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

A basolateral Ba²⁺-sensitive KCl cotransporter has previously been proposed as participating in basolateral K⁺ recycling and transepithelial 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 cotransporter plays a role in transepithelial K⁺ reabsorption and whether dietary Mg²⁺ deficiency, known to regulate the KCl cotransporter in erythrocytes, also regulates KCl transport in the TAL.

The effects of a low-Mg²⁺ diet were investigated on urinary and plasma K⁺ concentration in control mice and Mg²⁺-deficient mice. Transepithelial 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-performance 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 (MTAL) segments.

In isolated perfused CTALs, basolateral Ba²⁺ and amiloride induced a large K⁺ net secretion towards the tubular lumen, paralleled by a 50% decrease in transepithelial NaCl reabsorption. KCC1 transcripts were found in the mouse CTAL and MTAL. A low-Mg²⁺ diet led to diminished urinary K⁺ excretion, lowered plasma K⁺ concentration and up-regulation of KCC1 transcripts in the TAL. For low-Mg²⁺ diet, this upregulation was associated with increased transepithelial K⁺ reabsorption in the in vitro-perfused CTAL.

Our study provides evidence that the KCl cotransporter, which is functionally expressed in the TAL, plays an important role in transepithelial K⁺ reabsorption. Direct inhibition of this transporter by Ba²⁺ and its indirect inhibition by amiloride lead to a strong transepithelial K⁺ secretion and diminished NaCl reabsorption in the TAL. Up-regulation of KCC1 mRNA by dietary Mg²⁺ restriction is associated with an increased K⁺ reabsorption in the in vitro perfused CTAL.