Modulatory effect of some steroid hormones, their glucuronides and ouabain-like compounds on Cavia cobaya kidney Na⁺,K⁺-ATPase activity

1. Ouabain-like compounds (approx. mol. wt 700, 2,000 and 4,000 Da) were purified from plasma of essential hypertensive patients. 2. Dose-response experiments performed with (a) steroid hormones, (b) their glucuronides and (c) ouabain-like compounds, emphasize a modulatory effect [activation of the Na,K-ATPase at very low concentrations of ligand, inhibition at higher levels; apparent K_i: (a) between 1 and 0.5 mM; (b) between 1 and 0.5 μM; and (c) between 10 and 1 nM; maximum enhancement of the enzymatic activity: (a) + 20%; (b) + 45%; and (c) + 100%]. 3. Displacement experiments of [³H]ouabain evidence a high competition of the ligands towards the cardioglycoside. The relative I₅₀s are: (a) between 1 and 0.5 mM; (b) between 10 and 1 μM; and (c) between 10 and 0.01 nM.