Inhibition of volume-activated I^- and taurine efflux from HeLa cells by P-glycoprotein blockers correlates with calmodulin inhibition

P-glycoprotein, an active transporter that pumps a diverse range of hydrophobic compounds out of cells, has recently been proposed to function as, or regulate, a volume-activated, anion-selective channel (Valverde, M. A., Diaz, M., Sepulveda, F. V., Gill, D. R., Hyde, S. C., and Higgins, C. F. (1992) Nature 355, 830-833). In this study a number of compounds known to inhibit P-glycoprotein-mediated drug pumping were tested for their effect on the osmotically activated release from HeLa cells of I^-, a known substrate of volume-activated anion channels, and taurine, a sulfonic amino acid that serves as an important organic osmolyte in many cell-types. Tamoxifen, 4- iodotamoxifen, and pyrrolidino-4-iodotamoxifen (idoxifene) were potent blockers of osmotically activated I^- and taurine efflux. Other known P- glycoprotein inhibitors (verapamil, cyclosporin A, pimozide, trifluoperazine, ICI 164, and ICI 182) were less effective. For all compounds tested the effect on taurine release was the same as that on I^- release, consistent with the hypothesis that swelling-activated taurine release is via anion- selective channels. There was no positive correlation between the effect of the inhibitors on osmotically activated solute release and their effect on P- glycoprotein-mediated drug transport. In contrast, there was a strong positive correlation between the IC₅₀ values for the effect of the inhibitors on volume-activated solute release and those for their effect on calmodulin. These data raise doubts as to whether the effect of P- glycoprotein inhibitors on volume-activated channels is a consequence of their interaction with P-glycoprotein and indicate a possible role for calmodulin, or a cell component having at least some physical similarities, in controlling channel activity.