BK-β1 subunit: immunolocalization in the mammalian connecting tubule and its role in the kaliuretic response to volume expansion

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Abstract
Large, Ca2+-activated K+channels (BK), comprised of α- and β-subunits, mediate K+secretion during high flow rates in distal nephron segments. Because the BK-β1 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-β1 in the kaliuretic response to volume expansion, the rate of K+excretion (UKV) vs. varied urinary flow rates were determined in wild-type and BK-β1 knockout mice (BK-β1−/−). 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 UKV 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-β1−/−mice, UKV did not vary with changing flow rates ( r2= 0.15, P = 0.08, n = 20). Using immunohistochemical techniques, we found that BK-β1 was strongly expressed in the apical membrane of the murine distal nephron and that 98% of BK-β1 protein detected by histochemistry colocalized with NCX, a marker of connecting tubules (CNT). Both BK-β1 and NCX colocalized with BK-α in separate experiments. Furthermore, we confirmed BK-β1 protein expression in the apical membrane of connecting tubules in rabbits. BK-β1 RNA from rabbit CNT was sequenced and was identical to previously published rabbit muscle sequences. These data show that the BK-β1 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.