Expression of HKα2 protein is increased selectively in renal medulla by chronic hypokalemia

Our laboratory has demonstrated by Northern analysis that chronic hypokalemia increases HK alpha(2) (i.e., alpha-subunit of the colonic H(+)-K(+)-ATPase) mRNA abundance in the rat. To determine whether the increase in mRNA correlated with an increase in HK alpha(2) protein, an antibody was raised against a synthetic peptide derived from amino acids 686-698 of the HK alpha(2) sequence. The anti-HK alpha(2) antibody hybridized to rat distal colon membranes which migrated at similar to 100 kDa (expected mobility of HK alpha(2)). HK alpha(2) protein was not detected in plasma membranes from rat whole kidney or stomach (100 mu g) derived from control animals. The antibody was then used to investigate changes in expression of HK alpha(2) in renal cortex, renal medulla, and distal colon in two pathophysiological conditions: 1) chronic hypokalemia (LK) and 2) chronic metabolic acidosis (CMA). In LK rats there was a marked, but selective, increase in the abundance of HK alpha(2) protein in membranes prepared from renal medulla. Nevertheless, a corresponding increase in HK alpha(2) protein abundance was not observed in membranes prepared from the distal colon of LK rats. HK alpha(2) protein abundance in CMA was indistinguishable from controls. Moreover, chronic hypokalemia had no effect on expression of alpha(1)-Na(+)-K(+)-ATPase or HK alpha(1) in kidney or distal colon under any experimental condition. Therefore, HK alpha(2) protein is tissue- and site-specifically upregulated in response to chronic hypokalemia but not by CMA. Furthermore, this regulatory response is localized to the renal medulla.