Marked up-regulation of the beta-bungarotoxin site in adrenal chromaffin cells by specific nicotinic antagonists.

The effect of nicotinic antagonists was studied on various parameters of adrenal medullary chromaffin cells in culture. Incubation of the cells in culture with d-tubocurarine or mecamylamine for 0.5-6 days resulted in up to an 8-fold increase in the binding of alpha-bungarotoxin (alpha-BGT) to the cells; other ganglionic blockers, on the other hand, such as hexamethonium and dihydro-beta-erythroidine, had no effect. This enhanced binding was due to an increase in the number of alpha-BGT sites with little change in affinity of the ligand for the receptor. The nature of the increase in the number of toxin-binding sites in chromaffin cells observed after treatment with d-tubocurarine or mecamylamine was subsequently investigated. A direct interaction of the drug with the alpha-BGT recognition site was not required for the increase in receptor number to occur; mecamylamine did not affect binding of alpha-BGT to chromaffin cells in culture in competition binding experiments, although d-tubocurarine did compete with alpha-BGT for binding to its recognition site. The reversal of the antagonist-induced increase in the alpha-BGT-binding sites by nicotine and carbachol suggested it was mediated through an interaction at an acetylcholine receptor recognition site. The depolarizing agent veratridine greatly attenuated the increase in the number of toxin-binding sites in response to antagonists; this effect of veratridine could be reversed by tetrodotoxin. These latter findings indicate that neuronal excitability can influence the observed increase in the number of alpha-BGT sites after exposure of the cultures to nicotinic antagonists. The antagonist-induced increase in the alpha-BGT sites in the cells was not associated with an increased functional responsiveness of the cells to acetylcholine. The present results demonstrate that the number of alpha-BGT-binding sites in adrenal medullary chromaffin cells can increase dramatically in response to some, but not other, nicotinic antagonists by an interaction at a nicotinic acetylcholine recognition site. The differential effect of antagonists at the nicotinic-like alpha-BGT site and the functional nicotinic receptor suggests these two parameters are distinct. The unusually large alteration in receptor number may be related to the unique localization of the adrenal medulla and could infer that these receptors have a role in this tissue.