The overall goal of this Project is to understand the complex processes that regulate contractility in vascular smooth muscle under physiological and pathophysiological conditions, which lead to high blood pressure, atherosclerosis, coronary restenosis, shock or cerebral vasospasm. Contractility is switched on in smooth muscle (SM) via phosphorylation of the regulatory light chain of myosin (RLC) and the level of phosphorylation is dependent upon the opposing activities of the Ca2+/calmodulin dependent myosin light chain kinase (MLCK) and myosin light chain phosphatase, both of which can be regulated by upstream signaling pathways. We, with Project 2, have generated MLCK null mice, which are embryonic lethal, starting at E15.5 with some reaching term, but prior to this embryonic aortae or umbilical vessels display RLC phosphorylation and normal force development in response to Ca 2+. We will test the hypothesis that ubiquitously expressed SM MLCKs are critical for contraction, migration, filament and sarcomere formation in smooth and cardiac muscle respectively, as well as A404SMC "progenitor" cells (Project 2, Core A) and transformed proepicardial cells (Project 3, Core A); or alternatively that a compensatory kinase(s) accounts for the MLCK activity in MLCK null embryos or that another kinase normally predominants during embryonic development. Preliminary studies suggest that formation of the coronary vessels is defective in MLCK null embryos, thus, we will test the hypothesis that MLCKs are critical for migration of the epicardial cells of the proepicardial organ, the precursors of the coronary vessels with Project 3. The myosin motors underlying cell migration and contractility are also regulated and contribute to the SM contractile phenotype. These mechanisms will be explored using kinetic analysis with a novel recently synthesized fluorescent 3'-amino derivative of ATP. We will test the hypothesis that AM. ADP strongly bound crossbridges play a significant role in maintaining tonic force at low actomyosin activity, slow shortening velocity and low levels of RLC phosphorylation, characteristic features of SM myosins. The proposed studies require extensive interactions with the other projects and Core A and match the central theme of this PPG.