Molecular Pharmacology, Vol 18, 28-32, Copyright © 1980 by the American Society for Pharmacology and Experimental Therapeutics

Further Characterization of Human Red Blood Cell Membrane Cholinergic Receptors

¹ Departments of Pharmacology and Psychiatry, University of Pittsburgh Medical School and Western Psychiatric Institute and Clinic, Pittsburgh, Pennsylvania 15261

Human red blood cells bind d,l-[³H]quinuclidinyl benzilate, a potent muscarinic cholinergic antagonist, with high affinity (apparent K_D=0.26 nM) and low capacity as determined by equilibrium binding experiments. Nonspecific binding determined as binding in the presence of atropine is hyperbolic, and saturable within the same concentration range as total binding. The d-but not the l-isomer of benzetimide weakly inhibits ligand binding, which parallels the stereoselectivity but not the potency of drug antagonism to muscarinic receptors in other tissues. While treatment of brain homogenates with Triton X-100 (0.5%) destroys all d,l-[³H]quinuclidinyl benzilate binding to that tissue, similar detergent treatment of red blood cell membranes fails to greatly alter either total or certain drug-displaced ligand binding. These results show that the red blood cell membrane binding site for d,l-[³H]quinuclidinyl benzilate differs in some important aspects of its binding characteristics, pharmacology, and physical properties from muscarinic receptors similarly studied in other nervous and nonnervous tissue. Further characterization of the red blood cell cholinergic ligand binding site is necessary to establish its identity as either a different form of muscarinic receptor or as an acceptor site on other membrane cholinergic enzymes or carrier proteins.