ABSTRACT
1. The interaction between ouabain and K+ and their effects (Na+ + K+)-ATPase activity were studied using microsomes from guinea pig and rat heart. 2. Microsomes were incubated in the presence of various concentrations of K+ and ouabain and ATPase activity was estimated by measuring the inorganic phosphate liberated. The experimental data were analyzed statistically by micro-computer, using a non-linear regressión program based on the steepest descent techinique. 3. The experimental data were best fitted by a model which assumes that ouabain acts like a mixed inhibitor with respect to the apparently cooperative K+ activation of (Na+ + K+)-ATPase. This quantitative approach provieded estimates (with approximate standard deviations) of al the parameters involved in the model. 4. The inhibition constant for the uncompetitive term of the effect was 7 - to 9 - fold higher than the inhibiton constant for the competitive term for both the guinea pig and rat heart preparations. 5. The present results indicate tahta graphical analyses are helpful for illustrative purposes but sugfgests that a computerized, non-linear regression program simultaneously analyzing all the non-linearized data shoul be used to quantify the complex kinetic parameters and to discriminate objectively among possible models