Effects of changes in tonicity of the extracellular solution on the size of vesicles in frog motor nerve terminals.

Frog nerve-muscle preparations were soaked and then fixed in solutions roughly isotonic to frog plasma, or in solutions that were markedly hypertonic or hypotonic. The hypertonic solution decreased the cross-sectional area of the muscle fibers but not of the synaptic vesicles. The hypotonic solution increased the cross-sectional area of the muscle fibres but did not produce comparable increases in the areas of the synaptic vesicles. Apparently the vesicles in situ do not behave as simple osmometers. This fact is significant for theories of exocytosis and for the mechanism of transmitter packaging in the vesicles.