Enhancement of smooth muscle contraction with protein phosphatase inhibitor 1: Activation of inhibitor 1 by cGMP-dependent protein kinase

New method for purification of phosphatase inhibitor 1 (PPI-1) was developed which avoid the phosphorylation of PPI-1 during the purification and provides a high yield of highly pure preparation. Using this preparation, it was shown that PPI-1 was stoichiometrically phosphorylated by cGMP-dependent protein kinase at Thr-35 and the phosphorylated PPI-1 potently inhibited protein phosphatase 1. Addition of the phosphorylated PPI-1 to β-escin-skinned single smooth muscle cells resulted in force development of the cells at the submaximal pCa²⁺. The results suggest that the phosphorylation of PPI-1 can be the mechanism for modifying the Ca²⁺ sensitivity of smooth muscle contractile response.