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 t he molecular mechanism for agonist induced force enhancement. force wa s measured in single beta-escin skinned smooth muscle cells stimulated to contract with Ca2+, myosin light chain (MLC) kinase, PKC and micro cystin-LR. The constituently active fragment of protein kinase C (PKM) increased both force and MLC phospholylation in cells previously stim ulated to contract at submaximal Ca2+. For cells contracted with satur ating Ca2+, PKM stimulation did not increase either force or MLC phosp horylation. For contractions stimulated with bath PKM and microcystin- LR, force rase significantly slower than contractions produced by Ca2 or MLC kinase, suggesting that PKM increases force by a decrease in t he rate of myosin dephosphorylation. Consistent with this hypothesis i s the finding that the rate oi force relaxation was slowed by PKM. Thi s is the first direct demonstration that activation of PKC increases f orce in smooth muscle, and these results suggest that in smooth muscle , agonist induced activation of PKC plays a role in force regulation v ia an inhibition of myosin light chain phosphatase activity. (C) 1996 Academic Press, Inc.