Regulation of choline transport through synaptosomal membrane in the sodium-containing and sodium-free media

The transfer of choline through neuronal membrane is a specific, high-affin ity and sodium-dependent process. It is a rate-limiting and regulatory step in acetylcholine synthesis. In synaptosomes it was observed that high-affi nity but Na+-independent choline uptake proceeds concurrently to Na+-depend ent transport. The aim of the study was to reveal some regulatory mechanism s of high-affinity choline transport through synaptic striatal membrane in Na+-containing and Na+-free media. We have studied choline transport kineti cs in synaptosomes as well as conformational transitions in isolated membra nes in the presence of hemicholinium-3 (HC-3), known inhibitor of choline t ransport. It was shown that the light rihythm changes kinetic parameters of high-affinity choline uptake in the two media studied. In the synaptosomes obtained from striata rats adaptated to 1hyght rithm the V-max of sodium-d ependent uptake prevails over the sodium-independent one. In the synaptosom es obtained from striata of non adopted rats it was noted opposite relation s. It was shown that HC-3 inhibited the Na+-dependent and Na+-independent t ransport in different ways. This fact seems to provide evidence for relativ e independence of the two processes. HC-3 effects on the membrane structure (microviscosity and polarity of both the bulk and anular lipids were) stud ied. The effects of HC-3 depends on the presence or absence of sodium ions in the medium. In the Na+-containing medium microviscosity of the bulk lipi ds is significantly lower in the presence of 0.1, 1 and 5 muM HC-3 as compa red to Na+-free medium. In the Na+-containing medium microviscosity of the anular lipids at the HC-3 concentrations of 0.1 muM is significantly lower as compared to control. Our data suggested an important role of sodium ions as regulators of synaptosomal choline uptake.