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String-shaped reconstituted smooth muscle (SM) fibers were prepared in rectangular wells by thermal gelation of a mixed solution of collagen and cultured SM cells derived from guinea pig stomach. The cells in the fiber exhibited an elongated spindle shape and were aligned along the long axis. The fiber contracted in response to KCl (140 mM), norepinephrine (NE; 10(-7) M), epinephrine (10(-7) M), phenylephrine (10(-6) M), serotonin (10(-6) M), and histamine (10(-5) M), but not acetylcholine (10(-5) M). Phentolamine (10(-7) M) produced a parallel rightward shift of the NE dose-response curve. Moreover, NE-induced contraction was partially inhibited by nifedipine and completely abolished by the intracellular Ca(2+) chelator 1,2-bis(2-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid acetoxymethyl ester, the myosin light chain kinase inhibitor ML-9, the Rho kinase inhibitor Y-27632, and papaverine. A [(3)H]quinuclidinyl benzilate binding study revealed that the loss of response to acetylcholine was due to the loss of muscarinic receptor expression during culture. The expression of contractile proteins in the fibers was similar to that in cultured SM cells. These results suggest that, although the fiber is not a model for fully differentiated SM, contractile mechanisms are maintained.

ATP-dependent movement of actin filaments on smooth muscle myosin was investigated by using the in vitro motility assay method in which myosin was fixed on the surface of a coverslip in a phosphorylated or an unphosphorylated state. Actin filaments slid on gizzard myosin phosphorylated with myosin light chain kinase (MLCK) at a rate of 0.35 micron/s, but did not slide at all on unphosphorylated myosin. The movement of actin filaments on phosphorylated myosin was stopped by perfusion of phosphatase. Subsequent perfusion with a solution containing MLCK, calmodulin, and Ca2+ enabled actin filaments to move again. The sliding velocities on monophosphorylated and diphosphorylated myosin by MLCK were not different. Actin filaments did not move on myosin phosphorylated with protein kinase C (PKC). The sliding velocity on myosin phosphorylated with both MLCK and PKC was identical to that on myosin phosphorylated only with MLCK. Gizzard tropomyosin enhanced the sliding velocity to 0.76 micron/s. Gizzard caldesmon decreased the sliding velocity with increase in its concentration. At a 5-fold molar ratio of caldesmon to actin, the movement stopped completely. This inhibitory effect of caldesmon was relieved upon addition of excess calmodulin and Ca2+.

We investigated effects of protein kinase inhibitors on the first cell division in sea urchin eggs on the assumption that phosphorylation of myosin is requisite for the formation and/or the contraction of the contractile ring. ML-7 or ML-9, which inhibits myosin light chain kinase (MLCK), inhibited cytokinesis with a half maximal inhibition at 0.1 -0.2 mM. The nuclear division was accomplished normally at 0.2-0.25 mM where the cytokinesis was completely blocked. Fluorescent staining of actin filaments with rhodaminelabeled phalloidin revealed that the contractile ring was not formed in the cleavage-inhibited eggs. H-7 which inhibits CAMP-dependent protein kinase, cGMP-dependent protein kinase and protein kinase C arrested the process of the division at mid-cleavage at 0.25-0.3 mM and at metaphase or anaphase at 0.5 mM. H-8 and HA1004, which inhibit CAMP-dependent and cGMP-dependent protein kinases did not show significant effect at millimolar order. In the presence of micromolar concentrations of staurosporine which preferentially inhibits protein kinase C and MLCK small mitotic apparatuses were formed, in which chromosomes did not form the metaphase plate. The role of phosphorylation in the cell division is discussed.