Heterogeneity Of High-Affinity Nicotinic [3H]Acetylcholine Binding Sites



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Studies were conducted on high-affinity, nicotinic binding of 3H-labeled acetylcholine ([3H]ACh) to membrane preparations derived from the TE671 human clonal line and the PC12 rat pheochromocytoma. For comparative purposes and to extend results obtained by others in previous studies, [3H]ACh binding to membrane preparations derived from rat brain and from the electric tissue of Torpedo calfornica also was characterized. In each case, specific [3H]ACh binding (K(D) values of about 10 nM) could be fit by linear Scatchard and logit-log curves (slope of the latter of about 1.0) indicating that binding occurred to a single class of noninteracting sites, except that a better fit to PC12 cell membrane binding data was obtained using a two-site model. Quantitation of high-affinity binding sites for [3H]ACh and radiolabeled α-bungarotoxin and studies of unlabeled toxin competition for [3H]ACh binding indicated that toxin and agonist sites on TE671 cell or Torpedo membranes were closely related, but that toxin and agonist sites had limited or no physical overlap on PC12 cell or rat brain membranes. Generally, drugs of the bis-onium and bis-choline series were more effective inhibitors of [3H]ACh binding to TE671 cell or Torpedo membranes, but nicotine and cytisine interacted with PC12 cell or rat brain sites with highest affinity. These results suggest that nicotinic agonists bind with high affinity to specific, membrane-bound sites on presumptive nicotinic ACh receptors expressed on each of these preparations. Moreover, the data are consistent with structural heterogeneity of these nicotinic receptor sites. This structural heterogeneity may account for observed differences in drug and curarimimetic neurotoxin sensitivities of functional nicotinic receptor responses and may be used to classify nicotinic receptor subtypes, which appear to be the products of a diverse family of nicotinic receptor genes.

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Journal of Pharmacology and Experimental Therapeutics







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