Dietary potassium restriction stimulates endocytosis of ROMK channel in rat cortical collecting duct

ROMK potassium channels are present in the cortical collecting ducts (CCDs) of the kidney and serve as the exit pathways for K⁺ secretion in this nephron segment. Dietary K⁺ restriction reduces the abundance of ROMK in the kidney. We have previously shown that ROMK undergoes endocytosis via clathrin-coated vesicles in Xenopus laevis oocytes and in cultured cells. Here, we examined the effect of dietary K⁺ restriction on endocytosis of ROMK in CCDs using double-labeling immunofluorescent staining and confocal microscopic imaging in whole kidney sections as well as in individually isolated tubules. We found that ROMK abundance in kidney cortex and CCDs was reduced in rats fed a K⁺-restricted diet compared with rats fed the control K⁺ diet. In the control animals, ROMK staining was preferentially localized to the apical membrane of CCDs. Compared with control tubules, ROMK staining in CCDs was markedly shifted toward intracellular locations in animals fed a K⁺-deficient diet for 48 h. Some of the intracellular distribution of ROMK colocalized with an early endosomal marker, early endosomal antigen-1 or with a late endosomal/lysosomal marker, lysosomal membrane glycoprotein-120. These results suggest that K ⁺ restriction reduces the abundance of ROMK in CCDs by increasing endocytosis and degradation of the channel protein. This decrease in the abundance of ROMK is likely important for maintaining K⁺ homeostasis during K⁺ deficiency.