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

Short peptide sequence enhances epithelial permeability through interaction with protein kinase C

We have identified a short peptide sequence (L-R5) acting as partial inhibitor of intracellular protein kinase C, capable of tight junction modulation in terms of reversible and non-toxic drug permeation enhancement. L-R5 is a pentapeptide with a cell-penetrating group at the N-terminus and of the sequence myristoyl-ARRWR. Apically applied in vitro, L-R5 transiently increased epithelial permeability within minutes, enhancing apical-to-basolateral (AB) transport of 4-kDa dextran and BCS class III drug naloxone. L-R5 was shown to be stable and effective at 37°C over a period of 24 hours. L-R5 was shown to be non-cytotoxic in consecutive exposure studies on primary human nasal epithelial cells by LDH release assay and ciliary beating frequency test. Finally, L-R5 by itself showed very low diffusion across epithelial monolayers, which is of advantage with regard to its expected negligible systemic bioavailability and side effects. Taken together, these data demonstrate the potential of short peptide partial inhibitor L-R5 to enhance the epithelial paracellular permeability via a reversible mechanism, and in a non-toxic manner.

Prosecretory effect of loperamide in ileal and colonic mucosae of mice displaying high or low swim stress-induced analgesia associated with high and low endogenous opioid system activity

BACKGROUND

Irritable bowel syndrome (IBS) is characterized by abdominal pain, bloating, and changes in bowel habit. The aim of this study was to characterize the effect of loperamide hydrochloride (LOP) and naloxone hydrochloride (NLX), an opioid agonist and antagonist, respectively, on electrolyte equilibrium in ileal and colonic mucosae and to estimate the possible influence of divergent activity of the endogenous opioid system (EOS) on IBS therapy.

METHODS

Two mouse lines bidirectionally selected for high (HA) and low (LA) swim stress-induced analgesia associated with high and low EOS activity were used in this study. To assess the effect of LOP and NLX on HA/LA lines in vivo, we used the castor oil-induced diarrhea model. Changes in electrolyte equilibrium were determined on the basis of short-circuit current (ΔI_sc ) in isolated mouse ileum and colon exposed to LOP and NLX and stimulated by forskolin (FSK), veratridine (VER), and bethanechol (BET).

KEY RESULTS

In vivo, we found that LOP significantly prolonged time to appearance of diarrhea in HA and LA lines. In vitro, LOP and NLX increased ΔI_sc in FSK- and VER-stimulated colonic tissue, respectively, in HA line. In the ileum, LOP increased ΔI_sc in FSK- and VER-stimulated tissue and decreased ΔI_sc in BET-stimulated tissues in HA line.

CONCLUSIONS & INFERENCES

Individual differences in EOS activity may play a crucial role in the response to the IBS-D therapy, thus some patients may be at an increased risk of side effects such as constipation or diarrhea.

Unexpected prosecretory action component of loperamide at mu-opioid receptors in the guinea-pig colonic mucosa in vitro

1. In a voltage clamp setting (Ussing chamber), the antidiarrhoeal drug, loperamide (Lop) slightly augmented prostaglandin E1 (PGE1) plus theophylline-stimulated net chloride secretion above control values at low concentrations (10(-10) and 10(-9) M) but inhibited it at higher concentrations (10(-6) and 10(-5) M). The apparently weak prosecretory action component of Lop was turned into a clear cut antisecretory effect by pretreatment with 2 x 10(-7) M naloxonazine plus 10(-7) M CTOP-NH2 (D-Phe-Cys-Tyr-D-Trp-Orn-Thr-Pen-Thr-NH2), two selective mu opioid receptor antagonists. This indicates a prosecretory effect of loperamide at mu opioid receptors. The antisecretory effect of low Lop concentrations, uncovered by mu opioid receptor blockade, was prevented by additional blockade of kappa opioid receptors by 5 x 10(-9) M nor-binaltorphimine (nor-BNI). 2. The nonselective opioid antagonist, naloxone, at 10(-6) M did not significantly reduce either PGE1 plus theophylline-stimulated net chloride secretion in Lop-free controls or the antisecretory action of Lop. By contrast, the partial agonist ethylketocyclazocine (EKC), which activates kappa but blocks mu opioid receptors, concentration-dependently inhibited PGE1 plus theophylline-stimulated net chloride secretion without any consistent prosecretory action component. Nor-BNI at 5 x 10(-8) M significantly blocked the antisecretory action of EKC. 3. It is concluded that, in the guinea-pig colonic mucosa under the present conditions, mu opioid receptors mediate enhancement and kappa opioid receptors inhibition of PGE1-stimulated net chloride secretion by low Lop concentrations. The two opposite actions are largely masked by superimposition. An opioid receptor-independent mechanism of action contributes to the antisecretory effect of Lop at high concentrations.

Synergistic opioid inhibition of colonic Cl- secretion by kappa-opioid receptor agonism plus mu-opioid receptor antagonism

On the basis of the novel concept of a dual prosecretory/antisecretory intestinal opioid system, a voltage-clamp experiment was performed with guinea-pig colonic mucosa, and net anion (Cl-) secretion was measured after stimulation with prostaglandin E1 (PGE1) plus theophylline. Synergism between kappa-opioid receptor agonism by U69593 and mu-opioid receptor antagonism by CTOP-NH2 (D-Phe-Cys-Tyr-D-Trp-Orn-Thr-Pen-Thr-NH2) was observed.

Mu opioid receptors modulate net chloride secretion in the guinea pig colonic mucosa in a dual fashion

The present voltage-clamp data from guinea pig colonic mucosa indicate that mu opioid receptors both enhance and inhibit net chloride secretion stimulated by PGE1 plus theophylline. Actually, enhancement by the mu opioid agonist DAGO (0.2 nmol/l) declines and finally turns into inhibition with increasing net chloride secretion observed in control preparations taken from the same animals. Moreover, inhibition by the mu opioid antagonist CTOP-NH2 (100 nmol/l) increases with increasing inhibition (or decreasing enhancement) by the mu opioid agonist DAGO. Thus, the mu opioid agonist DAGO and the mu opioid antagonist CTOP-NH2 display opposite effects on net chloride secretion only to a limited degree but otherwise show parallel, either stimulatory or inhibitory, effects. To explain this complex function of mu opioid receptors in the control of net chloride secretion, a working hypothesis is being proposed.