Regulation of sodium balance is a critical factor in the maintenance of euvolemia, and dysregulation of renal sodium excretion results in disorders of altered intravascular volume, such as hypertension. The amiloridesensitive epithelial sodium channel (ENaC) is thought to be the only mechanism for sodium transport in the cortical collecting duct (CCD) of the kidney. However, it has been found that much of the sodium absorption in the CCD is actually amiloride insensitive and sensitive to thiazide diuretics, which also block the Na-Cl cotransporter (NCC) located in the distal convoluted tubule. In this study, we have demonstrated the presence of electroneutral, amiloride-resistant, thiazide-sensitive, transepithelial NaCl absorption in mouse CCDs, which persists even with genetic disruption of ENaC. Furthermore, hydrochlorothiazide (HCTZ) increased excretion of Na + and Cl -in mice devoid of the thiazide target NCC, suggesting that an additional mechanism might account for this effect. Studies on isolated CCDs suggested that the parallel action of the Na + -driven Cl -/HCO 3 -exchanger (NDCBE/SLC4A8) and the Na + -independent Cl -/HCO 3 -exchanger (pendrin/SLC26A4) accounted for the electroneutral thiazide-sensitive sodium transport. Furthermore, genetic ablation of SLC4A8 abolished thiazide-sensitive NaCl transport in the CCD. These studies establish what we believe to be a novel role for NDCBE in mediating substantial Na + reabsorption in the CCD and suggest a role for this transporter in the regulation of fluid homeostasis in mice.

Mutations in the serine-threonine kinase WNK4 [with no lysine (K) 4] cause pseudohypoaldosteronism type II, a Mendelian disease featuring hypertension with hyperkalemia. In the kidney, WNK4 regulates the balance between NaCl reabsorption and K ؉ secretion via variable inhibition of the thiazide-sensistive NaCl cotransporter and the K ؉ channel ROMK. We now demonstrate expression of WNK4 mRNA and protein outside the kidney. In extrarenal tissues, WNK4 is found almost exclusively in polarized epithelia, variably associating with tight junctions, lateral membranes, and cytoplasm. Epithelia expressing WNK4 include sweat ducts, colonic crypts, pancreatic ducts, bile ducts, and epididymis. WNK4 is also expressed in the specialized endothelium of the blood-brain barrier. These epithelia and endothelium all play important roles in Cl ؊ transport. Because WNK4 is known to regulate renal Cl ؊ handling, we tested WNK4's effect on the activity of mediators of epithelial Cl ؊ flux whose extrarenal expression overlaps with WNK4. WNK4 proved to be a potent inhibitor of the activity of both the Na ؉ -K ؉ -2Cl ؊ cotransporter (NKCC1) and the Cl ؊ ͞base exchanger SLC26A6 (CFEX) (>95% inhibition of NKCC1-mediated 86 Rb influx, P < 0.001; >80% inhibition of CFEX-mediated [ 14 C] formate uptake, P < 0.001), mediators of Cl ؊ flux across basolateral and apical membranes, respectively. In contrast, WNK4 showed no inhibition of pendrin, a related Cl ؊ ͞base exchanger. These findings indicate a general role for WNK4 in the regulation of electrolyte flux in diverse epithelia. Moreover, they reveal that WNK4 regulates the activities of a diverse group of structurally unrelated ion channels, cotransporters, and exchangers. P olarized epithelia regulate the transepithelial flux of electrolytes and solutes, thereby maintaining the proper ionic composition and volume of different fluid compartments (1). Among these, Cl Ϫ is the most abundant biological anion and the predominant permeating anionic species in all organisms. Transepithelial Cl Ϫ flux can occur by either transcellular or paracellular routes. Cl Ϫ channels, cotransporters, and exchangers mediate transcellular movement across both apical and basolateral membranes; paracellular pathways allow selective Cl Ϫ flux across tight junctions (TJs) (1). The proper regulation of transepithelial Cl Ϫ transport is required for a wide variety of physiologic functions including the regulation of intravascular volume and blood pressure, clearance of lung water, and establishment of intralumenal flow required for the passage of bile acids, exocrine pancreatic enzymes, sweat, and semen via their respective ducts.We have recently identified a pathway that regulates renal Cl Ϫ flux. By positional cloning, we demonstrated that mutations in the serine-threonine kinases WNK1 [with no lysine (K) 1] and WNK4 result in increased renal Cl Ϫ reabsorption and impaired K ϩ secretion, manifesting as the inherited syndrome of hypertension and hyperkalemia, pseudohypoaldosteronism type II (PHAII; ref. 2). WNK1 an...

No abstract

Hyperoxaluria is a major risk factor for kidney stones and has no specific therapy, although colonization is associated with reduced stone risk. interacts with colonic epithelium and induces colonic oxalate secretion, thereby reducing urinary oxalate excretion, an unknown secretagogue. The difficulties in sustaining colonization underscore the need to identify the derived factors inducing colonic oxalate secretion. We therefore evaluated the effects of culture conditioned medium (CM) on apical C-oxalate uptake by human intestinal Caco-2-BBE cells. Compared with control medium, CM significantly stimulated oxalate uptake (>2.4-fold), whereas CM from did not. Treating the CM with heat or pepsin completely abolished this bioactivity, and selective ultrafiltration of the CM revealed that the -derived factors have molecular masses of 10-30 kDa. Treatment with the protein kinase A inhibitor H89 or the anion exchange inhibitor 4,4'-diisothiocyano-2,2'-stilbenedisulfonic acid completely blocked the CM-induced oxalate transport. Knockdown of the oxalate transporter SLC26A6 also significantly restricted the induction of oxalate transport by CM. In a mouse model of primary hyperoxaluria type 1, rectal administration of CM significantly reduced (>32.5%) urinary oxalate excretion and stimulated (>42%) distal colonic oxalate secretion. We conclude that -derived bioactive factors stimulate oxalate transport in intestinal cells through mechanisms including PKA activation. The reduction in urinary oxalate excretion in hyperoxaluric mice treated with CM reflects the retention of biologic activity and the therapeutic potential of these factors.