Effect of trifluoperazine on renal epithelioid Madin-Darby canine kidney cells

Functional significance of cell volume regulatory mechanisms

To survive, cells have to avoid excessive alterations of cell volume that jeopardize structural integrity and constancy of intracellular milieu. The function of cellular proteins seems specifically sensitive to dilution and concentration, determining the extent of macromolecular crowding. Even at constant extracellular osmolarity, volume constancy of any mammalian cell is permanently challenged by transport of osmotically active substances across the cell membrane and formation or disappearance of cellular osmolarity by metabolism. Thus cell volume constancy requires the continued operation of cell volume regulatory mechanisms, including ion transport across the cell membrane as well as accumulation or disposal of organic osmolytes and metabolites. The various cell volume regulatory mechanisms are triggered by a multitude of intracellular signaling events including alterations of cell membrane potential and of intracellular ion composition, various second messenger cascades, phosphorylation of diverse target proteins, and altered gene expression. Hormones and mediators have been shown to exploit the volume regulatory machinery to exert their effects. Thus cell volume may be considered a second message in the transmission of hormonal signals. Accordingly, alterations of cell volume and volume regulatory mechanisms participate in a wide variety of cellular functions including epithelial transport, metabolism, excitation, hormone release, migration, cell proliferation, and cell death.

Chlorpromazine activates chloride currents in Xenopus oocytes

Xenopus oocytes are frequently used for in vivo expression of DNA and RNA, especially those encoding ion channel proteins. Accordingly, it is important to understand the expression and control of endogenous conductances. Ionic currents were studied in native Xenopus oocytes with two-microelectrode voltage-clamp technique to characterize the actions of chlorpromazine (CPZ) and trifluroperazine (TFP), two widely used antipsychotic drugs. External application of CPZ or TFP markedly stimulated endogenous conductances in a dose-dependent and reversible fashion. The current-voltage (I-V) relationship was non linear and dependent on the presence of external chloride. The CPZ-activated currents were inhibited by Cl- channel blockers. Although the removal of external Ca2+ had no effect on CPZ-induced conductances, the injection of BAPTA, a Ca2+ chelator, abolished endogenous activity. Thapsigargin also inhibited channel activity suggesting that CPZ acts through intraoocyte Ca2+ release. The calmodulin inhibitors, calmidazolium and W-7, failed to mimic the action of CPZ. These data provide evidence for external or internal phenothiazine receptors which when activated by CPZ induces Ca(2+)-dependent Cl- channel activity in endogenous native oocytes.