Trifluoperazine enhancement of Ca2+-dependent inactivation of L-type Ca2+ currents in Helix aspersa neurons.
The effects of trifluoperazine hydrochloride (TFP), a calmodulin antagonist, on L-type Ca2+ currents (L-type ICa2+) and their Ca(2+)-dependent inactivation, were studied in identified Helix aspersa neurons, using two microelectrode voltage clamp. Changes in [Ca2+]i were measured in unclamped fura-2 loaded neurons. Bath applied TFP produced a reversible and dose-dependent reduction in amplitude of L-type ICa2+ (IC50 = 28 microM). Using a double-pulse protocol, we found that TFP enhances the efficacy of Ca(2+)-dependent inactivation of L-type ICa2+. Trifluoperazine sulfoxide (50 microM), a TFP derivative with low calmodulin-antagonist activity, did not have any effects on either amplitude or inactivation of L-type ICa2+. TFP (20 microM) increased basal [Ca2+]i from 147 +/- 37 nM to 650 +/- 40 nM (N = 7). The increase in [Ca2+]i was prevented by removal of external Ca2+ and curtailed by depletion of caffeine-sensitive intracellular Ca2+ stores. Since TFP may also block protein kinase C (PKC), we tested the effect of a PKC activator (12-C-tetradecanoyl-phorbol-13-acetate) on L-type Ca2+ currents. This compound produced an increase in L-type ICa2+ without enhancing Ca(2+)-dependent inactivation. The results show that 1) TFP reduces L-type ICa2+ while enhancing the efficacy of Ca(2+)-dependent inactivation. 2) TFP produces an increase in basal [Ca2+]i which may contribute to the enhancement of Ca(2+)-dependent inactivation. 3) PKC up-regulates L-type ICa2+ without altering the efficacy of Ca(2+)-dependent inactivation. 4) The TFP effects cannot be attributed to its action as PKC blocker.
Cruzblanca
H
H
Departamento de Neurobiología, Instituto Mexicano de Psiquiatría, México.
Gamiño
S M
SM
Bernal
J
J
Alvarez-Leefmans
F J
FJ
D000818:Animals; D002121:Calcium Channel Blockers / Q000494:pharmacology*; D002147:Calmodulin / Q000037:antagonists & inhibitors*; D004789:Enzyme Activation; D005456:Fluorescent Dyes; D016257:Fura-2; D006372:Helix, Snails; D009474:Neurons / Q000187:drug effects*; D018408:Patch-Clamp Techniques; D011493:Protein Kinase C / Q000187:drug effects; D013755:Tetradecanoylphorbol Acetate / Q000494:pharmacology; D014268:Trifluoperazine / Q000627:therapeutic use*
D002121:Calcium Channel Blockers; D002147:Calmodulin; D005456:Fluorescent Dyes; D014268:Trifluoperazine; D011493:Protein Kinase C; D013755:Tetradecanoylphorbol Acetate; D016257:Fura-2