REVIEW ARTICLE


Carotid Body AT4 Receptor Expression and its Upregulation in Chronic Hypoxia Alternate Title: Carotid Body AT4 Receptor in Hypoxia



Man-Lung Fung*, 1, Siu-Yin Lam1, Tung-Po Wong2, Yung-Wui Tjong1, Po-Sing Leung2
1 Department of Physiology, University of Hong Kong, Pokfulam, Hong Kong
2 Department of Physiology, Chinese University of Hong Kong, Shatin, Hong Kong SAR, China


2007 Bentham Science Publishers Ltd.

open-access license: This is an open access article distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/2.5/), which permits unrestrictive use, distribution, and reproduction in any medium, provided the original work is properly cited.

* Address correspondence to this author at the Department of Physiology, The University of Hong Kong, Faculty of Medicine Building, 21 Sassoon Road, Pokfulam, Hong Kong; Tel: +852-2819-9234; Fax: +852-2855-9730; E-mail: fungml@hkucc.hku.hk


Abstract

Hypoxia regulates the local expression of angiotensin-generating system in the rat carotid body and the me-tabolite angiotensin IV (Ang IV) may be involved in the modulation of carotid body function. We tested the hypothesis that Ang IV-binding angiotensin AT4 receptors play a role in the adaptive change of the carotid body in hypoxia. The expression and localization of Ang IV-binding sites and AT4 receptors in the rat carotid bodies were studied with histochemistry. Specific fluorescein-labeled Ang IV binding sites and positive staining of AT4 immunoreactivity were mainly found in lobules in the carotid body. Double-labeling study showed the AT4 receptor was localized in glomus cells containing tyrosine hydroxylase, suggesting the expression in the chemosensitive cells. Intriguingly, the Ang IV-binding and AT4 immunoreactivity were more intense in the carotid body of chronically hypoxic (CH) rats (breathing 10% oxygen for 4 weeks) than the normoxic (Nx) control. Also, the protein level of AT4 receptor was doubled in the CH comparing with the Nx group, supporting an upregulation of the expression in hypoxia. To examine if Ang IV induces intracellular Ca2+ response in the carotid body, cytosolic calcium ([Ca2+]i) was measured by spectrofluorimetry in fura-2-loaded glomus cells dissociated from CH and Nx carotid bodies. Exogenous Ang IV elevated [Ca2+]i in the glomus cells and the Ang IV response was significantly greater in the CH than the Nx group. Hence, hypoxia induces an upregulation of the expression of AT4 receptors in the glomus cells of the carotid body with an increase in the Ang IV-induced [Ca2+]i elevation. This may be an additional pathway enhancing the Ang II action for the activation of chemoreflex in the hypoxic response during chronic hypoxia.

Keywords: Angiotensin IV, angiotensin IV receptor, chemoreceptor, type-I cells.