Studies on Physiological Roles of Protein Phosphatases in Mammalian Smooth Muscle Tissues

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In mammalian smooth muscle tissues, the composition and physiological functions of protein phosphatases have been studied with the use of okadaic acid (OA), a potent inhibitor of type 2A and type 1 protein phosphatases, and inhibitor 2, an intrinsic inhibitory factor of type 1 phosphatase. We have obtained the following results :[1] In the extracts of smooth muscle tissues of guinea-pig, we have identified the following four distinct protein phosphatase activities : an inhibitor 2-sensitive (type 1) phosphatase activity which is susceptible to OA, a Mg^<2+>-dependent and OA-insensitive (type 2C) phosphatase activity, and two type 2A-like phosphatase activities with different susceptibilities to OA. We have also obtained results indicating that the type 2A-like fraction with lower affinity to okadaic acid may be responsible for dephosphorylation of myosin light chains to induce relaxation of smooth muscle.[2] In isolated myocytes of rabbit trachea, we have shown that the openstate probability of Ca^<2+>-dependent K^+-channel is reversibly increased by either extracellular application of isoprenaline or intracellular application of protein kinase A. We have also observed that this effect is significantly enhanced and prolonged in the presence of okadaic acid.[3] So far the dephosphorylation of p-nitrophenyl phosphate (pNPP) by protein phosphatase preparations has been thought to be due to contamination by alkaline phosphatases. We have found that the activity of protein phosphatases are very susceptible to OA, to the same extent as is their activity against phosphorylated proteins. As OA has been shown to have no effect on alkaline phosphatases, this result indicates that the activity against pNPP is intrinsic to the protein phosphatases.