Specificity of the volume-sensitive sodium pump inhibitor isolated from human peritoneal dialysate in chronic renal...

Compromised renal function predisposes to volume-dependent hypertension. Increased plasma levels of a sodium pump inhibitor as a possible pathogenetic factor have been demonstrated by many investigators in such patients, but efforts to identify the responsible agent have led to many, diverse candidates. Our premise in this study is that candidacy must depend on the satisfaction of rigorous criteria, including a specific action of the agent on the sodium pump. These criteria included reversibility, concentration dependence, receptor mediation, and an action at the appropriate step in the enzyme cycle. These criteria were applied to a potent [Na,K]ATPase inhibitor we have identified in the peritoneal dialysate of patients with chronic renal failure, present only during extracellular fluid volume expansion, the levels of which are correlated with the blood pressure rise that results from excessive NaCl and water intake. In microsomes that contained both [Na,K]ATPase and other ATPases, this candidate inhibited only the Na and K dependent, ouabain-sensitive ATPase. It displaced ouabain from the cardioglycoside binding site and its binding was linked to inhibition. Inhibition was produced by slowing the pump's dephosphorylation step, the exact action of all cardioglycosides. Finally, the candidate cross reacted with a digoxin Fab fragment and this Fab reversed its inhibition of [Na,K]ATPase. Together, these experiments demonstrate that the PD candidate specifically, and reversibly, inhibits the sodium pump via the cardioglycoside binding site, and hence, meets this crucial criterion for candidacy.