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Role of BKβ1 in Na⁺ reabsorption by cortical collecting ducts of Na⁺-deprived mice

Department of Cellular and Integrative Physiology, University of Nebraska Medical Center, Omaha, Nebraska

Submitted 3 April 2009 ; accepted in final form 18 May 2009

On a low-Na⁺ diet (LNa⁺), urinary Na⁺ loss is prevented by aldosterone-induced Na⁺ reabsorption through epithelial Na⁺ channels (ENaC) in the connecting tubules (CNT) and cortical collecting ducts (CCD). However, the mechanism whereby K⁺ loss is minimized and Na⁺ reabsorption is maximized in the face of a reduced lumen-to-bath Na⁺ gradient is not fully understood. The large-conductance calcium-activated potassium channel (BK)β1 subunit (gene: Kcnmb1), which has a role in K⁺ secretion in the CNT, is absent in the CCD in mice on a control diet. We hypothesized that BK/β1 helps to maximize Na⁺ reabsorption during Na⁺ deficiency. With LNa⁺, the Na⁺ clearance of Kcnmb1-mutant mice (Kcnmb1^–/–) was 45% greater and the plasma Na⁺ concentration and osmolality were significantly reduced compared with wild-type mouse (WT) controls. On LNa⁺, Kcnmb1^–/– exhibited exacerbated volume depletion (higher Hct and weight loss) compared with WT. LNa⁺, which did not affect the mean arterial blood pressure (MAP) of WT, significantly reduced MAP of Kcnmb1^–/–. The plasma aldosterone concentration of Kcnmb1^–/– on LNa⁺ was significantly elevated compared with Kcnmb1^–/– on a control diet but was not different from WT on LNa⁺. Immunohistochemical staining revealed that BK and BKβ1, which were absent in the principal cells (PCs) of the CCD, were localized on the basolateral membrane (BSM) of PCs of WT on LNa⁺. Moreover, BK was absent from the BSM of PCs of Na⁺-deficient Kcnmb1^–/–. We conclude that part of the mechanism to maximize Na⁺ reabsorption during Na⁺ deficiency is the placement of BK/β1 channels in the BSM of CCD PCs.

BK channel; maxi K; mineralocorticoids; adrenal cortex; potassium secretion