Quantification and distribution of big conductance Ca-activated K channels in kidney epithelia

Big conductance Ca²⁺ activated K⁺ channels (BK channels) is an abundant channel present in almost all kind of tissue. The accurate quantity and especially the precise distribution of this channel in kidney epithelia are, however, still debated. The aim of the present study has therefore been to examine the presence of BK channels in kidney epithelia and determine the actual number and distribution of these channels. For this purpose, a selective peptidyl ligand for BK channels called iberiotoxin or the radiolabeled double mutant analog ¹²⁵I-IbTX-D19Y/Y36F has been employed. The presence of BK channels were determined by a isotope flux assay where up to 44% of the total K⁺ channel activity could be inhibited by iberiotoxin indicating that BK channels are widely present in kidney epithelia. Consistent with these functional studies, ¹²⁵I-IbTX-D19Y/Y36F binds to membrane vesicles from outer cortex, outer medulla and inner medulla with B_max values (in fmol/mg protein) of 6.8, 2.6 and 21.4, respectively. These studies were performed applying rabbit kidney epithelia tissue. The distinct distribution of BK channels in both rabbit and rat kidney epithelia was confirmed by autoradiography and immunohistochemical studies. In cortical collecting ducts, BK channels were exclusively located in principal cells while no channels could be found in intercalated cells. The abundant and distinct distribution in kidney epithelia talks in favor for BK channels being important contributors in maintaining salt and water homeostasis.