Involvement of inwardly rectifying K+ channels in secretory responses of human ileal mucosa

Bicarbonate and functional CFTR channel are required for proper mucin secretion and link cystic fibrosis with its mucus phenotype

Ileal mucus in CftrΔ508 mice is more adherent, denser, and less penetrable than that of WT mice, but addition of bicarbonate normalizes the properties of CftrΔ508 mucus.

Cystic fibrosis (CF) is caused by a nonfunctional chloride and bicarbonate ion channel (CF transmembrane regulator [CFTR]), but the link to the phenomenon of stagnant mucus is not well understood. Mice lacking functional CFTR (CftrΔ508) have no lung phenotype but show similar ileal problems to humans. We show that the ileal mucosa in CF have a mucus that adhered to the epithelium, was denser, and was less penetrable than that of wild-type mice. The properties of the ileal mucus of CF mice were normalized by secretion into a high concentration sodium bicarbonate buffer (∼100 mM). In addition, bicarbonate added to already formed CF mucus almost completely restored the mucus properties. This knowledge may provide novel therapeutic options for CF.

Influence of ATP-dependent K+ channels on nicotine-induced inhibition of withdrawal in morphine-dependent mice

In the present study, we have investigated the effect of nicotine and diazoxide, a potassium channel opener and glibenclamide, a potassium channel (K(ATP)) blocker on naloxone-precipitated physical withdrawal signs, including jumping and diarrhea. Then, the interactions of nicotine with diazoxide and glibenclamide were tested. Mice were rendered dependent on morphine by subcutaneous (s.c.) injections of morphine sulphate 3 times a day for 3 days, and jumping behavior and diarrhea were induced by intraperitoneal (i.p.) administration of naloxone 2 h after the 10th injection of morphine sulphate on day 4. Nicotine was administered 15 min and diazoxide and glibenclamide 30 min before naloxone injection. Nicotine (0.01-1 mg/kg, s.c.) and (0.1-1 mg/kg) reduced withdrawal jumping and diarrhea respectively. Diazoxide (8-64 mg/kg, i.p.) decreased jumping behavior significantly, but had no significant effect on diarrhea. On the other hand glibenclamide (0.25-1 mg/kg i.p.) and (1 mg/kg) augmented jumping and diarrhea respectively. The response of nicotine on jumping or on diarrhea was potentiated by diazoxide and decreased by glibenclamide pretreatment. The isobolographic analysis revealed synergistic interaction between diazoxide and nicotine on decreasing physical withdrawal signs including jumping and diarrhea in morphine-dependent mice. According to these results the interaction of nicotine with the K(ATP) channel opener and blocker in morphine physical withdrawal signs could be explained by direct and indirect effects of nicotine on membrane potassium currents.

K+-ATP-channel-related protein complexes: potential transducers in the regulation of epithelial tight junction permeability

K+-ATP channels are composed of an inwardly rectifying Kir6 subunit and an auxiliary sulfonylurea receptor (SUR) protein. The SUR subunits of Kir6 channels have been recognized as an ATPase, which appears to work as a mechanochemical device like other members of the ABC protein family. Thus, in spite of just gating ions, Kir6/Sur might, in addition, regulate completely different cellular systems. However, so far no model system was available to directly investigate this possibility. Using highly specific antibodies against Kir6.1-SUR2A and an in vitro model system of the rat small intestine, we describe a new function of the Kir6.1-SUR2A complex, namely the regulation of paracellular permeability. The Kir6.1-SUR2A complex localizes to regulated tight junctions in a variety of gastrointestinal, renal and liver tissues of rat, pig and human, whereas it is absent in the urothelium. Changes in paracellular permeability following food intake was investigated by incubating the lumen of morphological well-defined segments of rat small intestine with various amounts of glucose. Variations in the lumenal glucose concentrations and regulators of Kir6.1/SUR2A activity, such as tolbutamide or diazoxide, specifically modulate paracellular permeability. The data presented here shed new light on the physiological and pathophysiological role K+-ATP channels might have for the regulation of tight junctions.

