Concanavalin A‐ and fetal‐calf‐serum‐induced rounding and myosin light chain phosphorylation in cultured smooth muscle cells

Rabbit aorta smooth muscle cells (SMC) in long‐term culture retracted in less than 10 min in response to a sequential order of stimulations by concanavalin A (Con A) and fetal calf serum (FCS). With additional continuous stimulation by FCS, the SMC took on a circular shape and were anchored to the substrate by retraction fibrils. This rounding was observed only when the cells were sequentially stimulated by Con A and FCS. Depletion of intracellular Ca 2+ stores by the addition of EGTA and Ca 2+ ionophores inhibited the rounding. Transient phosphorylation of MLC 20 was observed in the initial stage during the SMC rounding. The extent of monophosphorylated MLC 20 increased for up to 5 min to a maximal value of 49%. The diphosphorylated form reached a maximal value of 29% within 2 min; then both forms of MLC 20 decreased. The process of the SMC rounding was inhibited by antimycin A or cytochalasins, in a dose‐dependent manner, findings which suggested a dependency on both metabolic energy and actin‐containing microfilaments. The smooth‐muscle‐relaxing agent, HA1077, also inhibited the process of SMC rounding. These observations suggest that a cellular contractile process might be involved in rounding of SMC.