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      Effects of cGMP on Coordination of Vascular Smooth Muscle Cells of Rat Mesenteric Small Arteries

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          Abstract

          Objective: We tested the hypothesis that cGMP can induce a state of only partial coordination of vascular smooth muscle cells (VSMC). Methods: This was done by studying the concentration-dependent effect of 8Br-cGMP on isometric and isobaric force development of noradrenaline-activated segments of rat mesenteric small arteries in which the endothelium was removed. We further measured the concentration-dependent effect of 8Br-cGMP on VSMC membrane potential, spatially resolved [Ca<sup>2+</sup>]<sub>i</sub> and VSMC membrane conductance. Results: With 300 µ M 8Br-cGMP, coordinated [Ca<sup>2+</sup>]<sub>i</sub> activity and vasomotion were seen as previously reported. At 10–30 µ M 8Br-cGMP, beating isometric tension oscillations were seen. Isobaric recordings revealed oscillations with different frequencies in different parts of the arteries. At these (10–30 µ M) 8Br-cGMP concentrations, membrane potential oscillations did not always concur with isometric tension oscillations, and [Ca<sup>2+</sup>]<sub>i</sub> oscillations were only synchronized locally within groups of cells. 8Br-cGMP concentration-dependently decreased the frequency of vasomotion and, in unsynchronized hyperpolarized VSMC, the frequency of [Ca<sup>2+</sup>]<sub>i</sub> waves. Conclusion: Our results demonstrated that cGMP can cause a partial coordination of the VSMC in the vascular wall (and at high concentrations near complete coordination). Furthermore, the cGMP concentration-dependent decrease of Ca<sup>2+</sup> wave frequency and of vasomotion frequency suggests that cGMP modifies oscillatory Ca<sup>2+</sup> release from the sarcoplasmic reticulum and supports the suggestion that this oscillatory release paces vasomotion.

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          Most cited references 14

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          Regulation of intracellular calcium by a signalling complex of IRAG, IP3 receptor and cGMP kinase Ibeta.

          Calcium release from the endoplasmic reticulum controls a number of cellular processes, including proliferation and contraction of smooth muscle and other cells. Calcium release from inositol 1,4,5-trisphosphate (IP3)-sensitive stores is negatively regulated by binding of calmodulin to the IP3 receptor (IP3R) and the NO/cGMP/cGMP kinase I (cGKI) signalling pathway. Activation of cGKI decreases IP3-stimulated elevations in intracellular calcium, induces smooth muscle relaxation and contributes to the antiproliferative and pro-apoptotic effects of NO/cGMP. Here we show that, in microsomal smooth muscle membranes, cGKIbeta phosphorylated the IP3R and cGKIbeta, and a protein of relative molecular mass 125,000 which we now identify as the IP3R-associated cGMP kinase substrate (IRAG). These proteins were co-immunoprecipitated by antibodies directed against cGKI, IP3R or IRAG. IRAG was found in many tissues including aorta, trachea and uterus, and was localized perinuclearly after heterologous expression in COS-7 cells. Bradykinin-stimulated calcium release was not affected by the expression of either IRAG or cGKIbeta, which we tested in the absence and presence of cGMP. However, calcium release was inhibited after co-expression of IRAG and cGKIbeta in the presence of cGMP. These results identify IRAG as an essential NO/cGKI-dependent regulator of IP3-induced calcium release.
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            Hypothesis for the initiation of vasomotion.

            Vasomotion is the regular variation in tone of arteries. In our study, we suggest a model for the initiation of vasomotion. We suggest that intermittent release of Ca(2+) from the sarcoplasmic reticulum (SR, cytosolic oscillator), which is initially unsynchronized between the vascular smooth muscle cells, becomes synchronized to initiate vasomotion. The synchronization is achieved by an ion current over the cell membrane, which is activated by the oscillating Ca(2+) release. This current results in an oscillating membrane potential, which synchronizes the SR in the vessel wall and starts vasomotion. Therefore, the pacemaker of the vascular wall can be envisaged as a diffuse array of individual cytosolic oscillators that become entrained by a reciprocal interaction with the cell membrane. The model is supported by experimental data. Confocal [Ca(2+)](i) imaging and isometric force development in isolated rat resistance arteries showed that low norepinephrine concentrations induced SR-dependent unsynchronized waves of Ca(2+) in the vascular smooth muscle. In the presence of the endothelium, the waves converted to global synchronized oscillations of [Ca(2+)](i) after some time, and vasomotion appeared. Synchronization was also seen in the absence of endothelium if 8-bromo-cGMP was added to the bath. Using the patch-clamp technique and microelectrodes, we showed that Ca(2+) release can activate an inward current in isolated smooth muscle cells from the arteries and cause depolarization. These electrophysiological effects of Ca(2+) release were cGMP dependent, which is consistent with the possibility that they are important for the cGMP-dependent synchronization. Further support for the model is the observation that a short-lasting current pulse can initiate vasomotion in an unsynchronized artery as expected from the model.
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              Nitric oxide synthase blockade enhances vasomotion in the cerebral microcirculation of anesthetized rats.

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                Author and article information

                Journal
                JVR
                J Vasc Res
                10.1159/issn.1018-1172
                Journal of Vascular Research
                S. Karger AG
                1018-1172
                1423-0135
                2005
                August 2005
                29 July 2005
                : 42
                : 4
                : 301-311
                Affiliations
                The Water and Salt Center, Institute of Physiology and Biophysics, University of Aarhus, Aarhus, Denmark
                Article
                86002 J Vasc Res 2005;42:301–311
                10.1159/000086002
                15925896
                © 2005 S. Karger AG, Basel

                Copyright: All rights reserved. No part of this publication may be translated into other languages, reproduced or utilized in any form or by any means, electronic or mechanical, including photocopying, recording, microcopying, or by any information storage and retrieval system, without permission in writing from the publisher. Drug Dosage: The authors and the publisher have exerted every effort to ensure that drug selection and dosage set forth in this text are in accord with current recommendations and practice at the time of publication. However, in view of ongoing research, changes in government regulations, and the constant flow of information relating to drug therapy and drug reactions, the reader is urged to check the package insert for each drug for any changes in indications and dosage and for added warnings and precautions. This is particularly important when the recommended agent is a new and/or infrequently employed drug. Disclaimer: The statements, opinions and data contained in this publication are solely those of the individual authors and contributors and not of the publishers and the editor(s). The appearance of advertisements or/and product references in the publication is not a warranty, endorsement, or approval of the products or services advertised or of their effectiveness, quality or safety. The publisher and the editor(s) disclaim responsibility for any injury to persons or property resulting from any ideas, methods, instructions or products referred to in the content or advertisements.

                Page count
                Figures: 8, Tables: 1, References: 25, Pages: 11
                Categories
                Research Paper

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