DIVERSE MECHANISMS OF CALCIUM MOBILIZATION BY PEROXISOME PROLIFERATORS IN RAT HEPATOCYTES

The ability of six peroxisome proliferators to modulate Ca2+ homeostas is was studied in freshly isolated rat hepatocytes. Clofibrate and bif onazole (0.5 mM) caused a transient increase in cytosolic-free Ca2+ co ncentration ([Ca2+](i)) by releasing the intracellular inositol 1,4,5- trisphosphate-sensitive Ca2+ pool. However, the mobilization of this p ool by clofibrate was only transient; a subsequent exposure of the cel ls to the endoplasmic reticulum Ca2+-ATPase inhibitor thapsigargin res ulted in a second release of the same Ca2+ store, indicating that this pool could refill from the cytosol, independently of extracellular Ca 2+. 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 [C a2+](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-tr isphosphate-sensitive Ca2+ pool and was not inhibited by prior treatme nt with the protonophore carbonyl cyanide 4-(trifluoromethoxy) phenylh ydrazone, but was slightly antagonized by prior exposure to the Ca2+ i onophore ionomycin, Pretreating the cells with nafenopin completely ab olished the response elicited by ciprofibrate, and vice versa. By cont rast to the other peroxisome proliferators, WY-14,643 and bezafibrate (1 mM) increased cytosolic free Ca2+ only by approximately 30 nM. In c onclusion, the structurally diverse peroxisome proliferators tested in this study all produced changes in [Ca2+](i) in hepatocytes but throu gh the redistribution of different internal Ca2+ pools. Further studie s are needed to determine whether any of the observed Ca2+ changes hav e a role in the pleiotropic effects elicited by peroxisome proliferato rs. (C) 1995 Academic Press, Inc.