Rapid changes of fluorescence intensity take place after fast mixing of quinacrine-labelled receptorrich membrane fragments from Torpedo marmorata with cholinergic effectors and are recorded in the time range of 1 -1000 ms, accessible to the stopped-flow technique. Agonists such as acetylcholine, phenyltrimethylammonium, carbamylcholine, suberyldicholine or choline cause the change, but not antagonists such as d-tubocurarine, flaxedil or Naja nigricollis cx-toxin. In addition, preincubation of the membrane fragments with N . nigricollis a-toxin blocks the response to agonists. Ceruleotoxin, a toxin from Bungarus ceruleus venom which blocks the postsynaptic response to acetylcholine at a site distinct from the a-toxin site, causes a decrease of the apparent amplitude of the fast signal. In the presence of 0.05 mM tetram, an acetylcholinesterase inhibitor, the kinetics of the fast response to acetylcholine do not change; in the second or minute range and in the case of acetylcholine, however, a decrease of fluorescence intensity occurs in the absence of acetylcholinesterase inhibitor showing that the fast fluorescence signal is reversible.Under the present experimental conditions the observed fast kinetics do not vary with the concentration of receptor sites or quinacrine but do vary with the concentration of agonist. The data can be analyzed in terms of the phenomenological reaction scheme A + B AB 3 A*B, when k, % k,, if the fluorescence signal monitors the isomerization from A to A*. The following values of the constants are found: for acetylcholine: k4=
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.