Trifluoperazine enhancement of Ca2+-dependent inactivation of L-type Ca2+ currents inHelix aspersa neurons

The effects of trifluoperazine hydrochloride (TFP), a calmodulin antagonist, on L-type Ca²⁺ currents (L-type ICa²⁺) and their Ca²⁺-dependent inactivation, were studied in identifiedHelix 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 μM). Using a double-pulse protocol, we found that TFP enhances the efficacy of Ca²⁺-dependent inactivation of L-type ICa²⁺. Trifluoperazine sulfoxide (50 μM), a TFP derivative with low calmodulin-antagonist activity, did not have any effects on either amplitude or inactivation of L-type ICa²⁺. TFP (20 μM) increased basal Ca²⁺]_i from 147±37 nM to 650±40nM (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-O-tetradecanoyl-phorbol-13-acetate) on L-type Ca²⁺ currents. This compound produced an increase in L-type ICa²⁺ without enhancing Ca²⁺-dependent inactivation. The results show that 1) TFP reduces L-type ICa²⁺ while enhancing the efficacy of Ca²⁺-dependent inactivation. 2) TFP produces an increase in basal Ca²⁺]_i which may contribute to the enhancement of Ca²⁺-dependent inactivation. 3) PKC up-regulates L-type ICa²⁺ without altering the efficacy of Ca²⁺ dependent inactivation. 4) The TFP effects cannot be attributed to its action as PKC blocker.