybenzamine per se inhibited the toxin-induced fluid accumulation. An alpha 2-selective agonist clonidine was slightly more potent than adrenaline, and was about 100-fold more potent than the alpha 1-selective agonist methoxamine in inhibiting the cholera-toxin-induced intestinal secretion. Clonidine, adrenaline and methoxamine failed to reduce the mucosal levels of cAMP, while these alpha-adrenoceptor agonists inhibited the toxin-induced fluid accumulation in the same preparations. These results suggest that the stimulation of alpha 2-adrenoceptors inhibit the cholera-toxin-induced intestinal secretion without reducing the whole mucosal levels of cAMP.