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BK-1 subunit: immunolocalization in the mammalian connecting tubule and its role in the kaliuretic response to volume expansion

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

Submitted 13 September 2004 ; accepted in final form 18 December 2004

Large, Ca²⁺-activated K⁺ channels (BK), comprised of - and -subunits, mediate K⁺ secretion during high flow rates in distal nephron segments. Because the BK-1 subunit enhances Ca²⁺ sensitivity of BK in a variety of cells, we determined its role in flow-induced K⁺ secretion and its localization in the mammalian nephron. To determine the role of BK-1 in the kaliuretic response to volume expansion, the rate of K⁺ excretion (U_KV) vs. varied urinary flow rates were determined in wild-type and BK-1 knockout mice (BK-1^–/–). When flow rate was varied by volume expansion (2 ml·h^–1·25 g body wt^–1) for 30 to 60 min in wild-type mice, we found that the U_KV increased significantly with increasing urine flow rates (r² = 0.50, P < 0.00001, n = 31), as demonstrated previously in distal nephron of rats and rabbits. However, in BK-1^–/– mice, U_KV did not vary with changing flow rates (r² = 0.15, P = 0.08, n = 20). Using immunohistochemical techniques, we found that BK-1 was strongly expressed in the apical membrane of the murine distal nephron and that 98% of BK-1 protein detected by histochemistry colocalized with NCX, a marker of connecting tubules (CNT). Both BK-1 and NCX colocalized with BK- in separate experiments. Furthermore, we confirmed BK-1 protein expression in the apical membrane of connecting tubules in rabbits. BK-1 RNA from rabbit CNT was sequenced and was identical to previously published rabbit muscle sequences. These data show that the BK-1 accessory subunit is present in the CNT segment of the mammalian distal nephron and has a significant role in the kaliuretic response to increased urinary flow induced by volume expansion.

Maxi K; distal nephron; flow-mediated K⁺ secretion

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