The Journal of General Physiology, Vol 85, 137-156, Copyright © 1985 by The Rockefeller University Press
Deuterium oxide and temperature effects on the properties of endplate channels at the frog neuromuscular junction
CA Lewis
The effects of deuterium oxide (D2O) and temperature on the properties of
endplate channels were studied in voltage-clamped muscle fibers from the
frog Rana pipiens. Studies were performed at temperatures of 8, 12, 16, and
20 degrees C. The single channel conductance (gamma) and mean channel
lifetime (tau) were calculated from fluctuation analysis of the
acetylcholine-induced end-plate currents. The reversal potential was
determined by interpolation of the acetylcholine-induced current- voltage
relation. The mean reversal potential was slightly more negative in D2O
Ringer's (-7.9 +/- 0.1 mV [+/- SEM]) compared with H2O Ringer's (-5.2 +/-
0.6 mV, P less than 0.01). The single channel conductance was decreased in
D2O. This decrease was greater than could be accounted for by the increased
viscosity of D2O solutions, and the amount of the decrease was greater at
higher temperatures. For example, gamma was 38.4 +/- 1.3 pS (+/- SEM) in
H2O Ringer's and 25.7 +/- 1.0 pS in D2O Ringer's for a holding potential of
-70 mV at 12 degrees C. The mean channel lifetime was significantly shorter
in D2O, and the effect was greater at lower temperatures. There was not a
strong effect of solvent on the temperature dependence of gamma. On the
other hand, the temperature dependence of the reciprocal mean channel
lifetime, alpha (where alpha = 1/tau), was strongly dependent upon the
solvent. The single channel conductances showed no demonstrable voltage
dependence over the range of -90 to -50 mV in both solvents. The reciprocal
mean channel lifetime showed a voltage dependence, which could be described
by the relation alpha = B exp(AV). The slope A was not strongly affected by
either temperature or the solvent. On the other hand, the intercept B was a
strong function of temperature and was weakly dependent upon the solvent,
with most values greater in D2O. The D2O effects on alpha were what would
be expected if they were due to the properties of D2O as a solvent (solvent
isotope effects), while the D2O effects on gamma must also include the
exchange of D for H in the vicinity of the selectivity filter (primary
and/or secondary kinetic isotope effects).
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