Deuterium oxide and temperature effects on the properties of endplate channels at the frog neuromuscular junction

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).