Characteristics of ouabain binding to isolated trout hepatocytes

Summary Na⁺, K⁺ exchanges were studied in isolated hepatocytes of the rainbow trout, Salmo gairdneri. Ouabain at 10^–4 M produced maximal inhibition (95%) of K⁺ uptake and enhanced intracellular Na⁺ accumulation, showing that active fluxes account for a very large proportion of Na⁺ and K⁺ exchanges. Inhibition of the Na–K pump by ouabain was significant at low concentrations (10^–8 M). When external K⁺ concentration was reduced from 7 mM to 0.5 mM, half maximum inhibition (IC₅₀) of K⁺ uptake was obtained at a 22-fold lower concentration of ouabain confirming that ouabain and potassium compete at the same pump site. Time-course analysis of ³H]ouabain binding indicated a two-component kinetics: one component saturable and dependent on K⁺ concentration in the medium, the other linear and independent of external K⁺. The ouabain binding site number, determined by Scatchard plots, remained constant (ca. 2.5·10⁵ per cell) and independent of the external K⁺ concentration (7, 0.5 or 0 mM), while the dissociation constant (K_D) decreased from 4.2 M to 7.3 nM when K⁺ was removed from the Hank's medium. These ouabain binding sites are characterized by an exceptionally low turnover rate (400 min^–1), as estimated from ouabain-sensitive K⁺ flux, in comparison to those described in other cell types of higher vertebrates. At each external K⁺ concentration studied, the inhibition of K⁺ uptake and ouabain binding measured as a function of ouabain concentration indicated a strict correlation between the degree of K pump inhibition and the amount of bound glycoside.