Abundant evidence has established a pivotal role for an H+, K+- ATPase in renal K+ homeostasis and acid-base balance. The regulatory response of the H+, K+-ATPases to chronic hypokalemia and chronic acidosis has been localized to the outer medullary and inner medullary collecting ducts (OMCD and IMCD). Two, and perhaps more, alpha-subunit isoforms have been localized to the mammalian kidney, but uncertainties remain with respect to participation of specific alpha isoforms in segmental K+ and acid-base homeostasis. Transport studies have been forced to rely on the effect of """"""""specific"""""""" inhibitors of the H+, K+-ATPase (such as Sch 28080) to identify that component of bicarbonate and/or K+ absorption attributable to this transporter. Since the H+, K+-ATPases exhibit differing sensitivities to ouabain and Sch 28080, it is not known with certainty if modulations in transport and the well established response to chronic metabolic acidosis and hypokalemia are the result of modulation in function of HKalpha1, HKalpha2, HKalpha4, or yet to be identified isoforms. This study is designed to elucidate the pathophysiologic factors which regulate at both molecular and functional levels, at H+, K+-ATPases in collecting duct segments, and in medullary collecting cells in culture. By transfecting mOMCD1, and mIMCD-3 cells with anti-sense HKalpah1, HKalpha2, and HKalpha4, we will define which isoform is responsible for the well-accepted adaptive response to chronic hypokalemia. We will then define the molecular equivalent of the H+, K+-ATPase enzymatic activity characterized as type III, which is upregulated by chronic K+ depletion. This approach will required the synthesis and screening of a subtraction cDNA library. Thirdly, we will determine if aldosterone or endothelin regulate H+, K+-ATPase function in mOMCD1 and mIMCD-3 cells in culture, and if so, we will delineate the alpha, H+, K+-ATPase iosoform responsible. Finally, we will define the contribution of the H+, K+-ATPases to net acid secretion in the OMCDis perfused in vitro during metabolic alkalosis without K+ depletion. Metabolic alkalosis with and without hypokalemia will then be simulated in mOMCD1, cells in culture to delineate whether hypokalemia or alkalemia per se upregulates the alpha H+, K+-ATPase, and if so, which alpha H+, K+-ATPase isoform responds specifically to each condition. These studies will help to elucidate the means by which K+ depletion can maintain metabolic alkalosis. The H+, K+- ATPase remains a candidate gene for abnormal structure and function in inherited and acquired forms of distal renal tubular acidosis. To understand this group of disorders more completely, fundamental studies which elucidate the molecular regulation of this family of transporters will be necessary to further our understanding of the pathophysiology of this disorder.
Showing the most recent 10 out of 35 publications
