[Effects of calcium, ethanol and hyperosmolarity on the distribution of intervals between MEPP's]

Distributions of intervals between MEPP's, recorded from the frog sartorius muscle in various experimental conditions, were compared, graphically and by X2, with the exponential distributions expected for a Poisson process. For each experiment, 200 intervals were measured. In isotonic Ringer solutions containing no Ca++, MEPP's fitted an exponential distribution; by adding Ca++ (1 mM, 2,5 MM, 15 mM), the fit between empirical and expected distributions became poorer, with an excess of short intervals. MEPP's showed the tendency to occur in bursts, and this was particularly evident at 15 mM Ca++. In the presence of 0,5% ethanol, also at high Ca++ the intervals between MEPP's showed a fair fit to the exponential distribution. In Ringer solutions made hypertonic by the addition of 50 mM sucrose, the fit was poor and there was a relative lack of short intervals. These different patterns of MEPP distribution could be explained by supposing that Ca++, ethanol and hypertonicity activate different mechanisms of transmitter release.