CELLULAR-LOCALIZATION AND REGULATION OF CHIF IN KIDNEY AND COLON

Channel inducing factor (CHIF) is a novel cDNA recently cloned from a rat distal colon cDNA library of dexamethasone-treated animals. While its expression in Xenopus oocytes evokes a potassium channel activity similar to that induced by Isk. (minK), its cellular role is not clear . CHIF exhibits significant homologies with proteins that are putative ly regulatory (phospholemman, gamma-subunit of Na+-K+-ATPase, Mat-8) w hile it differs from the small-conductance potassium channel Isk. We h ave studied the tissue specificity of CHIF expression in rat by in sit u hybridization. CHIF is selectively present in the distal parts of th e nephron (medullary and papillary collecting ducts and end portions o f cortical collecting tubule) and in the epithelial cells of the dista l colon. No expression of CHIF was found in renal proximal tubule, loo p of Henle and distal tubule, proximal colon, small intestine, lung, c horoid plexus, salivary glands, or brain. To gain some insight into CH IF function, we have investigated, using in situ hybridization and rib onuclease protection assay, whether CHIF mRNA expression could be alte red in some situations. In the distal colon, corticosteroid hormones, sodium restriction, low-potassium diet, and metabolic acidosis signifi cantly increased CHIF mRNA expression. In the kidney, metabolic acidos is was the only condition that showed an increase in CHIF mRNA express ion. Some of these treatments also altered the expression of the colon ic H+-K+-ATPase mRNA. In summary, CHIF mRNA is selectively expressed i n the medullary collecting duct of the kidney and in the epithelium of the distal colon; its expression varies differently in these two targ et tissues after alterations in corticosteroid status, potassium deple tion, and metabolic acidosis. The precise cell-specific functions of C HIF remain to be established.