Phosphatase toward MAP kinase is regulated by osmolarity in Madin-Darby canine kidney (MDCK) cells

We have reported that MAP kinase and its activator were activated by increase in extracellular osmolarity in Madin-Darby canine kidney (MDCK) cells [J. Clin. Invest. 93 (1994) 2387-2392]. The activation of MAP kinase quickly disappeared when cells in hypertonicity were shifted to isotonicity. Present study was planned to elucidate the mechanism for the inactivation of MAP kinase when osmolarity decreased. Combination of two different phosphatase inhibitors, 10(-6) M okadaic acid and 0.2 mM sodium orthovanadate, blocked the inactivation of MAP kinase after the decrease in osmolarity. We also demonstrated that phosphatase toward MAP kinase was activated in response to the decrease in osmolarity. These results suggest that MAP kinase is inactivated by phosphatase that is activated when osmolarity decreased.