Activation of protein kinase C increases acetylcholine release from frog motor nerves by a direct action on L-type Ca(2+) channels and apparently not by depolarisation of the terminal.
The effects of the dihydropyridine Ca(2+) channel antagonist nimodipine and the protein kinase C inhibitors staurosporine and calphostin C on the changes in the electrophysiological indices of quantal acetylcholine release induced by a 4-beta-phorbol ester were studied at the frog neuromuscular junction. 4-beta-Phorbol 12-myristate 13-acetate (200 nM) caused an increase in the frequency of miniature endplate potentials and miniature endplate currents and in the quantal content of endplate potentials and endplate currents. These effects were not replicated by 4-alpha-phorbol 12,13-didecanoate (200 nM). Nimodipine (1 microM) itself had no effect on the frequency of miniature endplate potentials and miniature endplate currents and it had no effect on the quantal content. Nimodipine inhibited by 83-98% the increase in these parameters induced by 4-beta-phorbol 12-myristate 13-acetate. The increase in the frequency of miniature endplate potentials and currents caused by KCl (12 mM) matched the increase caused by 20 min exposure to 4-beta-phorbol 12-myristate 13-acetate. Nimodipine did not reduce the increase in frequency caused by KCl. Unlike 4-beta-phorbol 12-myristate 13-acetate, KCl (12 mM) prevented neuromuscular transmission. The effects of prior exposure of muscles to staurosporine (5 microM) on 4-beta-phorbol 12-myristate 13-acetate-induced increases in quantal acetylcholine release were inconsistent. In some pretreated fibres, 4-beta-phorbol 12-myristate 13-acetate caused increases in miniature endplate potential frequency and quantal content which were as great as the largest values encountered in fibres that had not been pretreated. In others, 4-beta-phorbol 12-myristate 13-acetate did not have a marked effect; the frequency of the spontaneous potentials and the quantal content of endplate potentials recorded in the presence of 4-beta-phorbol 12-myristate 13-acetate were sometimes less than their respective control values. Pretreatment with calphostin C (500 nM) was more consistent; it prevented by 93-100% the 4-beta-phorbol 12-myristate 13-acetate-induced increases in the frequency of miniature endplate potentials and quantal content.Overall, from these results we suggest that activation of protein kinase C increases quantal acetylcholine release by opening quiescent L-type Ca(2+) channels in motor nerve terminals at resting potential and apparently not by depolarisation.
Arenson
M S
MS
Section of Molecular Pharmacology, Division of Biomedical Sciences, School of Medicine and Dentistry, Queen Mary and Westfield College, University of London, Mile End Road, London E1 4NS, UK. m.s.arenson@qmw.ac.uk
Evans
S C
SC
D000109:Acetylcholine / Q000378:metabolism*; D000818:Animals; D002121:Calcium Channel Blockers / Q000494:pharmacology; D020746:Calcium Channels, L-Type / Q000187:drug effects / Q000378:metabolism*; D004791:Enzyme Inhibitors / Q000494:pharmacology; D009046:Motor Neurons / Q000187:drug effects / Q000201:enzymology / Q000378:metabolism*; D009281:Naphthalenes / Q000494:pharmacology; D009469:Neuromuscular Junction / Q000187:drug effects / Q000201:enzymology / Q000378:metabolism*; D009553:Nimodipine / Q000494:pharmacology; D010704:Phorbols / Q000494:pharmacology; D011189:Potassium Chloride / Q000494:pharmacology; D017729:Presynaptic Terminals / Q000187:drug effects / Q000201:enzymology / Q000378:metabolism*; D011493:Protein Kinase C / Q000187:drug effects / Q000378:metabolism*; D011896:Rana temporaria; D019311:Staurosporine / Q000494:pharmacology; D009435:Synaptic Transmission / Q000187:drug effects / Q000502:physiology*; D013755:Tetradecanoylphorbol Acetate / Q000031:analogs & derivatives / Q000494:pharmacology
D002121:Calcium Channel Blockers; D020746:Calcium Channels, L-Type; D004791:Enzyme Inhibitors; D009281:Naphthalenes; D010704:Phorbols; C061465:calphostin complex; C020922:4-O-methyl-12-O-tetradecanoylphorbol 13-acetate; D009553:Nimodipine; D011189:Potassium Chloride; D011493:Protein Kinase C; D019311:Staurosporine; D000109:Acetylcholine; D013755:Tetradecanoylphorbol Acetate; C033085:phorbol