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      Cyclic AMP-Independent CGRP 8–37-Sensitive Receptors Mediate Adrenomedullin-Induced Decrease of CaCl 2-Contraction in Pregnant Rat Mesenteric Artery

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          Objectives: We tested the hypothesis that adrenomedullin reduces calcium influx independent of potassium channels in depolarized endothelium-denuded mesenteric artery from pregnant rats. Results: Adrenomedullin reduced the CaCl<sub>2</sub>-induced contraction, while the receptor antagonist calcitonin gene-related peptide (CGRP)<sub>8–37</sub>, but not adrenomedullin<sub>22–52</sub>, reversed these effects. Adenylate cyclase inhibition by SQ22536 did not prevent adrenomedullin effects on CaCl<sub>2</sub>-induced contraction. Adrenomedullin did not inhibit depolarization-induced calcium entry to isolated vascular smooth muscle. Inhibition of myosin light-chain (MLC) phosphatase by calyculin A reversed the effects of adrenomedullin on contraction caused by submillimolar concentrations of CaCl<sub>2</sub>, while adrenomedullin still inhibited contraction caused by higher concentrations of CaCl<sub>2</sub>. However, the ratio of phosphorylated to total myosin phosphatase target 1, the regulatory subunit of MLC phosphatase, did not change with adrenomedullin, indicating a lack of MLC phosphatase activation. Interestingly, sodium fluoride, a nonspecific protein phosphatase inhibitor, completely blocked the effect of adrenomedullin on CaCl<sub>2</sub>-induced contraction. Adrenomedullin inhibited calcium mobilization from intracellular stores induced by thapsigargin. Conclusion: Adrenomedullin inhibits CaCl<sub>2</sub>-induced contraction, without affecting calcium influx, through a CGRP<sub>8–37</sub>-sensitive receptor, but not using the cyclic adenosine monophosphate pathway, probably through activation of protein phosphatases. Inhibition of intracellular calcium release is an additional role played by adrenomedullin in calcium homeostasis in vascular smooth muscle.

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          Most cited references 19

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          Signal transduction by G-proteins, rho-kinase and protein phosphatase to smooth muscle and non-muscle myosin II.

           A P Somlyo (2000)
          We here review mechanisms that can regulate the activity of myosin II, in smooth muscle and non-muscle cells, by modulating the Ca2+ sensitivity of myosin regulatory light chain (RLC) phosphorylation. The major mechanism of Ca2+ sensitization of smooth muscle contraction and non-muscle cell motility is through inhibition of the smooth muscle myosin phosphatase (MLCP) that dephosphorylates the RLC in smooth muscle and non-muscle. The active, GTP-bound form of the small GTPase RhoA activates a serine/threonine kinase, Rho-kinase, that phosphorylates the regulatory subunit of MLCP and inhibits phosphatase activity. G-protein-coupled release of arachidonic acid may also contribute to inhibition of MLCP acting, at least in part, through the Rho/Rho-kinase pathway. Protein kinase C(s) activated by phorbol esters and diacylglycerol can also inhibit MLCP by phosphorylating and thereby activating CPI-17, an inhibitor of its catalytic subunit; this mechanism is independent of the Rho/Rho-kinase pathway and plays only a minor, transient role in the G-protein-coupled mechanism of Ca2+ sensitization. Ca2+ sensitization by the Rho/Rho-kinase pathway contributes to the tonic phase of agonist-induced contraction in smooth muscle, and abnormally increased activation of myosin II by this mechanism is thought to play a role in diseases such as high blood pressure and cancer cell metastasis.
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            Vascular actions of calcitonin gene-related peptide and adrenomedullin.

            This review summarizes the receptor-mediated vascular activities of calcitonin gene-related peptide (CGRP) and the structurally related peptide adrenomedullin (AM). CGRP is a 37-amino acid neuropeptide, primarily released from sensory nerves, whilst AM is produced by stimulated vascular cells, and amylin is secreted from the pancreas. They share vasodilator activity, albeit to varying extents depending on species and tissue. In particular, CGRP has potent activity in the cerebral circulation, which is possibly relevant to the pathology of migraine, whilst vascular sources of AM contribute to dysfunction in cardiovascular disease. Both peptides exhibit potent activity in microvascular beds. All three peptides can act on a family of CGRP receptors that consist of calcitonin receptor-like receptor (CL) linked to one of three receptor activity-modifying proteins (RAMPs) that are essential for functional activity. The association of CL with RAMP1 produces a CGRP receptor, with RAMP2 an AM receptor and with RAMP3 a CGRP/AM receptor. Evidence for the selective activity of the first nonpeptide CGRP antagonist BIBN4096BS for the CGRP receptor is presented. The cardiovascular activity of these peptides in a range of species and in human clinical conditions is detailed, and potential therapeutic applications based on use of antagonists and gene targeting of agonists are discussed.
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              Adrenomedullin stimulates two signal transduction pathways, cAMP accumulation and Ca2+ mobilization, in bovine aortic endothelial cells.

              The biological action of adrenomedullin, a novel hypotensive peptide, on bovine aortic endothelial cells, was examined. The specific binding of adrenomedullin to these cells was observed, and adrenomedullin was found to induce intracellular cAMP accumulation in a dose-dependent manner. EC50 for the cAMP accumulation was about 100 times lower than the apparent IC50 for the binding assay. Adrenomedullin also induced increase of intracellular free Ca2+ in endothelial cells in a dose-dependent manner. The Ca2+ response to adrenomedullin was biphasic with an initial transient increase due to the release from thapsigargin-sensitive intracellular Ca2+ storage and a prolonged increase by influx through the ion channel on the plasma membrane. This intracellular free Ca2+ increase resulted from phospholipase C activation and inositol 1,4,5-trisphosphate formation, and seemed to cause nitric oxide synthase activation by monitoring intracellular cGMP accumulation. Both cAMP accumulation and Ca2+ increased responses to adrenomedullin were mediated by cholera toxin-sensitive G protein, but the two signal transduction pathways were independent. Thus, the results suggest that adrenomedullin elicits the hypotensive effect through at least two mechanisms, a direct action on vascular smooth muscle cells to increase intracellular cAMP and an action on endothelial cells to stimulate nitric oxide release, with both leading to vascular relaxation.

                Author and article information

                J Vasc Res
                Journal of Vascular Research
                S. Karger AG
                December 2007
                26 September 2007
                : 45
                : 1
                : 33-44
                Department of Obstetrics and Gynecology, University of Texas Medical Branch, Galveston, Tex., USA
                109075 J Vasc Res 2008;45:33–44
                © 2007 S. Karger AG, Basel

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                Page count
                Figures: 8, References: 29, Pages: 12
                Research Paper


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