1. A study was made of the effects of phorbol ester (phorbol 12-myristate 13-acetate, PMA, 0.1 μM) on the two components of evoked transmitter release, namely the fast synchronous and the slow asynchronous components, from the giant presynaptic terminal of the chick ciliary ganglion. The excitatory postsynaptic currents (EPSCs) were recorded under whole-cell voltage clamp of the postsynaptic neuron. 2. The decay time constant of the slow component was prolonged by replacing Ca2+ with Sr2+. In 5 mM [Sr2+](o) the fast component decayed with a time constant of 2.6 ± 1.4 ms whereas the slow component decayed with a time constant of 19 ± 7 ms. 3. When stimulated with twin pulses with a short interpulse interval, the fast component of the second EPSC was often depressed whereas the slow component was usually facilitated. Both components were positively dependent on [Sr2+](o) in a saturable manner, but the fast component approached its maximum at a lower [Sr2+](o) than the slow component. 4. PMA potentiated both the fast and slow components to a similar extent and with a similar time course. For each component, the effect of PMA was less potent at high [Sr2+](o) than at low [Sr2+](o). For either the fast or the slow component the PMA-induced potentiation was accompanied by a reduction in the paired-pulse ratio (PPR). 5. Despite the different dissociation constant for dextran-conjugated fura-2, the fluorescent ratio for intraterminal [Sr2+] ([Sr2+](i)) decayed to the baseline after the nerve-evoked increment with a time course similar to that for [Ca2+](i), suggesting that intraterminal Sr2+ is buffered less efficiently than Ca2+. PMA did not increase the [Sr2+](i) transients produced by stimulation of the presynaptic oculomotor nerve. 8. It is suggested that protein kinase C (PKC) modulates both the fast and slow components through common molecular mechanisms that upregulate the Sr2+ sensitivity of the vesicle fusion probability.
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