Binding of [3H]quinuclidinyl benzilate to intestinal mucus. An artifact in identification of epithelial cell muscarinic receptors.

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 x 10(-7) M, compared to an IC50 of 1.4 x 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 x 10(-7) M) was about 300-fold greater than the IC50 determined with a control tissue, heart muscle (IC50, 1.2 x 10(-9) M). [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.