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) and disruption of Ca2+ homeostasis as potential mechanisms underlying effects of the polychlorinated biphenyl (PCB) congener 2,2'-dichlorobiphenyl (DCB) on inositol phosphate (IP) signaling in cerebellar granule cells. DCB (100 microM) increased basal IP accumulation in cerebellar granule cells when the extracellular free Ca2+ concentration ([Ca2+]e) was 0.75 mM but not when [Ca2+]e was 1 microM. Ionomycin (0.1 to 30 microM), a Ca2+ ionophore, also increased basal IP accumulation and [Ca2+]i in a concentration-dependent manner in cerebellar granule cells in the absence of DCB; increases in basal IP accumulation induced by 100 microM DCB were not additive with ionomycin. Ionomycin also disrupted carbachol (CARB, 1 mM)-stimulated IP accumulation. A 30-min preincubation with 0.3 or 1.0 microM ionomycin decreased CARB-stimulated IP accumulation, whereas simultaneous addition of 1.0 and 10 microM ionomycin with CARB increased and decreased, respectively, IP accumulation. DCB caused concentration-dependent increases in intracellular free Ca2+ concentration ([Ca2+]i) in cerebellar granule cells under experimental conditions identical to those used to measure IP accumulation. Following a one-half hour exposure to DMSO, 50 or 100 microM DCB, the [Ca2+]i was 36, 103, and 453 nM, respectively. We examined whether direct or indirect activation of PKC underlies DCB-induced inhibition of agonist-stimulated IP accumulation. DCB (100 microM) did not alter PKC activity in cytosolic or membrane fractions of granule cell homogenates. In intact cells, 50 nM phorbol 12-myristate, 13-acetate (PMA) inhibited CARB-stimulated IP accumulation by 80%, an effect which was blocked completely by the PKC inhibitor bisindolylmaleimide (2 microM; BIM). However, inhibition of CARB-stimulated IP accumulation (90%) induced by 100 microM DCB was not relieved by BIM. These results suggest that (1) perturbations of Ca2+ homeostasis may underlie DCB effects on IP accumulation, (2) at a time which corresponds to addition of agonists in IP accumulation assays, [Ca2+]i is elevated in cerebellar granule cells exposed to DCB, and (3) activation of PKC is not a mechanism by which DCB inhibits agonist-stimulated IP accumulation.