Abstract

Some properties of functional nicotinic acetylcholine receptors (nAcChoR) expressed by the PC12 rat pheochromocytoma or the TE671 human medulloblastoma were studied by the use of an isotopic rubidium ion efflux assay. The assay involves active uptake of 86Rb+ via a ouabain-sensitive mechanism to load cells with isotopic tracer and the subsequent release of ion from cells through agonist-activated opening of nAcChoR-coupled ion channels. For either cell line the rate of receptor-mediated ion efflux is time-dependent and falls as duration of exposure to agonist increases. However, dose-response curves for agonist activation of receptor function (or for antagonist blockade of agonist activation) are temporally invariant. Dose-response curves have characteristic shapes for a given agonist, and the relative and absolute potencies of agonists differ between TE671 and PC12 cells. Most notably, nicotine and cytisine are more potent activators of receptor function on the PC12 cell line whereas TE671 cell nAcChoR are more sensitive to activation by isoarecolone and suberyldicholine. PC12 cell nAcChoR are more sensitive to blockade by the classic "ganglionic" blockers, mecamylamine and hexamethonium, and by the molluscan substance, neosurugatoxin, than are nAcChoR on TE671 cells. The results, illustrating differences in the pharmacological profiles of drugs acting as functional nAcChoR on TE671 and PC12 cells, suggest that structural differences exist in nAcChoR active site(s) and are consistent with the notion that functional nAcChoR are a heterogeneous family of macromolecules.