The time course of smooth muscle contraction can be divided into two phases, the initial phase is associated with force development, whereas the sustained phase is associated with force maintenance. Cumulative evidence suggests that the two phases are regulated by different signaling pathways and that ρ-kinase (ROCK) and protein kinase C (PKC) play an important role in regulating isometric force in sustained contractions. Since the maintenance of sustained force is critical to the function of vascular smooth muscle, unraveling the complex mechanism of force maintenance is crucial for understanding the cell biology of the muscle. The present study examined the effects of ROCK and PKC on the level of phosphorylation of the 20-kD myosin light chain (MLC20) and isometric force during a sustained contraction. We used partial activation and inhibition of ROCK and PKC to reduce the isometric force by 50% of the maximal isometric force in fully activated muscle, F. We then examined the level of MLC20 phosphorylation in each case. We found that in partially activated muscle the level of MLC20 phosphorylation required to maintain 50% F was much lower than that required in muscles where 50% reduction in F was achieved by partial inhibition of ROCK and PKC. The results can be explained by a model containing a contractile apparatus and a cytoskeletal scaffold where force generated by the contractile apparatus is transmitted to the extracellular domain through the cytoskeleton. The results indicate that ROCK and PKC play an important role in force transmission through the cytoskeleton. The study supports a model that the maintenance of sustained force during a contraction of arterial smooth muscle is dependent on the intracellular transmission of force through the cytoskeleton and that ρ-kinase and protein kinase C plays an important role in the regulation of cytoskeletal integrity and its efficiency in force transmission.
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