Journal of Applied Physiology, Vol 70, Issue 5 2066-2071, Copyright © 1991 by American Physiological Society
Depolarization reverses age-related decrease of spontaneous transmitter release
W. B. Alshuaib and M. A. Fahim
Andrus Gerontology Center, University of Southern California, Los Angeles 90089-0191.
The effect of increasing extracellular Ca concentration on spontaneous
transmitter release was studied at soleus nerve terminals of young (10 mo)
and old (24 mo) C57BL/6J mice depolarized by high extracellular K
concentration ([K]o). By using intracellular recording, miniature end-plate
potentials (MEPPs) were first recorded in a normal [K]o Krebs solution.
Subsequently, MEPPs were recorded in high [K]o Krebs solutions with four
different Ca concentrations: Ca-free/ethylene glycol-bis(beta-aminoethyl
ether)-N,N,N',N'-tetraacetic acid and 0.5, 1.5, and 2.5 mM Ca. In both the
normal [K]o Krebs and the Ca-free-high [K]o Krebs solutions, MEPP frequency
was lower at old than at young nerve terminals. In the three high [K]o
Krebs solutions with Ca, MEPP frequency was progressively higher at old
than at young nerve terminals with higher Ca concentrations. Periodic
oscillations were observed in MEPP frequency of depolarized nerve
terminals. The period of oscillation was inversely proportional to
spontaneous transmitter release. These results demonstrate that when the
nerve terminal is depolarized, permeability of the terminal membrane to Ca
increases because of opening of voltage-dependent Ca channels. In the
present study resting MEPP frequency was lower at old than at young
terminals. On depolarization, MEPP frequency became higher at old than at
young terminals. The study demonstrates that voltage-dependent Ca entry
increases during aging at the soleus nerve terminal.
