CORRELATION BETWEEN HIGH-TEMPERATURE DEPENDENCE OF SMOOTH-MUSCLE MYOSIN LIGHT-CHAIN PHOSPHATASE-ACTIVITY AND MUSCLE-RELAXATION RATE

Q(10) values of the protein phosphatases that can dephosphorylate the regulatory light chain of smooth muscle myosin were determined. Six ph osphatases were examined, i.e. skeletal muscle protein phosphatase 1c; protein phosphatase 2Ac; smooth muscle phosphatases (SMP) I, II, and IV; and myosin-associated protein phosphatase (MAP phosphatase). Among them, SMP-IV and MAP phosphatase, which can dephosphorylate intact sm ooth muscle myosin, showed extremely high Q(10) values (5.3 and 5.2, r espectively). On the other hand, the Q(10) values of other tested phos phatases were within the range of the normal enzyme reaction (Q(10) = 2.0). The rate of dephosphorylation of the myosin light chain in alpha -toxin-skinned strips was measured at different temperatures. The resu lts provided a Q(10) of 5.1, which was quite similar to those values o btained for SMP-IV and MAP phosphatase. These results suggest that the physiological myosin light chain phosphatases are SMP-IV and/or MAP p hosphatase, i.e. type 1 protein phosphatases. The temperature dependen ce of maximum force, the steady-state extent of myosin light chain pho sphorylation, and the relaxation rate of alpha-toxin-permeabilized rab bit postal vein smooth muscle strips were measured. Both maximum force and the extent of myosin light chain phosphorylation were significant ly higher at lower temperature (15 degrees C) than at higher temperatu re (25 degrees C) under all pCa conditions tested, i.e. >8, 6.3, and 5 . The temperature dependence of the relaxation rate was much steeper ( decreased 4 times by lowering the temperature from 25 to 15 degrees C) than that of the initial rate of increase in force development (decre ased 1.4 times by lowering the temperature from 25 to 15 degrees C). T hese results are consistent with the and to values of myosin light cha in phosphatases (Q(10) = 5) and myosin light chain kinase (Q(10) = 1.7 ) and further show that the smooth muscle type 1 phosphatases are resp onsible for the dephosphorylation of smooth muscle myosin in situ.