Protein kinase C increases force and slows relaxation in smooth muscle: Evidence for regulation of the myosin light chain phosphatase

To determine if activation of protein kinase C (PKC) participates in the molecular mechanism for agonist induced force enhancement, force was measured in single β-escin skinned smooth muscle cells stimulated to contract with Ca²⁺, myosin light chain (MLC) kinase, PKC and microcystin-LR. The constituently active fragment of protein kinase C (PKM) increased both force and MLC phosphorylation in cells previously stimulated to contract at submaximal Ca²⁺. For cells contracted with saturating Ca²⁺, PKM stimulation did not increase either force or MLC phosphorylation. For contractions stimulated with both PKM and microcystin-LR, force rose significantly slower than contractions produced by Ca²⁺ or MLC kinase, suggesting that PKM increases force by a decrease in the rate of myosin dephosphorylation. Consistent with this hypothesis is the finding that the rate of force relaxation was slowed by PKM. This is the first direct demonstration that activation of PKC increases force in smooth muscle, and these results suggest that in smooth muscle, agonist induced activation of PKC plays a role in force regulation via an inhibition of myosin light chain phosphatase activity.