KCNQ1-dependent transport in renal and gastrointestinal epithelia - PubMed (original) (raw)

KCNQ1-dependent transport in renal and gastrointestinal epithelia

Volker Vallon et al. Proc Natl Acad Sci U S A. 2005.

Abstract

Mutations in the gene encoding for the K+ channel alpha-subunit KCNQ1 have been associated with long QT syndrome and deafness. Besides heart and inner ear epithelial cells, KCNQ1 is expressed in a variety of epithelial cells including renal proximal tubule and gastrointestinal tract epithelial cells. At these sites, cellular K+ ions exit through KCNQ1 channel complexes, which may serve to recycle K+ or to maintain cell membrane potential and thus the driving force for electrogenic transepithelial transport, e.g., Na+/glucose cotransport. Employing pharmacologic inhibition and gene knockout, the present study demonstrates the importance of KCNQ1 K+ channel complexes for the maintenance of the driving force for proximal tubular and intestinal Na+ absorption, gastric acid secretion, and cAMP-induced jejunal Cl- secretion. In the kidney, KCNQ1 appears dispensable under basal conditions because of limited substrate delivery for electrogenic Na+ reabsorption to KCNQ1-expressing mid to late proximal tubule. During conditions of increased substrate load, however, luminal KCNQ1 serves to repolarize the proximal tubule and stabilize the driving force for Na+ reabsorption. In mice lacking functional KCNQ1, impaired intestinal absorption is associated with reduced serum vitamin B12 concentrations, mild macrocytic anemia, and fecal loss of Na+ and K+, the latter affecting K+ homeostasis.

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Figures

Fig. 1.

Fig. 1.

_kcnq1_-/- mice present signs of macrocytic anemia. _kcnq1_-/- presented lower hematocrit, red blood cell counts, and hemoglobin concentrations but greater mean erythrocyte volume, which is associated with lower serum vitamin B12 levels. n = 9 mice per genotype. *, P < 0.05 versus kcnq1+/+.

Fig. 2.

Fig. 2.

_kcnq1_-/- mice present normal urinary excretion but fecal loss of Na+ and K+ and tend toward hypokalemia. Mice were kept on low-K+, high-K+, and control diets. (A) Food intake was greater in _kcnq1_-/- on all three diets. bw, Body weight. (B and C) On a low-K+ diet, _kcnq1_-/- presented lower plasma K+ concentrations despite lower plasma aldosterone concentrations. (D and E) In contrast with normal urinary excretion (U), _kcnq1_-/- lost Na+ and K+ through feces (F) out of proportion with food intake when on the control diet. Under the high-K+ diet, both genotypes showed similar plasma K+ and aldosterone concentrations associated with pronounced increases in urinary K+ excretion. n = 6-9 pairs of mice per genotype for metabolic cages and n = 12-18 mice for plasma parameters. *, P < 0.05 versus kcnq1+/+.

Fig. 3.

Fig. 3.

PT cell membrane potential and reabsorption is unaltered under basal conditions but impaired in response to increasing luminal glucose in _kcnq1_-/- mice. (A) Basal delivery of fluid, K+, and glucose to last proximal (lp) and first distal (dist) surface loop and urine (u) were normal in _kcnq1_-/-. n = 8 or 9 distal and 22-29 PT in six mice per genotype. (B) Evidence for net fluxes of K+ into early PT in both genotypes. Individual collections are shown. (C) Glucose concentrations rapidly dropped along PT irrespective of genotype, indicating low basal loads to KCNQ1-expressing mid and late PT. Individual collections are shown. (D) Basal potential differences across basolateral cell membrane of mid to late PT (PDbl) were not different between genotypes. Adding 5 mM luminal glucose elicited depolarization only in _kcnq1_-/-. n = 5 or 6 tubules in four mice per genotype. (E) Loading mid PTs with ATF containing 5 mM glucose revealed lower reabsorption of Na+, fluid, and glucose up to the distal tubule in _kcnq1_-/-. n = 12 or 13 nephrons in three mice per genotype. *, P < 0.05 versus kcnq1+/+.

Fig. 4.

Fig. 4.

Pharmacological inhibition of KCNQ1 reduces PT Na+ reabsorption in the presence of luminal glucose. During loop of Henle perfusion from mid PT, fluid was collected from the first superficial distal tubular loop in rats. Collections were performed first during perfusion with ATF containing 0, 5, or 20 mM glucose (basal, b). Tubular absorption was reassessed in a paired fashion (experimental, exp) with the same solution (open circles) or the addition of 10 μM HMR1556 to inhibit KCNQ1 (filled circles). n = 5-8 nephrons per perfusion in eight rats. *, P < 0.05 versus time control. ND, not determined.

Fig. 5.

Fig. 5.

Loss of gastric pH recovery in _kcnq1_-/- mice. (A) Representative original photographs (oil immersion ×40) and pH recovery tracings after an formula image pulse in histamine-prestimulated gastric glands of kcnq1+/+ (Upper) and _kcnq1_-/- (Lower). Gastric glands of _kcnq1_-/- appear wider and show impaired realkalinization under Na+-free conditions (H+/K+-ATPase-mediated) but readily normalize their intracellular pH after the readdition of Na+. Nig, nigericin (for calibration). (B) Realkalinization is blunted with both histamine and carbachol (each 100 μM) prestimulation in _kcnq1_-/-. n = 81-124 cells in six or seven glands of five or six mice per genotype. *, P < 0.05 versus kcnq1+/+.

Fig. 6.

Fig. 6.

Reduced glucose (Gluc)- and phenylalanine (Phe)-induced currents in jejunum of _kcnq1_-/- mice. (A) Representative original miniUssing-chamber recordings in response to phenylalanine or glucuse (each 20 mM) of kcnq1+/+ (Upper) and _kcnq1_-/- (Lower). Ctr, control. (B) For both phenylalanine and glucose, the resultant change in equivalent short circuit current is reduced by half in _kcnq1_-/- versus kcnq1+/+. n = 6 mice per genotype. *, P < 0.05 versus kcnq1+/+.

Fig. 7.

Fig. 7.

Forskolin-induced jejunal Cl- secretion is reduced but not abolished in _kcnq1_-/- mice. (A) Representative original tracings from jejunal Cl- secretion experiments with basolateral application of 2.5 μM forskolin (FSK) and 10 μM chromanol 293B (293B) in kcnq1+/+ (Upper) and _kcnq1_-/- (Lower). (B) Forskolin-induced Cl- secretion was reduced by 50% in _kcnq1_-/- versus kcnq1+/+. Adding chromanol 293B (293B) blunted the effect of forskolin in kcnq1+/+ but was ineffective in _kcnq1_-/-. Activation of Ca2+-activated Cl- secretion by 100 μM carbachol (CCH) was not different between genotypes. n = 9 mice per genotype. *, P < 0.05 versus kcnq1+/+.

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