The Journal of Cell Biology, Vol 69, 521-538, Copyright © 1976 by The Rockefeller University Press
Changes in the structure of neuromuscular junctions caused by variations in osmotic pressure
AW Clark
Neuromuscular junctions of the frog, Rana pipiens, were examined for
structural modifications produced by exposure to increased and reduced
osmotic pressure (pi). Preparations exposed to increased pi for varying
lengths of time were fixed with either OSO4-Veronal with and without
calcium, glutaraldehyde-phosphate, or glutaraldehyde-formaldehyde-
phosphate as primary fixatives. The greatest difference between the
fixatives was seen in preparations exposed to increased pi for 5 min,
corresponding to the time when miniature endplate potential frequency is
highest. The 5-min OSO4 calcium-free preparations appeared comparatively
normal, while those fixed with OSO4 and 2 mM CaCl2 or aldehyde-phosphate
had wide infoldings of the presynaptic membrane and a reduced number of
synaptic vesicles. Aldehyde-phosphate had the same effect on mouse
diaphragm. Another series of frog preparations were conditioned to elevated
pi and then returned to normal Ringer's for varying times before fixation
in OSO4-phosphate. Preparations fixed 2 min after their return to normal
Ringer's showed marked disruption of the presynaptic membrane as well as
apparently rupturing vesicles. If fixed after 10 min, terminals were
depleted of vesicles although the presynaptic membrane had returned to its
normal position and appearance.
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