title: Thromboxane A2-induced bi-directional regulation of cerebral arterial tone.
abstract: Myosin light chain phosphatase plays a critical role in modulating smooth muscle contraction in response to a variety of physiologic stimuli. A downstream target of the RhoA/Rho-kinase and nitric oxide (NO)/cGMP/cyclic GMP-dependent kinase (cGKI) pathways, myosin light chain phosphatase activity reflects the sum of both calcium sensitization and desensitization pathways through phosphorylation and dephosphorylation of the myosin phosphatase targeting subunit (MYPT1). As cerebral blood flow is highly spatio-temporally modulated under normal physiologic conditions, severe perturbations in normal cerebral blood flow, such as in cerebral vasospasm, can induce neurological deficits. In nonpermeabilized cerebral vessels stimulated with U-46619, a stable mimetic of endogenous thromboxane A2 implicated in the etiology of cerebral vasospasm, we observed significant increases in contractile force, RhoA activation, regulatory light chain phosphorylation, as well as phosphorylation of MYPT1 at Thr-696, Thr-853, and surprisingly Ser-695. Inhibition of nitric oxide signaling completely abrogated basal MYPT1 Ser-695 phosphorylation and significantly increased and potentiated U-46619-induced MYPT1 Thr-853 phosphorylation and contractile force, indicating that NO/cGMP/cGKI signaling maintains basal vascular tone through active inhibition of calcium sensitization. Surprisingly, a fall in Ser-695 phosphorylation did not result in an increase in phosphorylation of the Thr-696 site. Although activation of cGKI with exogenous cyclic nucleotides inhibited thromboxane A2-induced MYPT1 membrane association, RhoA activation, contractile force, and regulatory light chain phosphorylation, the anticipated decreases in MYPT1 phosphorylation at Thr-696/Thr-853 were not observed, indicating that the vasorelaxant effects of cGKI are not through dephosphorylation of MYPT1. Thus, thromboxane A2 signaling within the intact cerebral vasculature induces "buffered" vasoconstrictions, in which both the RhoA/Rho-kinase calcium-sensitizing and the NO/cGMP/cGKI calcium-desensitizing pathways are activated.
authors.affiliation: Department of Molecular Physiology and Biological Physics, Robert M. Berne Cardiovascular Research Center, University of Virginia, Charlottesville, Virginia 22908, USA.
authors.forename: Ronald L
authors.initials: RL
authors.lastname: Neppl
authors.forename: Lubomir T
authors.initials: LT
authors.lastname: Lubomirov
authors.forename: Ko
authors.initials: K
authors.lastname: Momotani
authors.forename: Gabriele
authors.initials: G
authors.lastname: Pfitzer
authors.forename: Masumi
authors.initials: M
authors.lastname: Eto
authors.forename: Avril V
authors.initials: AV
authors.lastname: Somlyo
mesh_terms: D019796:15-Hydroxy-11 alpha,9 alpha-(epoxymethano)prosta-5,13-dienoic Acid / Q000494:pharmacology*; D000818:Animals; D001783:Blood Flow Velocity / Q000187:drug effects; D002536:Cerebral Arteries / Q000378:metabolism* / Q000503:physiopathology; D006152:Cyclic GMP / Q000378:metabolism; D017869:Cyclic GMP-Dependent Protein Kinases / Q000378:metabolism; D004789:Enzyme Activation / Q000187:drug effects; D008297:Male; D009119:Muscle Contraction / Q000187:drug effects; D043263:Myosin-Light-Chain Phosphatase / Q000378:metabolism; D009569:Nitric Oxide; D010766:Phosphorylation / Q000187:drug effects; D054645:Protein Phosphatase 1 / Q000378:metabolism; D011817:Rabbits; D051381:Rats; D015398:Signal Transduction / Q000187:drug effects*; D013928:Thromboxane A2 / Q000378:metabolism / Q000494:pharmacology*; D014661:Vasoconstriction / Q000187:drug effects; D014662:Vasoconstrictor Agents / Q000493:pharmacokinetics*; D020301:Vasospasm, Intracranial / Q000139:chemically induced / Q000378:metabolism* / Q000503:physiopathology; D054460:rho-Associated Kinases / Q000378:metabolism; D020742:rhoA GTP-Binding Protein / Q000378:metabolism
chemical_list: D014662:Vasoconstrictor Agents; D009569:Nitric Oxide; D013928:Thromboxane A2; D019796:15-Hydroxy-11 alpha,9 alpha-(epoxymethano)prosta-5,13-dienoic Acid; D054460:rho-Associated Kinases; D017869:Cyclic GMP-Dependent Protein Kinases; C499850:Ppp1r12a protein, rat; D054645:Protein Phosphatase 1; D043263:Myosin-Light-Chain Phosphatase; D020742:rhoA GTP-Binding Protein; D006152:Cyclic GMP