Physiology and pathophysiology of potassium channels in gastrointestinal epithelia

Activation of PPARgamma by rosiglitazone attenuates intestinal Cl- secretion

The thiazolidinedione (TZD) drugs rosiglitazone (Ro) and pioglitazone (Po) are PPARgamma agonists in widespread clinical use as insulin-sensitizing agents in Type 2 diabetes. On the basis of recent evidence implicating PPARgamma as a positive modulator of intestinal epithelial differentiation, we hypothesized that TZD drugs might attenuate intestinal secretory function. To evaluate this possibility, we examined the effects of Ro and Po on electrogenic Cl- secretion [short-circuit current (I(sc))] in mouse intestinal segments and in cultured human intestinal epithelial cells (HT29-Cl.19A). As hypothesized, oral administration of Ro (20 mg.kg(-1).day(-1)) to mice for 8 days markedly reduced intestinal I(sc) responses to cAMP (forskolin)- and Ca2+ (carbachol)-dependent stimuli. In these Ro-treated mice, cholera toxin-induced intestinal fluid accumulation was reduced 65%. With continued Ro treatment, the I(sc) response to carbachol recovered significantly, whereas that to forskolin remained attenuated. Treatment of HT29 cells for 5 days with 10 muM Ro or Po in vitro brought about a similar hyposecretory state. In HT29 cells, the loss of cAMP-dependent Cl- secretion was attributable to a reduced expression of CFTR Cl- channel, KCNQ1 K+ channel, and Na-K-2Cl cotransporter-1 proteins. The transient loss of Ca2+-dependent Cl- secretion involved an impairment of basolateral Ca2+-stimulated K+ channel activity without a detectable loss of K(Ca)3.1 channel protein. Our results establish TZD drugs as important modulators of intestinal Cl- secretory function.

A polyether biotoxin binding site on the lipid-exposed face of the pore domain of Kv channels revealed by the marine toxin gambierol

Gambierol is a marine polycyclic ether toxin belonging to the group of ciguatera toxins. It does not activate voltage-gated sodium channels (VGSCs) but inhibits Kv1 potassium channels by an unknown mechanism. While testing whether Kv2, Kv3, and Kv4 channels also serve as targets, we found that Kv3.1 was inhibited with an IC(50) of 1.2 +/- 0.2 nM, whereas Kv2 and Kv4 channels were insensitive to 1 microM gambierol. Onset of block was similar from either side of the membrane, and gambierol did not compete with internal cavity blockers. The inhibition did not require channel opening and could not be reversed by strong depolarization. Using chimeric Kv3.1-Kv2.1 constructs, the toxin sensitivity was traced to S6, in which T427 was identified as a key determinant. In Kv3.1 homology models, T427 and other molecular determinants (L348, F351) reside in a space between S5 and S6 outside the permeation pathway. In conclusion, we propose that gambierol acts as a gating modifier that binds to the lipid-exposed surface of the pore domain, thereby stabilizing the closed state. This site may be the topological equivalent of the neurotoxin site 5 of VGSCs. Further elucidation of this previously undescribed binding site may explain why most ciguatoxins activate VGSCs, whereas others inhibit voltage-dependent potassium (Kv) channels. This previously undescribed Kv neurotoxin site may have wide implications not only for our understanding of channel function at the molecular level but for future development of drugs to alleviate ciguatera poisoning or to modulate electrical excitability in general.

KCNQ1 loss-of-function mutation impairs gastric acid secretion in mice

The KCNQ1 channel is abundantly expressed in the gastric parietal cells. Although the functional coupling of KCNQ1 with the H(+)/K(+)-ATPase has already been confirmed on the basis of pharmacological kinetics, the effect of a KCNQ1 loss-of-function mutation on gastric acidification remains unclear. In this study, parietal cells and gastric glands from both C57BL/6 J mice (normal control) and J343 mice (mice with a KCNQ1 loss-of-function mutation) were isolated to study the effects of KCNQ1 on gastric acidification. We found that the mutation limited intracellular acidification of parietal cells and H(+) secretion of the stomach in response to histamine. Thus, a KCNQ1 loss-of-function mutation may impair gastric acid secretion.