OPPOSING EFFECTS OF PHORBOL ESTERS ON TRANSMITTER RELEASE AND CALCIUMCURRENTS AT FROG MOTOR-NERVE ENDINGS

1. Phorbol esters activate protein kinase C (PKC) and also increase th e secretion of neurotransmitter substances by an unknown mechanism. To evaluate whether the stimulatory effects of such agents on acetylchol ine (ACh) secretion occur as a consequence of stimulation of Ca2+ entr y, we made electrophysiological measurements of ACh secretion (i.e. en dplate potentials, EPPs) and the component of the prejunctional perine ural voltage change associated with nerve terminal calcium currents (p erineural calcium current) at frog neuromuscular junctions. 2. In the first series of experiments, modest concentrations of K+ channel block ers were employed so that simultaneous measurements of EPP amplitudes and perineural calcium currents could be made. In these experiments, 1 2-O-tetradecanoylphorbol 13-acetate (TPA; 162 nM) and phorbol 12,13-di butyrate (PDBu; 100-200 nM) each increased ACh release but simultaneou sly decreased the calcium component of the prejunctional perineural cu rrent. TPA and PDBu also inhibited perineural calcium currents in the presence of higher concentrations of K+ channel blockers. 3. Blockade of Ca2+ channels by Cd2+ prevented the action of PKC stimulators on pe rineural waveforms. 4. The inactive compound 4-alpha-phorbol 12-myrist ate 13-acetate (150 nM) did not affect EPP amplitudes or perineural cu rrents. 5. The extracellular [Ca2+]-ACh release relationship was incre ased in maximum by PDBu without any change in the potency of Ca2+ to s upport evoked ACh release. 6. The results demonstrate that phorbol est ers increase neurotransmitter secretion whilst simultaneously decreasi ng the nerve ending calcium currents that promote evoked release. The results, which suggest that the optimal control point for secretion mi ght not be the calcium channel but rather a component of the secretory apparatus, are discussed in conjunction with the possible target site s for phorbol esters in the nerve ending.