INCREASED MYOSIN LIGHT-CHAIN KINASE CONTENT IN SENSITIZED CANINE SAPHENOUS-VEIN

Our previous studies revealed that smooth muscle from sensitized canin e saphenous vein (SCSV) demonstrated greater active shortening capacit y, maximum shortening velocity, and prolonged relaxation vis-a-vis the control muscle. These changes could be responsible for the in vivo hy perreactivity of venous smooth muscle observed in anaphylactic shock. Because smooth muscle cross-bridge cycling is regulated by myosin ligh t chain kinase (MLCK)-dependent phosphorylation of the 20-kDa myosin l ight chain (MLC(20)), we studied MLC(20) and MLCK phosphorylation in h omogenates of SCSV and veins from littermate control dogs. We found th at phosphorylation of MLC(20) in SCSV homogenate was higher (42.26 +/- 5.10%) compared with control homogenates (26.69 +/- 3.30%; P < 0.05); MLCK content was significantly higher in SCSV homogenates [0.169 +/- 0.019 (SE) mu g/mg protein] than in control homogenates (0.075 +/- 0.0 04 mu g/mg protein; P < 0.05). Total MLCK activity increased from 6.16 +/- 0.60 X 10(-5) nmol P-i . mg fresh weight of tissue(-1). min(-1) i n control homogenates to 12.50 +/- 2.50 X 10(-5) nmol P-i . mg fresh w eight of tissue-l min-l in sensitized homogenates (P < 0.05). Specific MLCK activity was, however, similar in sensitized and control homogen ates. The results of our study suggest that elevation bf MLCK content in the homogenate could account for the increased contractility of the SCSV in anaphylactic shock.