Background: Although coronary artery spasm plays an important role in a wide variety of ischemic heart diseases, the intracellular mechanism for the spasm remains to be clarified. We examined the role of myosin light chain (MLC) phosphorylations, a key mechanism for contraction of vascular smooth muscle, in our swine model with interleukin-1β (IL-1β). Methods and Results: IL-1β was applied chronically to the porcine coronary arteries from the adventitia to induce an inflammatory/proliferative lesion. Two weeks after the operation, intracoronary serotonin repeatedly induced coronary hyperconstrictions at the IL-1β-treated site both in vivo and in vitro, which were markedly inhibited by fasudil, an inhibitor of protein kinases, including protein kinase C and MLC kinase. Western blot analysis showed that during serotonin-induced contractions, MLC monophosphorylation was significantly increased and sustained in the spastic segment compared with the control segment, whereas MLC diphosphorylation was noted only in the spastic segment. A significant correlation was noted between the serotonin- induced contractions and MLC phosphorylations. Both types of MLC phosphorylation were markedly inhibited by fasudil. In addition, MLC diphosphorylation was never induced by a simple endothelium removal in the normal coronary artery, whereas enhanced MLC phosphorylations in the spastic segment were noted regardless of the presence or absence of the endothelium. Conclusions: These results indicate that enhanced MLC phosphorylations in the vascular smooth muscle play a central role in the pathogenesis of coronary spasm in our swine model.
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