Dependence of double-pulse facilitation on amplitude and duration of the depolarization pulses at frog's motor nerve terminals.

Motor nerve terminals of the frog were depolarized by pairs of pulses with 5 to 10 ms interval and the resulting quantal transmitter releases were determined. In 'fixed pulse facilitation', Fc, the second pulse was kept constant, and the effect of a varying pre-pulse was measured, comparing the thus facilitated release after the fixed pulse to control release after the fixed pulse alone. If depolarization in the pre-pulse was increased from threshold to almost saturation level of release, Fc had a maximum, Fc, at about 1/10 the saturation level of release, as reported before. In 'double-pulse facilitation', Fd, two identical pulses were applied, and the facilitated release after the second pulse was compared to control release after the first pulse. On increasing pulse duration from 0.4 to 2.5 ms, at fixed depolarization levels, Fd had a peak at short pulse duration and low release, and declined with increasing pulse duration and release. This dependence is expected if facilitation is caused by 'residual Ca'. Alternatively, if at fixed duration depolarization in the pulses was increased from threshold level, in most preparations Fd rose to a maximum at low depolarization and release, declined to a minimum at the depolarization level of Fc, and rose again for larger depolarizations. In some preparations, and for short pulses, the peak of Fd at low depolarizations was not observed, but always Fd increased with depolarization beyond Fc. The complicated dependence of Fd on depolarization can be explained by the residual Ca theory, if at depolarizations larger than that which produced Fc and the minimum of Fd, Ca-inflow decreases.(ABSTRACT TRUNCATED AT 250 WORDS)