Extracellular calcium is required for the polychlorinated biphenyl-inducedincrease of intracellular free calcium levels in cerebellar granule cell culture

Recent studies from the laboratory indicate that polychlorinated biphenyl ( PCB) congeners can alter signal transduction and calcium homeostasis in neu ronal preparations. These effects were more pronounced for the ortho-substi tuted, non-coplanar congeners, although the mechanisms underlying these eff ects are not clear. In the present study the time-course and concentration- dependent effects of coplanar and non-coplanar PCBs on intracellular free c alcium concentration ([Ca2+](i)) in cerebellar granule cell cultures were c ompared using the fluorescent probe fura-2. The ortho-substituted congeners 2,2'-dichlorobiphenyl (DCB) and 2,2',4,6,6'-pentachlorobiphenyl (PeCB) cau sed a gradual increase of [Ca2+](i) while the non-ortho -substituted congen ers 4,4'-DCB and 3,3',4,4',5-PeCB had no effect. The increase of [Ca2+](i) produced by 2,2'-DCB was time- and concentration-dependent. Further studies examined possible mechanisms for this rise in [Ca2+](i). In contrast to th e muscarinic agonist carbachol, the effects of 2,2'-DCB on [Ca2+](i) were n ot blocked by thapsigargin and required the presence of extracellular calci um. The effects of ortho-substituted PCBs may depend on their ability to in hibit calcium sequestration as 2,2'-DCB significantly inhibited Ca-45(2+)-u ptake by microsomes and mitochondria while 3,3',4,4',5-PeCB had no effect. In addition, 2,2'-DCB significantly increased the binding of [H-3]inositol 1,4,5-trisphosphate to receptors on cerebellar microsomes, suggesting anoth er possible mechanism by which ortho-substituted PCBs can mobilize [Ca2+](i ). These results show that PCBs increase [Ca2+](i) in vitro via a mechanism that requires extracelluar calcium, and support previous structure-activit y studies indicating that ortho-substituted PCBs are more potent than non-o rtho -substituted PCBs. (C) 1999 Elsevier Science Ireland Ltd. All rights r eserved.