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

在哺乳动物平滑肌组织中，蛋白磷酸酶的组成和生理功能已经用冈田酸（OA）（2A型和1型蛋白磷酸酶的有效抑制剂）和抑制剂2（1型磷酸酶的内在抑制因子）进行了研究。[1]在豚鼠平滑肌组织提取物中，我们鉴定出以下四种不同的蛋白磷酸酶活性：对OA敏感的抑制剂2敏感的（1型）磷酸酶活性，Mg^<2+>依赖的和OA不敏感的（2C型）磷酸酶活性，和两种对OA敏感性不同的2A型样磷酸酶活性。我们还得到的结果表明，与冈田酸亲和力较低的2A型样组分可能是负责肌球蛋白轻链去磷酸化，诱导平滑肌松弛。[2]在离体兔气管肌细胞中，我们发现细胞外应用异丙肾上腺素或细胞内应用蛋白激酶A都能可逆地增加Ca^2+依赖性K^+通道的开放概率。我们还观察到，在冈田酸的存在下，这种作用显著增强和延长。[3]到目前为止，蛋白磷酸酶制剂对磷酸对硝基苯酯（pNPP）的脱磷酸作用被认为是由于碱性磷酸酶的污染。我们已经发现蛋白磷酸酶的活性对OA非常敏感，其程度与它们对磷酸化蛋白的活性相同。由于OA已显示对碱性磷酸酶没有影响，该结果表明针对pNPP的活性是蛋白磷酸酶固有的。

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.