Fig. 1. Ca²⁺-dependent facilitation of spontaneous ACh secretion by RA. The continuous trace depicts the membrane currents recorded from an innervated myocyte in day-1 Xenopus cell culture, using the whole-cell recording method (V_H=–70 mV, filtered at 10 kHz). Downward events are inward currents resulted from quantal ACh secretion. Bath application of RA at 30 µM dramatically enhanced spontaneous transmitter release, as seen by a marked increase in the frequency of spontaneous synaptic events. Samples of current events are shown below at higher time resolution. Scale bars are 1 nA, 20 seconds, and 1 nA, 50 milliseconds for the slow and fast traces, respectively. (B) Pretreatment of the culture with BAPTA-AM at 30 µM significantly abolished RA-induced facilitation of SSC frequency. (C) Summary of the effect of a Ca²⁺ chelator. Values are the mean and s.e.m. from 6-17 separate experiments. The SSC frequency from a single synapse was counted for a 6-minute period in control and a 6-minute period after RA application. The data were then averaged and normalized to control of the same synapse (n=6-17). The horizontal reference dotted line, which defines basal activity as `1' is shown for comparison. *P<0.05 compared with the control group (Student's t-test).