STUDIES ON THE SUBCELLULAR-DISTRIBUTION OF (NA+ + K+)-ATPASE, K+-STIMULATED ATPASE AND HCO3--STIMULATED ATPASE ACTIVITIES IN MALPIGHIAN TUBULES OF LOCUSTA-MIGRATORIA L

The present study confirms previous reports of the presence of (Na+ + K+)-ATPase and anion-stimulated ATPase activity in Malpighian tubules of Locusta. In addition, the presence of a K+-stimulated, ouabain-insensitive ATPase activity has been identified in microsomal fractions. Differential and sucrose density-gradient centrifugation of homogenates has been used to separate membrane fractions which are rich in mitochondria, apical membranes and basolateral membranes; as indicated by the presence of succinate dehydrogenase and the presence or absence of non-specific alkaline phosphatase activity, respectively. Relatively high specific (Na+ + K+)-ATPase activity was associated with the basolateral membrane-rich fractions with only low levels of this activity being associated with the apical membrane-rich preparation. K+-stimulated ATPase activity was also associated, predominantly, with the basolateral membrane-rich fractions. However, comparison of the distribution of this activity with that of the (Na+ + K+)-ATPase suggests that the two enzymes did not co-separate. The possibility that the K+-stimulated ATPase was not associated with the basolateral plasma membrane is discussed. Anion-stimulated ATPase activity was found in the apical and basolateral membrane-rich fractions and in the fraction containing mainly mitochondria. Nevertheless, the fact that this bicarbonate-stimulated activity did not co-separate with succinate dehydrogenase activity suggests that it was not exclusively mitochondrial in origin. These results are consistent with physiological studies indicating a basolateral (Na+ + K+)-ATPase but do not support the K+-stimulated ATPase as a candidate for the apical electrogenic pump. The possible role of the bicarbonate-stimulated ATPase activity in ion transport across both the basolateral and apical cell membranes is discussed.