AJP - Cell Physiology, Vol 245, Issue 5 308-C315, Copyright © 1983 by American Physiological Society
Stereospecific glucose transport across motor nerve terminal membrane: an electrophysiological study
Y. Shimoni and R. Rahamimoff
Spontaneous transmitter release at the neuromuscular junction of the frog
and rat was monitored during exposures to hyperosmotic solutions containing
different sugars. Raising the osmolarity of the medium with D-glucose
causes a marked, but transient, increase in the frequency of miniature
end-plate potentials (MEPPs): after the initial elevation in frequency
there is a subsequent decline towards the control levels, in spite of a
continuous perfusion with the hyperosmotic solution. This decline occurs
more rapidly in the frog. Two nonmetabolized analogues of glucose,
2-deoxy-D-glucose and 3-O-methylglucose, cause a transient hyperosmotic
increase in MEPP frequency, which is very similar to the effect of
D-glucose. The elevation of MEPP frequency with hyperosmotic glucose is
stereospecific. Hyperosmotic solutions of L-glucose cause a sustained
increase in transmitter release in the rat and frog. Insulin dramatically
reduces the response of the frog nerve terminal to hyperosmotic D-glucose.
Phenolphthalein, a glucose transport blocker, reduces or eliminates the
secondary decline in MEPP frequency. It is suggested that the transient
nature of the response to hyperosmotic solutions reflects the penetration
of the hyperosmotic agent into the nerve terminal. The rate of decline of
the MEPP frequency presumably indicates the rate of transport, which
determines the rate of osmotic equilibration. This rate can then serve as
an index of the relative permeability of the functioning presynaptic
membrane to different sugars.
