SODIUM-PUMP ISOFORM SPECIFICITY FOR THE DIGITALIS-LIKE FACTOR ISOLATED FROM HUMAN PERITONEAL DIALYSATE

We have isolated a labile, specific sodium pump inhibitor or digitalis -like factor from the peritoneal dialysate of volume-expanded renal fa ilure patients whose levels correlated closely with Volume status and brood pressure. This study characterizes the inhibitory profile of thi s agent compared with that of ouabain against the three alpha-isoforms of the sodium pump. We prepared microsomal Na,K-ATPase from rat tissu es representing the highest proportion of one of the alpha-isoforms. B oth Northern and Western blot analyses confirmed that kidney had predo minantly the alpha(1)-isoform, skeletal muscle the alpha(2)-isoform, a nd fetal brain the alpha(3)-isoform. Ouabain (5x10(-6) mol/L) produced little inhibition of kidney Na,K-ATPase (3.4+/-2.0%) but significant inhibition of skeletal muscle (37.2+/-3.7%, P<.001) and fetal brain (3 8.8+/-3.5%, P<.001) activity. Ln contrast, the labile digitalis like f actor, causing comparable inhibition of fetal brain Na,K-ATPase activi ty (33.3+/-4.7%), produced markedly greater inhibition of kidney (42.5 +/-5.6%, P<.001) and moderately greater inhibition of skeletal muscle pump activity (57.7+/-6.3%, P<.05). In addition, the labile digitalis- like factor produced a marked concentration-dependent inhibition of th e a(2)- and a(3)-isoforms (r=.79, P=.00005). Experiments combining the labile digitalislike factor and ouabain confirmed that digitalis-like factor, unlike ouabain, was an effective inhibitor of all three isofo rms in rat, in particular a(2). The different pattern of isoform sensi tivity displayed by the labile digitalis-like factor and ouabain furth er differentiates the two agents and raises some interesting possibili ties about the functional implications of the endogenous factor.