Early disturbance of microvascular function precedes chemotherapy-induced intestinal injury

Intestinal injury 4-48 hr after cytotoxic therapy (etoposide phosphate, 100 mg/kg body weight [bw], intravenously [i.v.]) was studied in rats using ligated intestinal loops. Chromium-51 ethylenediaminetetraacetic acid ((51)Cr-EDTA) and rubidium-86 chloride ((86)RbCl) were deposited intraluminally to determine the extent of the increase in intestinal permeability and ion channel disruption. Evans Blue (EB) was used for detection of endothelial leakage. Intestinal morphology was documented. Endothelial dysfunction, as observed by an increased extravasation of EB, was evident already 4 hr after cytotoxic therapy. Intestinal epithelial injury, as observed by an increase in (51)Cr-EDTA permeation and a decrease in (86)Rb absorption, occurred after 48 hr. Finally, histology disclosed a reduced crypt cell proliferation, displayed as a decrease in Ki67-positive cells. The findings suggest that, in the development of intestinal injury after cytotoxic therapy, endothelial disruption is an early event, whereafter epithelial dysfunction and crypt stem cell arrest occur. This knowledge could be of importance in the design of future intervention trials.

Clinical endocrinology and metabolism. Receptors for gut peptides

Most gut peptides exert their effects through G protein-coupled receptors, a family of about 700 membrane proteins, 87 of which are presently known to have peptide ligands. Three additional gut peptide receptors are not G protein-coupled receptors but regulate intracellular cyclic GMP accumulation. The aim of this review is to illustrate how the sequencing of the human genome and other recent advances in genomics has contributed to our understanding of the role of peptides and their receptors in gastrointestinal function. Recent discoveries include the identification of receptors for the peptides motilin and neuromedin U, and new physiological ligands for the PTH2 receptor, the CRF(2) receptor and the growth hormone secretagogue receptor. Knockout mice lacking specific peptide receptors or their ligands provide informative animal models in which to determine the functions of the numerous peptide-receptor systems in the gut and to predict which of them may be the most fruitful for drug development. Some peptide-receptor signalling systems may be more important in disease states than they are in normal physiology. For example, substance P, galanin, bradykinin and opioids play important roles in visceral pain and inflammation. Other peptides may have developmental roles: for example, disruption of endothelin-3 signalling prevents the normal development of the enteric nervous system and contributes to the pathogenesis of Hirschsprung disease.

Evaluation of Second Generation Amiloride Analogs as Therapy for Cystic Fibrosis Lung Disease

Epithelial sodium channel (ENaC) blockers have been proposed as a therapy to restore mucus clearance (MC) in cystic fibrosis (CF) airways. The therapeutic effects of the first generation ENaC blocker, amiloride, in CF patients, however, were minimal. Because the failure of amiloride reflected both its low potency and short duration of action on airway surfaces, we investigated whether the increased potency of benzamil and phenamil would produce more favorable pharmacodynamic properties. In vitro potency, maximal efficacy, rate of recovery from maximal block of ENaC, and rate of drug absorption were compared for amiloride, benzamil, and phenamil in cultured human and ovine bronchial epithelial cells. In both human and ovine bronchial epithelia, the rank order of potency was benzamil > phenamil >> amiloride, the maximal efficacy was benzamil = phenamil = amiloride, the recovery to baseline sodium transport was phenamil < benzamil << amiloride, and the rate of drug absorption was phenamil > benzamil >> amiloride. Based on greater potency, benzamil was compared with amiloride in in vivo pharmacodynamic studies in sheep, including tracheal mucus velocity (TMV) and MC. Benzamil enhanced MC and TMV, but acute potency or duration of effect did not exceed that of amiloride. In conclusion, our data support the hypothesis that ENaC blocker aerosol therapy increases MC. However, rapid absorption of benzamil from the mucosal surface offset its greater potency, making it equieffective with amiloride in vivo. More potent, less absorbable, third generation ENaC blockers will be required for an effective aerosol CF pharmacotherapy.