The effect of temperature on the amplitude distributions of miniature endplate potentials in the mouse diaphragm

Abstract

The effect of temperature (11 to 45°C) on miniature endplate potential (MEPP) distributions was examined at the mouse diaphragm. MEPP distributions were composed of two populations ('skew-MEPPS' and ‘bell-MEPPs’) and the mode of the skew-MEPP population (subminiature endplate potential, sub-MEPP) had an amplitude equal to 1/10 to 1/15 the mean of the normally distributed bell-MEPP class. The overall MEPP frequency increased with a Q 10 of 2.2 between 11 and 30°C, and an Arrhenius plot indicated two temperature-sensitive reactions between 11 and 45°C. Below 30°C, the activation energy was 10.4 kcal/mole K and between 30 and 45°C the activation energy was 38.7 kcal/ mole K. The proportion of skew-MEPPs between different fibers was independent of the overall MEPP frequency and varied between 1% and 31% at temperatures of 11–34°C. However, as the temperature was lowered below 24°C, the decrease in frequency of skew-MEPPs was more than that of bell-MEPPs. Conversely, an increase in temperature from 24 to 34°C increased the bell-MEPP frequency but either reduced or had little effect on the frequency of skew-MEPPs. Following heat challenges (T > 40°C), MEPP distributions contained a large percentage of skew-MEPPs and the profile of the MEPP distribution became uniform. Before complete cessation of spontaneous activity with multiple heat challenges, MEPP amplitude distributions were either uniform or were composed primarily of sub-MEPPs. MEPP time courses were slower after heat challenges, and in some preparations, inflections were observed on many MEPP rising phases. Amplitude histograms of these inflections yielded distributions similar to control distributions of skew-MEPPs. The presence of inflections on MEPPs following heat challenges supports the hypothesis that skew-MEPPs and bell-MEPPs are composed of subunits. These results suggest that skew- and bell-MEPPs are caused by the release of transmitter by different temperature-sensitive mechanisms.