Assessment of a strontium capture technique for cytochemical localization of potassium‐dependent phosphatase in Malpighian tubules of Locusta

Abstract

A strontium capture method, using p‐nitrophenyl phosphate as substrate, was used to determine the subcellular localization of (Na+ + K+)‐ATPase activity in Malpighian tubules of Locusta migratoria L. Ultrastructural studies revealed that (Na+ + K+)‐ATPase activity was restricted to the basolateral plasma membranes with little evidence of activity associated with the apical microvilli. In contrast, alkaline phosphatase activity was specifically associated with the apical cell membrane. Biochemical assays of fixed and non‐fixed tubule homogenates were used to evaluate the p‐nitrophenyl phosphate‐strontium procedure for localization of the phosphatase component of (Na+ + K+)‐ATPase. No significant potassium‐dependent, ouabain‐sensitive p‐nitrophenyl phosphatase activity was demonstrated in homogenates under conditions necessary for the cytochemical procedure, viz fixation, pH 9.0 and the presence of strontium. The significance of the biochemical results are discussed in relation to the validity of such cytochemical techniques for (Na+ + K+)‐ATPase localization.