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”,F _c, 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,F _c had a maximum, \(\hat F_c \) , at about 1/10 the saturation level of release, as reported before. In ‘double-pulse facilitation’,F _d, 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,F _d 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 preparationsF _d rose to a maximum at low depolarization and release, declined to a minimum at the depolarization level of \(\hat F_c \) , and rose again for larger depolarizations. In some preparations, and for short pulses, the peak ofF _d at low depolarizations was not observed, but alwaysF _d increased with depolarization can be explained by the residual Ca theory, if at depolarizations larger than that which produced \(\hat F_c \) and the minimum ofF _d, Ca-inflow decreases. The same was concluded before from the decline ofF _c with increasing depolarization in this range. While thus forF _c andF _d, the effects of changes in pulse amplitude and duration on facilitation can be explained by the residual Ca theory, the further increase of release with simultaneous decline of Ca-inflow at large depolarizations indicates that in addition to [Ca]_i another potential dependent activator controls release. Surprisingly, at 0°C no facilitation could be measured in spite of large release. For low pulse amplitudes, release had a large Q₁₀ of 10 to 40 in the range from 0°C to 20°C.