The ouabain inhibition of sugar transport in kidney cortex cells

Abstract

The inhibitory effect of ouabain on the active, Na-dependent transport of α-methyl- d -glucoside by slices of rabbit kidney cortex was studied: A Ki of 0.13 m m was found. The inhibition at low ouabain concentrations (0.02 m m ) could be abolished by increasing [K+]0 to 25 m m , whereas at higher concentrations of ouabain (up to 0.5 m m ), increased [K+]0 was ineffective. Evidence for binding of ouabain by kidney cortex was obtained: (a) preincubation of slices in the presence of ouabain produced a subsequent inhibition of cation and α-methyl-glucoside transport in ouabain-free media; (b) more than 80% of ouabain-3H taken up by the tissue could not be washed out. Binding of ouabain by the tissue showed the following characteristics: (a) at low concentrations of the cardiac glycoside (1–100 μ m ) the saturable binding component was depressed by the absence of Na+, and was abolished by increased [K+]0, 0o, and denaturation of tissue proteins at 80°; (b) binding of ouabain by the nonsaturable tissue component was linearly related to the concentration of the substance in the medium. The maximum number of ouabain-binding sites in rabbit kidney, cortex, corrected for the nonspecific binding of the glycoside, was determined to be 6×1015/g wet wt, and the association constant was of the order of 3×105 m −1; in rats, the value for the maximum number of ouabain-binding sites was at least one order of magnitude lower than in rabbits. The above results correlate with the characteristics of the interaction of ouabain with the (Na++K+) ATPase. It is concluded that the membrane ATPase participates (directly or indirectly) in the Na-dependent transport system for α-methylglucoside.