Abstract
The effects of altering calcium/magnesium ratios (Ca ++/Mg ++) were tested on the rat diaphragm cut muscle preparation. Decreasing Ca ++/Mg ++ did not alter MEPPs, but evoked release was significantly depressed. The decrease in quantal content (m) was a function of Ca ++/Mg ++, and a plot of log m vs. log Ca ++/Mg ++ indicated a power relationship of 3.69. Although low Ca ++/Mg ++ caused a significant reduction in the probability of release (p), the quantal distribution was still best described by binomial statistics. Mobilization measured at high frequency (150 Hz) was reduced by decreasing Ca ++/Mg ++. Verapamil decreased m and n, but did not alter mobilization or p. The highest concentration employed (300 uM) decreased MEPP frequency and produced conduction block to high frequency EPP trains. Thus it appears that altering the effective calcium concentration has different effects on transmitter release as compared to verapamil.
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