| The mechanism dealing with smooth muscle contraction can be simply described as that smooth muscle myosin light chains are phosphorylated by smooth muscle myosin light chain kinase (MLCK) in Ca2+-calmodulin (CaM) dependent way (CDPM), and the interaction between phosphorylated myosin and actin induces smooth muscle contraction. Since the rise of intra-cellular Ca2+ and the phsphorylation of myosin is happened in a micro-second range, the phenomena that the tension of smooth muscle may be retained as [Ca2+]i fall down to the basic level and myosin dephosphorylated are difficult to be explained by traditional hypothesis of CDPM. These phenomena suggested that other mechanisms may also be involved in the regulation of smooth muscle contraction. This study is tried to partially reveal the characterization of Ca2+-CaM independent phosphorylation way (CIPM) by MLCK in gizzard, uterus smooth muscle and cardiac muscle and factors involved in the regulation. The author hopes that these results can provide some valuable information to further study the mechanism of CIPM. Part one The characterization of CIPM by MLCK in gizzard smooth muscle Part one indicated that smooth muscle myosin may also be phosphorylated by MLCK in the absence of Ca2+-CaM. The major differences between CIPM and CDPM are as follows. (1) With the increase of incubation-temperature, the extent of CIPM by MLCK showed no apparent change. However, the extent of CDPM by MLCK apparently decreased with the increased temperature. (2) With the increase of incubation-time, the extent of CIPM by MLCK showed no apparent changes; in contrast, the extent of CDPM by MLCK was apparently decreased. (3) The increase of ionic strength from 60 to 360 mmol KCl showed no apparent effects on CIPM. However, when ionic strength was over 120 mmol/L, Ca2+-CaM dependent di-phosphorylation of myosin was not observed. (4) CDPM was significantly inhibited by ML-9 (MLCK inhibitor). By contrast, no inhibitory effects on CIPM by MLCK were observed in the same assay condition. (5) The Mg2+-ATPase activity of CIPM was higher than that of dephosphorylated myosin but lower than that of CDPM (***P <0.001,** P <0.01 or * P <0.05). Part two The characterization of CIPM by MLCK in uterus The characterization of Ca2+-CaM independent phosphorylation of uterus myosin light chains by MLCK is as following. (1) CIPM by MLCK is much lower than that of CDPM by MLCK. (2) The extent of CIPM is more stable than that of CDPM, and CIPM was less influenced by the changes of incubation-time, incubation-temperature, and increase of the concentration of KCl. (3) Both CIPM and CDPM of uterus were less influenced by achidonic acid (AA) compared to those in gizzard smooth muscle. (4) CIPM by MLCK was less efficient, less energy consumption than that of CDPM by MLCK. (5) It was not observed that CIPM in uterus was affected by oxcytocin. The results suggested that CIPM by MLCK in uterus smooth muscle share similarity to that of CIPM by MLCK in gizzard smooth muscle. Part three Characterization of cardiac CIPM by MLCK The characterizations of cardiac CIPM by MLCK and CDPM by MLCK are as follows. (1) The extent of CIPM by MLCK is lower than that of CDPM. CIPM was characterized by less efficient, fewer energy consumption comparing with that of CDPM. (2) CIPM by MLCK in cardiac muscle was more influenced by changes of ionic strength than that of CDPM by MLCK. The extent of CIPM was less influenced by the changes of AA concentration than that of CDPM and this is different from that of CIPM in smooth muscle. (3) Cardiac CIPM and CDPM were more stable than those in smooth muscle. Cardiac CIPM and CDPM by MLCK were less influenced by changes of incubation time, incubation-temperature. CIPM by MLCK in cardiac muscle can be observed only under acid assay condition and these differences are statistically significant (** P < 0.01, or * P < 0.05). Part four The effects of histamine on CIPM by MLCK in different muscle The effects of histamine in high, medium and low concentration on CIPM and CDPM in gizzard, uterus sm...
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