The mechanosensitive BKα/β1 channel localizes to cilia of principal cells in rabbit cortical collecting duct (CCD)

Abstract

Within the CCD of the distal nephron of the rabbit, the BK (maxi K) channel mediates Ca- 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 Ca 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 Ca 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.

Intercalated cell BKα subunit is required for flow-induced K+ secretion

“…ICs represent less than 30% of the cells present in the CCD and a very small subset of cells present in the cortex of the mouse kidney, and BK channels are present in both PCs and ICs of the CCD (24). In fact, we did not detect a difference in steady state abundance of BKα message in isolated CCDs (10-12.5 mm total length per sample) between floxed control and IC-BKα-KO mice by quantitative PCR (qPCR); the relative expression of BKα in CCDs from 3 KO versus 3 control mice was 1.4 ± 0.5 (P = 0.18).…”

“…In vitro microperfused CCDs isolated from IC-BKα-KO mice fail to exhibit FIKS. The impact of the cell-specific disruption of BKα on basal and flow-induced net K + secretion (JK) and Na + absorption (JNa) was studied in microperfused CCDs isolated from male and female mice fed a HK diet to maximize BK channel expression (23,24). In CCDs from control mice (n = 7, including 4 males and 3 females), an increase in tubular fluid flow rate from 0.9 (slow) to 5.5 (fast) nL/min per mm was associated with a significant increase in JNa (from 7.7 ± 8.3 to 42.7 ± 9.6 pmol/min per mm; P < 0.01) and JK (from -1.6 ± 1.4 to -5.9 ± 1.6 pmol/min per mm; P < 0.01) ( Figure 5, A and D).…”

“…A major question -as yet unresolved, to our knowledge -is whether ICs and/or PCs mediate BK channel-mediated FIKS and/or K + adaptation in the ASDN. In support of a role for ICs in this process are the observations that (a) electrophysiologic (8)(9)(10)26) and immunolocalization (23,24,(27)(28)(29) studies reveal a higher density of BK channels in ICs versus PCs in the ASDN; (b) when rats are fed a high K + (HK) diet for as little as 18 hours, a significant fraction of K + excretion becomes amiloride insensitive and, BK channels are expressed in intercalated cells (ICs) and principal cells (PCs) in the cortical collecting duct (CCD) of the mammalian kidney and have been proposed to be responsible for flow-induced K + secretion (FIKS) and K + adaptation. To examine the IC-specific role of BK channels, we generated a mouse with targeted disruption of the pore-forming BK α subunit (BKα) in ICs (IC-BKα-KO).…”

“…Both ROMK and BK channels play major roles in the renal adaptation to dietary K + (17,20,(22)(23)(24). Expression of ROMK is restricted to principal cells (PCs), cells with robust basolateral Na + /K + -ATPase activity that have traditionally been considered to mediate electrogenic epithelial Na + channel-dependent (ENaC-dependent) transepithelial Na + absorption and K + secretion (1)(2)(3)(4).…”

Aldosterone controls primary cilium length and cell size in renal collecting duct principal cells

“…There has been some experimental evidence that suggests a connection between primary cilia and the sodium transport by CD cells. Drug‐induced removal of primary cilia abolishes fluid flow‐induced calcium signaling in Madin Darby Canine Kidney (MDCK) cells as well as the flow dependency of Na + reabsorption in rabbit cortical CD is almost abolished . In CD principal cells, lengthening of the cilia is correlated with decreased intracellular calcium entry, elevated intracellular cyclic adenosine monophosphate (cAMP) levels, and activation of protein kinase A .…”