The Na+ and K+ content of isolated Torpedo synaptosomes and its effect on choline uptake.

The Na+ and K+ concentrations in isolated Torpedo marmorata synaptosomes were determined. Synaptosomes made according to the method of Israël et al. have high internal Na+ (290 mM) and low internal K+ (30 mM) concentrations. Modification of the homogenisation media permitted the isolation of synaptosomes which could maintain transmembrane ion gradients (internal Na+, 96 mM; K+, 81 mM); 0.1 mM-ouabain abolished these gradients. The trans-membrane Na+ gradient started to dissipate after 15 min at 20 degrees C. Inclusion of ATP in the homogenisation medium enabled the synaptosomes to maintain the Na+ gradient for about 90 min. The presence of these transmembrane ion gradients stimulated choline uptake sevenfold. It is concluded that (a) by selecting the isolation media, Torpedo synaptosomes can be prepared with transmembrane ion gradients; (b) these gradients are ouabain-sensitive and stimulate choline uptake; (c) the synaptosomes require additional ATP to maintain the ion gradients.