Hsiao-Ping H. Moore

Fast cation flux from Torpedo californica membrane preparations: Implications for a functional role for acetylcholine receptor dimers☆☆☆

Abstract Closed membrane vesicles derived from the innervated face of Torpedo californica electroplax respond to the cholinergic agonist carbamylcholine by a rapid efflux of cations. This response is detected by release of 22Na enclosed within the vesicles and is considerably faster than previously reported for this in vitro system. It is considered likely that the rapid response is analogous to physiological phenomena since it has the appropriate pharmacological characteristics; it desensitizes upon prolonged contact with the agonist and it has a dose-response curve in a physiological range. It is further shown that a dimeric form of the acetylcholine receptor, stabilized by chemical modification methods, is fully active in terms of the carbamylcholine elicited response.

Rapid cation flux from Torpedo californica membrane vesicles: comparison of acetylcholine receptor enriched and selectively extracted preparations.

Abstract Rapid efflux of 22 Na from within closed vesicles derived from Torpedo californica electroplax membranes has been studied as an in vitro assay of acetylcholine receptor functionality. The most highly purified membrane preparations contained major polypeptides of M.W. 43 and 90 × 10 3 daltons in addition to the four peptides characteristic of the acetylcholine receptor (40, 50, 60, 65 × 10 3 daltons). Removal of these extra peptides by base extraction did not significantly alter the characteristics of carbamylcholine induced 22 Na efflux: the agonist dose response curve was similar, preequilibration with agonist caused desensitization, the irreversible antagonist α-Bungarotoxin blocked the efflux and the reversible blockade by the neurotoxin perhydrohistrionicotoxin was also retained. The dose response curve for perhydrohistrionicotoxin corresponded closely to its known binding characteristics for base extracted membranes.