Diverse mechanisms of calcium mobilization by peroxisome proliferators in rat hepatocytes

The ability of six peroxisome proliferators to modulate Ca2+ homeostasis was studied in freshly isolated rat hepatocytes. Clofibrate and bifonazole (0.5 mM) caused a transient increase in cytosolic-free Ca2+ concentration ([Ca2+]i) by releasing the intracellular inositol 1,4,5-trisphosphate-sensitive Ca2+ pool. However, the mobilization of this pool by clofibrate was only transient; a subsequent exposure of the cells to the endoplasmic reticulum Ca(2+)-ATPase inhibitor thapsigargin resulted in a second release of the same Ca2+ store, indicating that this pool could refill from the cytosol, independently of extracellular Ca2+. By contrast, bifonazole-exposed hepatocytes no longer responded to a stimulation by thapsigargin. Bifonazole also strongly inhibited Ca2+ influx. Ciprofibrate and nafenopin (0.5 mM) produced increases in [Ca2+]i that were sustained, even in the absence of extracellular Ca2+. The [Ca2+]i response was not due to release of the inositol 1,4,5-trisphosphate-sensitive Ca2+ pool and was not inhibited by prior treatment with the protonophore carbonyl cyanide 4-(trifluoromethoxy) phenylhydrazone, but was slightly antagonized by prior exposure to the Ca2+ ionophore ionomycin. Pretreating the cells with nafenopin completely abolished the response elicited by ciprofibrate, and vice versa. By contrast to the other peroxisome proliferators, WY-14,643 and bezafibrate (1 mM) increased cytosolic free Ca2+ only by approximately 30 nM. In conclusion, the structurally diverse peroxisome proliferators tested in this study all produced changes in [Ca2+]i in hepatocytes but through the redistribution of different internal Ca2+ pools. Further studies are needed to determine whether any of the observed Ca2+ changes have a role in the pleiotropic effects elicited by peroxisome proliferators.