A, distribution of m broadened as stimulation frequency increased. B, this broadening was suppressed by HS. C and E, the number of releasable quanta, n, increased during frequency facilitation. This increase was suppressed by HS. D and F, HS increased release probability, p. at the lowest stimulation frequency, f = 1 Hz. C and D show the data from Experiment 1; E and F show the average from 15 experiments. * Significance at 5 %.