INTRACELLULAR MECHANISMS INVOLVED IN THE REGULATION OF VASCULAR SMOOTH-MUSCLE TONE

Vascular smooth muscle contraction is thought to occur by a mechanism similar to that described for striated muscles, i.e., via a cross-bridge cycling - sliding filament mechanism. This symposium focused on Ca2+ signalling and the role of intracellular free Ca2+ concentration, [Ca2+](i), in regulating vascular tone: how contractile stimuli leading to an increase in [Ca2+](i) trigger vasoconstriction and how relaxant signals reduce [Ca2+](i) causing vasodilation. M.P. Walsh opened the symposium with an overview emphasizing the central role of myosin phosphorylation - dephosphorylation in the regulation of vascular tone and identifying recent developments concerning regulation of [Ca2+](i), Ca2+ sensitization and desensitization of the contractile response, Ca2+-independent protein kinase C induced contraction, and direct regulation of cross-bridge cycling by the thin filament associated proteins caldesmon and calponin. The remainder of the symposium focused on three specific areas related to the regulation of vascular tone: Ca2+ signalling in relation to smooth muscle structure, structure-function relations of myosin, and the role of cyclic GMP (cGMP) dependent protein kinase. G.J. Kargacin described how smooth muscle cells are structured and how second messenger signals such as Ca2+ might be modified or influenced by this structure. J. Kendrick-Jones then discussed the results of mutagenesis studies aimed at understanding how the myosin light chains, particularly the phosphorylatable (Ca2+-calmodulin dependent) regulatory light chains, control myosin. The vasorelaxant effects of signalling molecules such as beta-adrenergic agents and nitrovasodilators are mediated by cyclic nucleotide dependent protein kinases, leading principally to a reduction in [Ca2+](i). T.M. Lincoln described the roles of cyclic nucleotide dependent protein kinases, in particular cyclic GMP dependent protein kinase, in vasodilation.