Figure 7. Apparent affinity for K⁺ of renal (Na⁺+K⁺)ATPase.

The (Na⁺+K⁺)ATPase activity was assayed at the K⁺ concentrations presented on the abscissa (starting with a contaminant concentration of 0.1 mM K⁺ according to flame photometric determinations performed after preparation of the solution was completed). The Na⁺ concentrations ranged from 150 to 100 mM to keep the sum of Na⁺ plus K⁺ concentrations equal to 150 mM in all tubes. The hyperbolic function v = V_max×[K⁺]/(K_0.5+[K⁺]) was adjusted to the experimental points obtained with the use of membranes isolated from control (open circles) and programmed (filled circles) offspring. Data are means ± S.E.M. of at least seven determinations carried out in triplicate with four different preparations from each group (C and P). The abbreviations of the expression correspond to activity at each K⁺ concentration (v), extrapolated maximal velocity (V_max) and to the K⁺ concentration at which v = V_max/2 (K_0.5).