Cation-induced asymmetry of choline flux across presynaptic plasma membranes |
| |
Authors: | Collin WB Campbell |
| |
Institution: | Institut de Biologie Marine, 2 Rue du Prof. Jolyet, 33120 Arachon France |
| |
Abstract: | Highly cholinergic synaptosomes from the optic lobes of Sepia officinalis retain their ability to concentrate K+ and extrude Na+ and to synthesise acetylcholien in vitro. Choline uptake is hemicholinium-3 and Na+ sensitive but is not obligatorily coupled to choline metabolism, or an energy supply as shown by the action of metabolic and ion pump inhibitors. The influx and efflux and/or steady-state distributions of choline in the presence of Na+, Li+, Rb+, Cs+ and mannitol were studied. The influx studies at different cis-choline concentrations revealed two systems for choline influx with different monovalent cation sensitivity and suggested a 1 : 1 interaction of choline with both mechanisms. Choline efflux was stimulated by trans-choline. Calculations of the internal/external concentration ratio expected if choline transport were coupled to the Na+ gradient gave a maximal value of about 102. A secondary active transport of choline, where Na+ is the driver solute provides an explanation for the cation sensitivity of the mechanism as well as for the method of coupling of choline transport to the varying demands of the nervous system for acetylcholine. |
| |
Keywords: | Presynaptic plasma membrane Choline flux Cation-induced asymmetry |
本文献已被 ScienceDirect 等数据库收录! |
|