REVERSAL OF SODIUM-PUMP INHIBITOR-INDUCED VASCULAR SMOOTH-MUSCLE CONTRACTION WITH DIGIBIND - STOICHIOMETRY AND ITS IMPLICATIONS

The possibility that a circulating sodium pump inhibitor contributes t o the pathogenesis of volume-dependent hypertension via an action on v ascular smooth muscle (VSM) is supported by multiple lines of investig ation, but remains controversial. We had two goals in this study. The first was to compare the pattern of contractile response of rabbit aor ta induced by two candidates, ouabain and a labile sodium pump inhibit or that we have identified in the peritoneal dialysate of volume-expan ded hypertensive patients with chronic renal failure. Our second goal was to examine the ability of Digibind, a Fab fragment of antisera dir ected against digoxin, to reverse VSM contraction induced by both agen ts. Ouabain induced a concentration-dependent contraction, which was d elayed in onset, was gradual, and reached a stable plateau after many hours. The labile sodium pump inhibitor induced a qualitatively simila r series of responses. Digibind rapidly reversed the contractile respo nses to both sodium pump inhibitors, with a rate of relaxation that ma tched that induced by physical removal of the pump inhibitor from the bath. For ouabain, the Digibind:ouabain stoichiometry was highly predi ctable. When Digibind was present in a molar concentration equivalent to that of ouabain, or less, it had no effect. When the Digibind conce ntration was twice that of ouabain, complete relaxation occurred. Alth ough the concentration:VSM response relationship for ouabain was steep , the concentration:effect interaction with Digibind was even more ste ep. The molar concentration of Digibind required to reverse the effect s of the labile endogenous inhibitor from peritoneal dialysate was con sistently lower than that for ouabain, which is compatible with either greater potency of the labile factor in VSM or greater affinity for D igibind. These findings are compatible with a role for one or more end ogenous sodium pump inhibitors as the determinant of vascular smooth m uscle tone in the volume-sensitive hypertension of renal disease.