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TJ Rimele, MS O'Dorisio and TS Gaginella
Physiological evidence suggests that muscarinic receptors mediate the secretory response of the intestinal mucosa of cholinergic agonists, but the question of whether muscarinic receptors are intrinsic to intestinal epithelial cells has remained unanswered. We therefore studied binding of the muscarinic ligand [3H]quinuclidinyl benzilate to isolated rat colonic epithelial cells. Specific binding in the membrane preparation reached equilibrium in 30 min at 37 degrees C, was linear with tissue protein concentration and was saturable. Estimates of the apparent equilibrium dissociation constant (KD) equalling 0.11 +/- 0.03 nM and of maximum receptor density (Bmax) equalling 103.6 +/- 9.0 fmol/mg of protein were determined from Scatchard plots of the binding data. The Hill coefficient for binding was 0.93 +/- 0.04. From kinetic analysis of data, association and dissociation rate constants of 8.6 X 10(7) M-1 min-1 and 6.0 X 10(-3) min-1, respectively, were calculated. Inhibition of specific binding was stereoselective and pharmacologically specific for muscarinic agents. Specific binding, vasoactive intestinal peptide-stimulated adenylate cyclase and Na+-K+- stimulated adenosine triphosphatase activities were highest in a mitochondrial-free fraction of rat colonocytes. Specific binding was lowest in the nuclear fraction, followed by the total particulate and mitochondrial fractions. The regional distribution of specific binding along the length of the intestine was also studied. Scatchard analysis revealed one specific binding site, with the same KD in the jejunum, ileum and colon. The receptor density was the same in the jejunum and ileum, but about 5-fold higher for the existence of muscarinic receptors on intestinal epithelial cells and suggest that the effects of muscarinic drugs on intestinal epithelial cell function are mediated by interaction with these receptors.
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