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1 Department of Molecular and Cellular Physiology, and 2 Division of Nephrology and Hypertension, Department of Internal Medicine, University of Cincinnati, Cincinnati, Ohio 45267-0576
The present study was undertaken to determine whether rat carotid bodies express adenosine (Ado) A2A receptors and whether this receptor is involved in the cellular response to hypoxia. Our results demonstrate that rat carotid bodies express the A2A and A2B Ado receptor mRNAs but not the A1 or A3 receptor mRNAs as determined by reverse transcriptase-polymerase chain reaction. In situ hybridization confirmed the expression of the A2A receptor mRNA. Immunohistochemical studies further showed that the A2A receptor is expressed in the carotid body and that it is colocalized with tyrosine hydroxylase in type I cells. Whole cell voltage-clamp studies using isolated type I cells showed that Ado inhibited the voltage-dependent Ca2+ currents and that this inhibition was abolished by the selective A2A receptor antagonist ZM-241385. Ca2+ imaging studies using fura 2 revealed that exposure to severe hypoxia induced elevation of intracellular Ca2+ concentration ([Ca2+]i) in type I cells and that extracellularly applied Ado significantly attenuated the hypoxia-induced elevation of [Ca2+]i. Taken together, our findings indicate that A2A receptors are present in type I cells and that activation of A2A receptors modulates Ca2+ accumulation during hypoxia. This mechanism may play a role in regulating intracellular Ca2+ homeostasis and cellular excitability during hypoxia.
polymerase chain reaction; in situ hybridization; immunohistochemistry; patch clamp; calcium current; fura 2-calcium imaging
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