Some investigators, utilizing electron spin resonance techniques or radiologic and binding assays, reported on the presence of muscarinic cholinergic receptors on the membranes of human erythrocytes. Recently, however, another report has questioned these findings. We investigated muscarinic receptors in hemoglobin-free erythrocyte membranes from rats by looking at the binding of the specific muscarinic antagonist 3H-quinuclidinyl benzilate (3H-QNB). We did not find any indication of saturable binding in the range of concentrations used (0.08-8.0 nM). Furthermore, no specific binding was observed in the presence of different concentrations of atropine. Thus, our results suggest that the binding of 3H-QNB to erythrocyte membranes is totally nonspecific in nature. The reasons for these discrepancies are not known, unless one assumes a difference between human erythrocytes and red blood cells from other species. Since cholinergic receptors in blood cells might represent a potential peripheral marker of central cholinergic function, a better understanding of these cholinergic binding sites is certainly warranted.
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