Aldosterone-Induced Increase in Protein Phosphatase Activity of Toad Bladder

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Abstract
A study has been carried out of the effect of aldosterone on the endogenous phosphorylation and dephosphorylation of membrane-bound and of soluble proteins from toad bladder. Membrane-bound protein D (apparent molecular weight, 49,000), a protein which may possibly be involved in the regulation of sodium transport across the mucosal epithelium of toad bladder, contained a substantial fraction of the radioactive phosphate incorporated into membrane proteins; moreover, it was the only protein to appear consistently in autoradiographs of polyacrylamide gels of phosphorylated membrane proteins. Pretreatment of toad bladder slices with aldosterone caused an increase in the endogenous dephosphorylation of membrane-bound protein D. A half-maximal increase in this dephosphorylation occurred at an aldosterone concentration of 20-40 nM. The increase in protein D phosphatase activity induced by aldosterone was prevented by inhibitors of RNA and protein synthesis as well as by spironolactone, a specific antagonist of aldosterone. The mineralocorticoid, 9alpha-fluorohydrocortisone, also increased protein D phosphatase activity, but testosterone did not. Aldosterone also increased the removal of [(32)P]phosphate from protein D in the cell sap. In contrast to the increase in protein D phosphatase activity, aldosterone had little effect on the phosphorylation of protein D by endogenous protein D kinase. In some experiments, effects of aldosterone and of cAMP, qualitatively similar to those found with protein D, were also observed on the phosphorylation and dephosphorylation of a protein with an apparent molecular weight of 37,000, in both the microsomal and cell sap fractions. No consistent effect of preincubation with aldosterone or of cAMP was observed on any membrane-bound or cell sap protein other than protein D and the 37,000 dalton protein.