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

Summary

The influence of four opioid antagonists on short circuit current (I_sc), transepithelial potential difference (Pd_o) and tissue conductance (G_t) 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 I_sc by amiloride (100 µmol/I), net anion secretion was regarded to equal the lumen-negative shift in I_sc upon addition of 1 µmol/l PGE₁ plus 100 gmol/l theophylline. It was significantly elevated by 100 nmol/l of the ϰ-selective antagonist nor-binaltorphimine (nor-BNI). This augmenting effect was totally abolished in amiloride-free buffer or by omission of chloride. 1 µmol/l TTX completely prevented the effect of both PGE₁ plus theophylline and nor-BNI. Both the ϰ agonist U 69593 (10 nmol/l) and the calcium channel agonist Bay K 8644 (1 µmol/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 ϰ agonist U 69593 and the ϰ antagonist nor-BNI. In contrast to the ϰ antagonist nor-BNI, the µ antagonist CTOP-NH₂ at 100 nmol/l significantly impaired, while the µ-selective agonist DAGO at 0.2 nmol/l augmented, net anion secretion stimulated by PGE₁ plus theophylline. The effect of CTOP-NHn2 was abolished in chloride-free buffer. The δ selective antagonist ICI 174.864 (up to 1 µmol/l) and the non-selective antagonist naloxone (up to 1 µmol/l) failed to influence stimulated net anion secretion. The failure of naloxone may be due to superimposition of the contrasting effects of µ and ϰ receptor blockade by a high naloxone concentration.

The antagonist effect of nor-BNI considered above was obtained at 9.6 nmol/l potassium but was no longer observed when the potassium concentration in the bathing solution was reduced to 4.8 mmol/l. It is suggested that both elevated potassium concentration and calcium channel activation by Bay K 8644 release endogenous ϰ agonists in the guinea pig colonic mucosa under the present conditions. Since CTOP-NH₂ impaired stimulated net anion secretion at both of the potassium concentrations, release of endogenous µ agonists and, hence, activation of µ opioid receptors seem to take place under both conditions. In conclusion, the present work demonstrates for the first time that activation of ϰ opiod receptors by endogenous opioid peptides impair, while activation of µ opioid receptors augment, stimulated chloride secretion in this tissue.