DISRUPTION OF INOSITOL PHOSPHATE ACCUMULATION IN CEREBELLAR GRANULE CELLS BY POLYCHLORINATED-BIPHENYLS - A CONSEQUENCE OF ALTERED CA2+ HOMEOSTASIS

The present study examined the activation of protein kinase C (PKC) an d disruption of Ca2+ homeostasis as potential mechanisms underlying ef fects of the polychlorinated biphenyl (PCB) congener 2,2'-dichlorobiph enyl (DCB) on inositol phosphate (IP) signaling in cerebellar granule cells. DCB (100 mu M) increased basal IP accumulation in cerebellar gr anule cells when the extracellular free Ca2+ concentration ([Ca2+](e)) was 0.75 mM but not when [Ca2+](e) was 1 mu M. Ionomycin (0.1 to 30 m u M), a Ca2+ ionophore, also increased basal IP accumulation and [Ca2](i) in a concentration-dependent manner in cerebellar granule cells i n the absence of DCB; increases in basal IP accumulation induced by 10 0 mu M DCB were not additive with ionomycin. Ionomycin also disrupted carbachol (CARB, 1 mM)-stimulated IP accumulation, A 30-min preincubat ion with 0.3 or 1.0 mu M ionomycin decreased CARB-stimuIated IP accumu lation, whereas simultaneous addition of 1.0 and 10 mu M ionomycin wit h CARB increased and decreased, respectively, IP accumulation. DCB cau sed concentration-dependent increases in intracellular free Ca2+ conce ntration ([Ca2+](i)) in cerebellar granule cells under experimental co nditions identical to those used to measure IP accumulation. Following a one-half hour exposure to DMSO, 50 or 100 mu M DCB, the [Ca2+](i) w as 36, 103, and 453 nM, respectively. We examined whether direct or in direct activation of PKC underlies DCB-induced inhibition of agonist-s timulated IP accumulation. DCB (100 mu M) did not alter PKC activity i n cytosolic or membrane fractions of granule cell homogenates. In inta ct cells, 50 nM phorbol 12-myristate, 13-acetate (PMA) inhibited CARB- stimulated IP accumulation by 80%, an effect which was blocked complet ely by the PKC inhibitor bisindolylmaleimide (2 mu M; BIM). However, i nhibition of CARB-stimulated IP accumulation (90%) induced by 100 mu M DCB was not relieved by BIM. These results suggest that (1) perturbat ions of Ca2+ homeostasis may underlie DCB effects on IP accumulation, (2) at a time which corresponds to addition of agonists in IP accumula tion assays, [Ca2+](i) is elevated in cerebellar granule cells exposed to DCB, and (3) activation of PKC is not a mechanism by which DCB inh ibits agonist-stimulated IP accumulation.