The results support previous findings that vasopressin produces dephosphorylation of a toad bladder membrane protein of 50000 mol. wt. At vasopressin concentrations of 50 mu./ml phosphorylation of this protein was 60 \m=+-\7% of the control level, and maximal natriferic and hydro-osmotic responses were observed in intact bladders. However, at concentrations of vasopressin of 10 mu./ml when a maximal natriferic response but no significant hydro-osmotic response were observed, there was no significant difference in phosphorylation of the proteins from control or vasopressin\x=req-\ treated bladders.We conclude that the 50000 mol. wt membrane protein is likely to be associated with the hydro-osmotic response of the toad bladder to vasopressin.

Summary1. A cAMP-dependent protein kinase enzyme has been isolated from toad bladder epithelium.2. This enzyme catalyses the phosphorylation of histones; a reaction stimulated by 0'1 ,uM cAMP. 3. Activity of this enzyme in the presence of 0-1 umM cAMP was significantly inhibited by 100 ,mM mercuderamide, bumetanide and frusemide. A small, though statistically insignificant inhibition was seen with the same concentrations of hydrochlorothiazide and ethacrynic acid. 4. The half maximal inhibition was achieved with mercuderamide, 5 pmM, and with frusemide and bumetanide 600 ,uM.

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