Calcium-sensing Receptor Decreases Cell Surface Expression of the Inwardly Rectifying K+ Channel Kir4.1

The Ca2+-sensing receptor (CaR) regulates salt and water transport in the kidney as demonstrated by the association of gain of function CaR mutations with a Bartter syndrome-like, salt-wasting phenotype, but the precise mechanism for this effect is not fully established. We found previously that the CaR interacts with and inactivates an inwardly rectifying K+ channel, Kir4.1, which is expressed in the distal nephron that contributes to the basolateral K+ conductance, and in which loss of function mutations are associated with a complex phenotype that includes renal salt wasting. We now find that CaR inactivates Kir4.1 by reducing its cell surface expression. Mutant CaRs reduced Kir4.1 cell surface expression and current density in HEK-293 cells in proportion to their signaling activity. Mutant, activated G alpha(q) reduced cell surface expression and current density of Kir4.1, and these effects were blocked by RGS4, a protein that blocks signaling via G alpha(i) and G alpha(q). Other alpha subunits had insignificant effects. Knockdown of caveolin-1 blocked the effect of G alpha(q) on Kir4.1, whereas knockdown of the clathrin heavy chain had no effect. CaR had no comparable effect on the renal outer medullary K+ channel, an apical membrane distal nephron K+ channel that is internalized by clathrin-coated vesicles. Co-immunoprecipitation studies showed that the CaR and Kir4.1 physically associate with caveolin-1 in HEK cells and in kidney extracts. Thus, the CaR decreases cell surface expression of Kir4.1 channels via a mechanism that involves G alpha(q) and caveolin. These results provide a novel molecular basis for the inhibition of renal NaCl transport by the CaR.