Voltage-clamp experiments reveal receptor type-dependent modulation of chloride secretion in the guinea pig colonic mucosa by intestinal opioids.

The influence of four opioid antagonists on short circuit current (Isc), transepithelial potential difference (Pdo) and tissue conductance (Gt) in the guinea pig colonic mucosa was investigated in vitro under both basal and PGE1 plus theophylline-stimulated conditions. The experiments aimed at identifying the opioid receptor type(s) endogenously activated to control chloride secretion. Under blockade of sodium-dependent Isc by amiloride (100 mumol/l), net anion secretion was regarded to equal the lumen-negative shift in Isc upon addition of 1 mumol/l PGE1 plus 100 mumol/l theophylline. It was significantly elevated by 100 nmol/l of the kappa-selective antagonist nor-binaltorphimine (nor-BNI). This augmenting effect was totally abolished in amiloride-free buffer or by omission of chloride. 1 mumol/l TTX completely prevented the effect of both PGE1 plus theophylline and nor-BNI. Both the kappa agonist U 69593 (10 nmol/l) and the calcium channel agonist Bay K 8644 (1 mumol/l) significantly depressed net anion secretion stimulated by PGE1 plus theophylline. Nor-BNI at 10 nmol/l prevented the suppressive effect of both Bay K 8644 and U 69593. This suggests release of endogenous opioids by the calcium channel agonist Bay K 8644 and competition between the kappa agonist U 69593 and the kappa antagonist nor-BNI. In contrast to the kappa antagonist nor-BNI, the mu antagonist CTOP-NH2 at 100 nmol/l significantly impaired, while the mu-selective agonist DAGO at 0.2 nmol/l augmented, net anion secretion stimulated by PGE1 plus theophylline. The effect of CTOP-NH2 was abolished in chloride-free buffer.(ABSTRACT TRUNCATED AT 250 WORDS)