Phosphorylation of protein phosphatase-1 inhibitors, inhibitor-1 and DARPP-32, in renal medulla.

Inhibitor-1 and DARPP-32 (dopamine and cAMP-regulated phosphoprotein, Mr 32 kDa) are each phosphorylated by cAMP-dependent protein kinase, resulting in their conversion to potent inhibitors of protein phosphatase-1. Protein phosphatase-1 is involved in the regulation of Na(+) reabsorption from renal tubule by modulating the activity of Na(+),K(+)-ATPase. In this study, we have investigated the regulation of inhibitor-1 and DARPP-32 phosphorylation in slices of renal medulla. Activation of cAMP-dependent protein kinase by forskolin and 8-bromo-cAMP increased the level of phosphorylated inhibitor-1. Okadaic acid (1 microM), used to inhibit protein phosphatase-2A, increased the level of phosphorylated inhibitor-1, but cyclosporin A had no effect. DARPP-32, like inhibitor-1, was phosphorylated by cAMP-dependent protein kinase and dephosphorylated only by protein phosphatase-2A. These data demonstrate that the phosphorylation of inhibitor-1 and DARPP-32 is regulated by the balance of phosphorylation by cAMP-dependent protein kinase and dephosphorylation by protein phosphatase-2A in renal medulla. Furthermore, the phosphorylation step is regulated by pharmacological stimuli such as activation of beta(1)-adrenoceptors and dopamine D1 receptors.