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Smooth Muscle Phosphatases II (SMP-II) which has been purified from turkey gizzards and previously classified as protein phosphatase 2C, is inactive in the absence of divalent cations. Study of the activation of SMP-II by Mg2+ and Mn2+ revealed differences in the modes of activation by these cations. The maximal activation elicited by Mg2+ is 1.5-2.5-fold higher than the maximal Mn2+ activation. However, the latter is achieved at a lower concentration than the maximal Mg2(+)-activation. Furthermore, at low cation concentrations (less than or equal to 2 mM), the Mn2(+)-activated activity is higher than the Mg2(+)-activated activity. In the presence of both cations, the effect of Mn2+ predominates suggesting that the affinity of the enzyme for Mn2+ is greater than for Mg2+. In contrast to Mg2+ and Mn2+, Ca2+ does not activate SMP-II but it was observed to antagonize the effects of Mg2+ and Mn2+. Ca2+ acts as a competitive inhibitor of Mg2+. However, the inhibitory effect at high Ca2+ concentrations is not completely reversed by increasing the Mg2+ concentration. Mn2+ activation is also inhibited by Ca2+ but to a lesser extent. Ca2+ cannot completely inhibit Mn2(+)-activation suggesting that SMP-II has greater affinity for Mn2+ than for Ca2+. The finding that Ca2+ inhibits the activation of SMP-II raises the possibility that Ca2+ may be a regulator of SMP-II in vivo.

Belik, J., Ewa Kerc, and Mary D. Pato. Rat pulmonary arterial smooth muscle myosin light chain kinase and phosphatase activities decrease with age. Am J Physiol Lung Cell Mol Physiol 290: L509-L516, 2006. First published October 7, 2005 doi:10.1152/ajplung.00145.2005.-We and others have shown that the fetal pulmonary arterial smooth muscle potential for contraction and relaxation is significantly reduced compared with the adult. Whether these developmental changes relate to age differences in the expression and/or activity of key enzymes regulating the smooth muscle mechanical properties has not been previously evaluated. Therefore, we studied the catalytic activities and expression of myosin light chain kinase (MLCK) and myosin light chain phosphatase (MLCP) catalytic (PP1c␦) and regulatory (MYPT) subunits in late fetal, early newborn, and adult rat intrapulmonary arterial tissues. In keeping with the greater force development and relaxation of adult pulmonary artery, Western blot analysis showed that the MLCK, MYPT, and PP1c␦ contents increased significantly with age and were highest in the adult rat. In contrast, their specific activities (activity/enzyme content) were significantly higher in the fetal compared with the adult tissue. The fetal and newborn pulmonary arterial muscle relaxant response to the Rho-kinase inhibitor Y-27632 was greater than the adult tissue. In addition to the 130-kDa isoform of MLCK, we documented the presence of minor highermolecular-weight embryonic isoforms in the fetus and newborn. During fetal life, the lung pulmonary arterial MLCK-and MLCPspecific activities are highest and appear to be related to Rho-kinase activation during lung morphogenesis. lung; transitional circulation; pulmonary vascular resistance VASCULAR RESISTANCE IS dynamically controlled by the contraction and relaxation of its smooth muscle layer. Although this process is modulated by a number of humoral and nonhumoral factors, the vessel maximum potential for constriction and dilation is constrained by the vascular smooth muscle mechanical properties. We have previously shown that the pulmonary and systemic vascular smooth muscle of the newborn sheep has a lesser potential for force development, and a longer relaxation half-time compared with the adult (6, 9). Similar agerelated systemic vascular smooth muscle differences have been reported in relation to its maximum contraction in the rabbit (27) and rat (22), as well as to the degree of nitric oxideinduced relaxation in the guinea pig (5).The smooth muscle mechanical properties are primarily dependent on the rates of phosphorylation and dephosphorylation of the 20-kDa light chains of myosin by myosin light chain kinase (MLCK) and myosin light chain phosphatase (MLCP), respectively. These enzymes are key smooth muscle proteins that regulate contraction and relaxation, respectively. We have previously reported that prenatally induced pulmonary hypertension is associated with a significant reduction in pulmonary arterial MLCK and MLCP content (8). These findin...