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

Abstract

Recent studies from the laboratory indicate that polychlorinated biphenyl (PCB) congeners can alter signal transduction and calcium homeostasis in neuronal preparations. These effects were more pronounced for the ortho-substituted, non-coplanar congeners, although the mechanisms underlying these effects are not clear. In the present study the time-course and concentration-dependent effects of coplanar and non-coplanar PCBs on intracellular free calcium concentration ([Ca 2+] i) in cerebellar granule cell cultures were compared using the fluorescent probe fura-2. The ortho-substituted congeners 2,2′-dichlorobiphenyl (DCB) and 2,2′,4,6,6′-pentachlorobiphenyl (PeCB) caused a gradual increase of [Ca 2+] i while the non- ortho-substituted congeners 4,4′-DCB and 3,3′,4,4′,5-PeCB had no effect. The increase of [Ca 2+] i produced by 2,2′-DCB was time- and concentration-dependent. Further studies examined possible mechanisms for this rise in [Ca 2+] i. In contrast to the muscarinic agonist carbachol, the effects of 2,2′-DCB on [Ca 2+] i were not blocked by thapsigargin and required the presence of extracellular calcium. The effects of ortho-substituted PCBs may depend on their ability to inhibit calcium sequestration as 2,2′-DCB significantly inhibited 45Ca 2+-uptake by microsomes and mitochondria while 3,3′,4,4′,5-PeCB had no effect. In addition, 2,2′-DCB significantly increased the binding of [ 3H]inositol 1,4,5-trisphosphate to receptors on cerebellar microsomes, suggesting another possible mechanism by which ortho-substituted PCBs can mobilize [Ca 2+] i. These results show that PCBs increase [Ca 2+] i in vitro via a mechanism that requires extracelluar calcium, and support previous structure-activity studies indicating that ortho-substituted PCBs are more potent than non- ortho-substituted PCBs.