Evolving mechanisms of vascular smooth muscle contraction highlight key targets in vascular disease

Vascular smooth muscle (VSM) plays an important role in the regulation of vascular function. Identifying the mechanisms of VSM contraction has been a major research goal in order to determine the causes of vascular dysfunction and exaggerated vasoconstriction in vascular disease. Major discoveries over several decades have helped to better understand the mechanisms of VSM contraction. Ca²⁺ has been established as a major regulator of VSM contraction, and its sources, cytosolic levels, homeostatic mechanisms and subcellular distribution have been defined. Biochemical studies have also suggested that stimulation of Gq protein-coupled membrane receptors activates phospholipase C and promotes the hydrolysis of membrane phospholipids into inositol 1,4,5-trisphosphate (IP₃) and diacylglycerol (DAG). IP₃ stimulates initial Ca²⁺ release from the sarcoplasmic reticulum, and is buttressed by Ca²⁺ influx through voltage-dependent, receptor-operated, transient receptor potential and store-operated channels. In order to prevent large increases in cytosolic Ca²⁺ concentration ([Ca²⁺]_c), Ca²⁺ removal mechanisms promote Ca²⁺ extrusion via the plasmalemmal Ca²⁺ pump and Na⁺/Ca²⁺ exchanger, and Ca²⁺ uptake by the sarcoplasmic reticulum and mitochondria, and the coordinated activities of these Ca²⁺ handling mechanisms help to create subplasmalemmal Ca²⁺ domains. Threshold increases in [Ca²⁺]_c form a Ca²⁺-calmodulin complex, which activates myosin light chain (MLC) kinase, and causes MLC phosphorylation, actin-myosin interaction, and VSM contraction. Dissociations in the relationships between [Ca²⁺]_c, MLC phosphorylation, and force have suggested additional Ca²⁺ sensitization mechanisms. DAG activates protein kinase C (PKC) isoforms, which directly or indirectly via mitogen-activated protein kinase phosphorylate the actin-binding proteins calponin and caldesmon and thereby enhance the myofilaments force sensitivity to Ca²⁺. PKC-mediated phosphorylation of PKC-potentiated phosphatase inhibitor protein-17 (CPI-17), and RhoA-mediated activation of Rho-kinase (ROCK) inhibit MLC phosphatase and in turn increase MLC phosphorylation and VSM contraction. Abnormalities in the Ca²⁺ handling mechanisms and PKC and ROCK activity have been associated with vascular dysfunction in multiple vascular disorders. Modulators of [Ca²⁺]_c, PKC and ROCK activity could be useful in mitigating the increased vasoconstriction associated with vascular disease.