Evidence supporting a role for KCl cotransporter in the thick ascending limb of Henle's loop

Background. A basolateral Ba2+-sensitive KCl cotransporter has previously b een proposed as participating in basolateral K+ recycling and transepitheli al NaCl reabsorption in the thick ascending limb of Henle's loop (TAL). The aim of the present study was to answer the question as to whether this cot ransporter plays a role in transepithelial K+ reabsorption and whether diet ary Mg2+ deficiency, known to regulate the KCl cotransporter in erythrocyte s, also regulates KCl transport in the TAL. Methods. The effects of a low-Mg2+ diet were investigated on urinary and pl asma K+ concentration in control mice and Mg2+-deficient mice. Transepithel ial Na+, Cl- and K+ net fluxes (J(Na), J(Cl), J(K)), determined in isolated perfused TALs with electron probe analysis or cation-exchange high-perform ance liquid chromatography (HPLC) and electrophysiological parameters (V-te , R-te), were measured in both animal groups. Expression of transcripts for the KCl cotransporter and its possible regulation by low-Mg2+ were studied by RT-PCR in microdissected mouse cortical TAL (CTAL) and medullary TAL (M TAL) segments. Results. In isolated perfused CTALs, basolateral Ba2+ and amiloride induced a large K+ net secretion towards the tubular lumen, paralleled by a 50% de crease in transepithelial NaCl reabsorption. KCC1 transcripts were found in the mouse CTAL and MTAL. A low-Mg2+ diet led to diminished urinary K+ excr etion, lowered plasma K+ concentration and up-regulation of KCC1 transcript s in the TAL. For low-Mg2+ diet, this upregulation was associated with incr eased transepithelial K+ reabsorption in the in vitro-perfused CTAL. Conclusions: Our study provides evidence that the KCl cotransporter, which is functionally expressed in the TAL, plays an important role in transepith elial K+ reabsorption. Direct inhibition of this transporter by Ba2+ and it s indirect inhibition by amiloride lead to a strong transepithelial K+ secr etion and diminished NaCl reabsorption in the TAL. Up-regulation of KCC1 mR NA by dietary Mg2+ restriction is associated with an increased K+ reabsorpt ion in the in vitro perfused CTAL.