SPECIFICITY OF THE VOLUME-SENSITIVE SODIUM-PUMP INHIBITOR ISOLATED FROM HUMAN PERITONEAL DIALYSATE IN CHRONIC-RENAL-FAILURE

Compromised renal function predisposes to volume-dependent hypertensio n. Increased plasma levels of a sodium pump inhibitor as a possible pa thogenetic factor have been demonstrated by many investigators in such patients, but efforts to identify the responsible agent have led to m any, diverse candidates. Our premise in this study is that candidacy m ust depend on the satisfaction of rigorous criteria, including a speci fic action of the agent on the sodium pump. These criteria included re versibility, concentration dependence, receptor mediation, and an acti on at the appropriate step in the enzyme cycle. These criteria were ap plied to a potent [Na,K]ATPase inhibitor we have identified in the per itoneal 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 Na Cl and water intake. In microsomes that contained both [Na,K]ATPase an d other ATPases, this candidate inhibited only the Na and K dependent, ouabain-sensitive ATPase. It displaced ouabain from the cardioglycosi de binding site and its binding was linked to inhibition. Inhibition w as produced by slowing the pump's dephosphorylation step, the exact ac tion of all cardioglycosides. Finally, the candidate cross reacted wit h 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 cardio glycoside binding site, and hence, meets this crucial criterion for ca ndidacy.