Abstract

Previous work had shown that incubation with the nicotinic antagonist d-tubocurarine resulted in a marked increase in alpha-bungarotoxin (alpha-BGT) binding in adrenal medullary chromaffin cells in culture; the possible molecular mechanisms involved in up-regulating the alpha-BGT sites were investigated. To determine whether changes in the extracellular K+ concentration could influence the number of toxin binding sites, the chromaffin cells were incubated in the presence of 2-50 mM K+; this resulted in an increase in alpha-BGT binding similar to that observed with the nicotinic antagonist. This enhanced binding was maximal with 20 mM K+ and was not due simply to a generalized ion effect, inasmuch as incubation of the cells with a concentration of Na+ of equivalent osmolarity did not alter alpha-BGT binding. Carbachol and the agonist nicotine completely prevented the K+-induced increase in the binding sites. In contrast to the marked up-regulation of the nicotinic alpha-BGT sites by K+, this agent did not increase the acetylcholine-induced release of [3H]noradrenaline from chromaffin cells in culture, further supporting the contention that the nicotinic alpha-BGT site and the functional nicotinic receptor are distinct. The increases in toxin binding due to K+ and d-tubocurarine were partially additive, suggesting that d-tubocurarine and K+ may share a common pathway, but only to a small degree. The calcium channel agonist BAY K 8644 and antagonist D600 had no effect on alpha-BGT binding either alone or in the presence of K+ or d-tubocurarine. On the other hand, forskolin, an activator of adenylate cyclase, and dibutyryl cAMP, an analog of cAMP, partially prevented the K+ and the d-tubocurarine-induced increases in toxin binding. These results suggest an involvement of cAMP in both the nicotinic antagonist-induced and K+-induced up-regulation of the sites. The observation that several mechanisms exist for the fine regulation of the nicotinic alpha-BGT binding sites in adrenal chromaffin cells could imply that this nicotinic receptor population plays a role in this tissue.