Protein kinase C modulates calcium channels in isolated presynaptic nerve terminals of rat hippocampus.
Nerve terminals ("synaptosomes") isolated from rat brain hippocampus were loaded with the fluorescent Ca2+ indicator fura-2 and were subjected to depolarization with an elevated K+ concentration in a stopped-flow spectrophotometer to measure the activity of voltage-gated Ca2+ channels in the presynaptic membrane. Three components of Ca2+ influx were seen, which were tentatively identified as two classes of voltage-dependent Ca2+ channels with different inactivation kinetics (tau of approximately 60 ms and 1 s, respectively) and Na+/Ca2+ exchange working in the "reverse" mode. The activity of both classes of voltage-dependent Ca2+ channels was slightly augmented by the phorbol ester phorbol 12-myristate 13-acetate (PMA), an activator of protein kinase C (PKC), but the effect of PMA was markedly enhanced by the protein phosphatase inhibitor okadaic acid (OKA). The PKC inhibitors calphostin C and dihydrosphingosine (DHS) caused a prompt decrease in voltage-dependent Ca2+ channel activity, but the effect of DHS could be slowed by coaddition of OKA. These results suggest that the activity of presynaptic voltage-dependent Ca2+ channels in the hippocampus is under a dynamic balance between PKC phosphorylation (leading to activation) and protein phosphatase dephosphorylation (leading to inactivation) and that both of these metabolic pathways are tonically active in the nerve terminals.
Bartschat
D K
DK
Department of Physiology, Eastern Virginia Medical School, Norfolk 23501, USA.
Rhodes
T E
TE
D000818:Animals; D002118:Calcium / Q000378:metabolism; D015220:Calcium Channels / Q000502:physiology*; D016257:Fura-2; D006624:Hippocampus / Q000502:physiology*; D007700:Kinetics; D009281:Naphthalenes; D010703:Phorbol Esters / Q000494:pharmacology; D010766:Phosphorylation; D011083:Polycyclic Compounds / Q000494:pharmacology; D011188:Potassium / Q000378:metabolism; D017729:Presynaptic Terminals / Q000502:physiology*; D011493:Protein Kinase C / Q000037:antagonists & inhibitors / Q000378:metabolism*; D051381:Rats; D012964:Sodium / Q000378:metabolism; D013050:Spectrometry, Fluorescence; D013110:Sphingosine / Q000031:analogs & derivatives / Q000494:pharmacology; D013755:Tetradecanoylphorbol Acetate / Q000494:pharmacology
D015220:Calcium Channels; D009281:Naphthalenes; D010703:Phorbol Esters; D011083:Polycyclic Compounds; C008876:phorbol-12,13-didecanoate; D012964:Sodium; D011493:Protein Kinase C; C058819:calphostin C; D013110:Sphingosine; D013755:Tetradecanoylphorbol Acetate; C005682:safingol; D011188:Potassium; D002118:Calcium; D016257:Fura-2