Abstract
The existence and localization of brain angiotensin receptors is well established. However, questions regarding the endogenous ligand for brain angiotensin type 1 (AT1) receptors necessitates re-examination of brain angiotensin receptor binding studies. To assess the ability of angiotensin II to bind to the brain AT1 receptor, radioligand binding studies of rat brain AT1 receptors were performed using both 125I-angiotensin II and 125I-sarcosine1, isoleucine8 angiotensin II. Determination of binding kinetics and competition by an AT1 receptor antagonist was carried out to reveal the identity of the membrane binding sites and to identify the bound 125I-labeled molecules. Initial analysis of 125I-angiotensin II binding to hypothalamic membranes using an established protocol revealed that a negligible amount of intact radioligand was bound to the membranes. In contrast, binding of 125I-sarcosine1, isoleucine8 angiotensin II was saturable, of high affinity, and primarily as intact radioligand. Sequential addition of four peptidase inhibitors—o-phenanthroline, puromycin, phenymethylsulfonyl fluoride, and glutamate phosphonate—to the assay buffer dramatically increased the binding of 125I-angiotensin II to rat brain membranes: more than 75% of the bound 125I was the intact radioligand, and the binding was of high affinity and saturable. Some, but not all, of the binding could be displaced by the AT1-selective antagonist losartan. This demonstrates that 125I-angiotensin II can bind to brain AT1 receptors and does not require conversion to 125I-angiotensin III to bind to brain AT1 receptors.
- Ang, angiotensin
- SI Ang II, sarcosine1, isoleucine8 angiotensin II
- AT1, angiotensin receptor type 1
- AT2, angiotensin receptor type 2
- PD123319, S-(+)-1-([4-(dimethylamino)-3-methylphenyl]methyl)-5-(diphenylacetyl)-4,5,6,7-tetrahydro-1H-imidazo(4,5-c)pyridine-6-carboxylic acid;
- GluP, glutamate phosphonate (4-amino-4-phosphonobutyric acid)
- JA-2, N-[1-(R,S)-carboxy-3-phenylpropyl]-Ala-Aib-Tyr-p-aminobenzoate;
- HPLC, high-performance liquid chromatography
- PMSF, phenylmethylsulfonyl fluoride.
Footnotes
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↵This work was supported by The Peptide Radioiodination Service Center of the University of Mississippi; and startup funds provided by the School of Pharmacy, Texas Tech University Health Sciences Center.
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Current affiliation: Department of Pharmaceutical Sciences, School of Pharmacy, Texas Tech University Health Sciences Center, Amarillo, Texas.
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Article, publication date, and citation information can be found at http://jpet.aspetjournals.org.
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ABBREVIATIONS:
- Received June 9, 2009.
- Accepted July 7, 2009.
- © 2009 by The American Society for Pharmacology and Experimental Therapeutics
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