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1 subunit: Immunolocalization in the mammalian connecting tubule and its role in the kaliuretic response to volume expansion
1 Department of Cellular and Integrative Physiology, University of Nebraska Medical Center, Omaha, NE, USA
* To whom correspondence should be addressed. E-mail: ssansom{at}unmc.edu.
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 flowinduced 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 (UKV) versus varied urinary flow rates were determined in wild-type and BK-1
knockout mice (BK-1-/-). When flow rate was varied by volume expansion (2ml/hr/25gBW) for 30 to 60 minutes 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-1-/- mice, UKV 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). BK-1 and NCX also colocalized with BK- in separate experiments. Furthermore, we
confirmed BK-1 protein expression in the apical membrane of connecting tubules in rabbits. 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.
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