Abstract
Within the CCD of the distal nephron of the rabbit, the BK (maxi K) channel mediates Ca2+- and/or stretch-dependent flow-induced K+ secretion (FIKS) and contributes to K+ adaptation in response to dietary K+ loading. An unresolved question is whether BK channels in intercalated cells (ICs) and/or principal cells (PCs) in the CCD mediate these K+ secretory processes. In support of a role for ICs in FIKS is the higher density of immunoreactive apical BKα (pore-forming subunit) and functional BK channel activity than detected in PCs, and an increase in IC BKα expression in response to a high-K+ diet. PCs possess a single apical cilium which has been proposed to serve as a mechanosensor; direct manipulation of cilia leads to increases in cell Ca2+ concentration, albeit of nonciliary origin. Immunoperfusion of isolated and fixed CCDs isolated from control K+-fed rabbits with channel subunit-specific antibodies revealed colocalization of immunodetectable BKα- and β1-subunits in cilia as well as on the apical membrane of cilia-expressing PCs. Ciliary BK channels were more easily detected in rabbits fed a low-K+ vs. high-K+ diet. Single-channel recordings of cilia revealed K+ channels with conductance and kinetics typical of the BK channel. The observations that 1) FIKS was preserved but 2) the high-amplitude Ca2+ peak elicited by flow was reduced in microperfused CCDs subject to pharmacological deciliation suggest that cilia BK channels do not contribute to K+ secretion in this segment, but that cilia serve as modulators of cell signaling.