Comparative effects of two polychlorinated biphenyl congeners on calcium homeostasis in rat cerebellar granule cells.

Some polychlorinated biphenyls (PCBs) have been reported to alter locomotor activity and decrease brain dopamine function in laboratory animals. PCBs with ortho- and/or parachlorine substitutions and varying number of chlorinations are known to decrease cell dopamine content in vitro and have been detected in brains of animals exposed to PCBs, suggesting that the neurotoxicity could be mediated by ortho-substituted congeners. Dopamine or other neurotransmitter uptake and release phenomena are dependent on the maintenance of intracellular Ca2+ homeostasis, and perturbations in Ca2+ homeostasis could lead to altered cell function and/or death. We compared the effects of two PCB congeners on Ca2+ homeostasis in cerebellar granule cells: 2,2'-dichlorobiphenyl (DCBP), a putative neurotoxic congener, and 3,3',4,4',5-pentachlorobiphenyl (PCBP), a presumed nonneurotoxic congener. In cerebellar granule cells (6-8 days in vitro), DCBP was cytotoxic as indicated by a significant increase in LDH leakage at 200 microM after 2 hr of exposure and at 100 microM after 4 hr exposure. PCBP, on the other hand, did not affect LDH leakage even at 200 microM for up to 4 hr. Although both congeners increased cerebellar granule cell [Ca2+]i, DCPB was more effective in increasing [Ca2+]i to a greater extent than PCBP. The increase in [Ca2+]i produced by both congeners was not transient, but a steady rise was observed with time. To understand cellular Ca(2+)-buffering capacity, Ca2+ sequestration and Ca2+ extrusion were studied in mitochondria, microsomes, and synaptosomes, isolated from adult rat cerebellum. DCBP was a potent inhibitor of 45Ca2+ uptake by mitochondria (IC50 = 6.17 +/- 0.53 microM) and microsomes (IC50 = 7.61 +/- 0.35 microM). PCBP inhibited Ca2+ sequestration by mitochondria (68% of control) and microsomes (72% of control), but the effects were much less than those produced by equivalent concentrations of DCBP. Synaptosomal Ca(2+)-ATPase was inhibited by DCBP, but not by PCBP. These results indicate that at concentrations where cytotoxicity in cerebellar granule cells was not observed, DCBP increased intracellular [Ca2+]i, and at the same concentrations, Ca2+ sequestration by intracellular organelles and Ca(2+)-ATPase in synaptic plasma membrane were inhibited. Although PCBP increased [Ca2+]i in cerebellar granule cells to some extent, it was not potent in affecting Ca2+ sequestration or Ca2+ extrusion in adult cerebellar components. Hence, PCBP-induced slight increase of [Ca2+]i levels in the cells might have been associated with effective Ca2+ sequestration by intracellular organelles, as seen in cerebellar preparations. The results of this study support the hypothesis that the position of chlorine substitution on the biphenyl ring and/or number of chlorine substitutions may have significant implications for predicting potential effects of PCB congeners in the nervous system, and perturbations in Ca2+ homeostasis might play a significant role in the neuroactivity of PCBs.