Binding of [3H]quinuclidinyl benzilate to intestinal mucus: An artifact in identification of epithelial cell muscarinic receptors |
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Authors: | Thomas J Rimele Timothy S Gaginella |
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Institution: | Division of Pharmacology, College of Pharmacy and Department of Medicine, College of Medicine, The Ohio State University, Columbus, OH 43210, U.S.A. |
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Abstract: | The widely used muscarinic receptor ligand 3H]quinuclidinyl benzilate (3H]QNB) was found to bind in a site-specific but artifactual manner to rat intestinal mucus, obscuring specific binding to muscarinic receptors on intestinal epithelial cells. Atropine inhibited 3H]QNB binding to mucus with an apparent IC50 of 2.1 × 10?7 M, compared to an IC50 of 1.4 × 10?8 M obtained with a homogenate of intestinal epithelial cells. Unlabeled QNB also inhibited binding of 3H]QNB to mucus but the apparent IC50(4 × 10?7 M) was about 300-fold greater than the IC50 determined with a control tissue, heart muscle (IC50, 1.2 × 10?9M). 3H]QNB binding was saturable over the concentration range of 1–7 nM in the heart, with an apparent kD of 0.76 nM. As expected from the high IC50 for QNB in the mucus binding experiments, binding to mucus was not saturable over the 1–15 nM concentration range. Based on pH profiles and temperature dependency of binding, it seems unlikely that mucin, the primary component of mucus, was responsible for 3H]QNB binding to the mucus. The findings have implications for studies which involve binding of 3H]QNB in particular and other ligands in general to mucus-secreting epithelial tissues. |
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Keywords: | Address correspondence to: T S Gaginella Ph D The Ohio State University College of Pharmacy 500 West 12th Ave Columbus OH 43210 U S A |
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