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      Alpha-Adrenoceptors in Vascular Smooth Muscle: All Is Not Well

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          Abstract

          Studies of binding interactions and contractile responses of vascular muscles at α<sub> 1</sub> and α<sub>2</sub>-adrenoceptors revealed the following. (1) Agonists at α<sub>1</sub> and α<sub>2</sub>-adrenoceptors may achieve selectivity by virtue of different efficacies despite similar affinities at the two receptors as well as by differing affinities. This implies that their potencies in binding studies may not correlate with potencies in response and that an agonist may produce positive or negative interactions by occupying both α<sub>1</sub> - and α<sub>2</sub>-receptors. (2) Agonists at α<sub>2</sub>-adrenoceptors have the ability in some vascular muscles to release internal Ca<sup>2+</sup> (implying an inositol triphosphate mechanism) as well as open Ca<sup>2+</sup> channels. However, their contractile abilities are not closely related to function of Na<sup>+</sup>/H<sup>+</sup> or Na<sup>+</sup>/Ca<sup>2+</sup> exchange sites. Amiloride derivatives probably inhibit contractile effects of α-agonists and K<sup>+</sup> elevation by an action at sites distal to the receptor or Ca channels. (3) The failure of α<sub>1</sub>-agonists to contract arteries in vitro is not related to the absence of these receptors but most likely to their uncoupling from contractile responses, possibly owing to changes related to the in vitro condition (loss of modulating endogenous substances present in vivo such as angiotensin II or endothelins or to changed physical conditions such as may alter function of stretch-activated channels).

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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
          978-3-8055-5380-3
          978-3-318-01726-7
          1018-1172
          1423-0135
          1991
          1991
          23 September 2008
          : 28
          : 1-3
          : 104-114
          Affiliations
          Division of Physiology and Pharmacology, Department of Biomedical Sciences, McMaster University, Hamilton, Canada
          Article
          158849 Blood Vessels 1991;28:104–114
          10.1159/000158849
          © 1991 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
          Pages: 11
          Categories
          Signal Recognition and Transduction in Vascular Smooth Muscle

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