The Time Course of Myosin Light-Chain Phosphorylation in Blood-induced Vasospasm

THE PHOSPHORYLATION OF an Mr 20,000 myosin light chain (MLC20) promotes the generation of contractile force through actin-myosin adenosine triphosphatase in most agonist-mediated vascular smooth muscle cell contraction. However, the role of calcium-mediated contractile processes in sustained arterial narrowing after subarachnoid hemorrhage remains unknown. In a femoral artery model of vasospasm, whole blood was applied to arteries in 54 rats for periods of 2 to 10 days; the contralateral artery treated with platelet-rich plasma served as matched control. During the early stage of vasospasm (Days 2–5), in the media of arteries exposed to blood, MLC20 phosphorylation (including diphosphorylated forms) increased significantly (30–38%; P < 0.05); total medial MLC20 during this interval was comparable to that in controls. After 5 days, however, total MLC20 decreased markedly (>90%; P < 0.01) compared with controls; phosphorylated MLC20 was undetectable during this interval. MLC20-mediated contractile processes may be prominent in the early stages of arterial narrowing after subarachnoid hemorrhage; later stages are associated with the loss of MLC20 and the possible persistence of arterial narrowing by other mechanisms.