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

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 (U_KV) 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 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-β1^-/- 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-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.