A study has been made of ouabain binding and the formation of phosphoprotein from ATP and inorganic phosphate (P
i
) with plasma membranes from rabbit and guinea-pig kidney cortex. The aim of the work was first to see whether apparently conflicting results in the literature arise from membranes being prepared by different methods and, secondly, to evaluate the results in relation to the Na pump mechanism. Three different methods were used to prepare membranes, types A, Au and B. The preparations differed markedly when ouabain binding was supported by Mg alone both in the amount bound and in the affinity. Mgdependent binding was influenced by 1 mM P
i
but the extent of stimulation varied according to the preparations. The main effect of P
i
was to decrease the equilibrium dissociation constant marginally for type A membranes but eightfold for type B membranes. In contrast, the maximum number of binding sites was little affected. The membrane affinity for ouabain in relation to Mg and P
i
therefore depended on the method of preparation. In the reaction with Mg-ATP, type Au and B membranes were both phosphorylated to about the same extent. On the other hand, they reacted differently with P
i
, type B membranes being phosphorylated (in the presence of Mg and ouabain) to the same extent as with ATP, whereas under the same conditions, type Au membranes gave only 15 % of the phosphorylation found with ATP. The phosphoprotein, however formed, whether from ATP or P
i
, or type Au or type B membranes, migrated in the same way on gel electrophoresis to give a relative molecular mass of approximately 90000. With each preparation, over a tenfold range of ATPase activity, there was a constant value of 1.2 in the ratio of the maximum phosphorylation by ATP compared with the maximum number of ouabain-binding sites. These results show that membranes prepared in different ways exhibit some consistent properties of the Na pump but also striking anomalies. In view of likely morphological differences in the preparations, it is concluded that the inconsistent features, notably the responses to Mg and P
i
, are an unreliable guide to the pump mechanism.
THE POLARITY OF THE PROXIMAL TUBULE CELL IN RAT KIDNEY