Abstract

Ortho-substituted polychlorinated biphenyls (PCBs) have been shown to alter microsomal Ca²⁺ transport by selective interaction with ryanodine receptors (RyRs) of muscle sarcoplasmic reticulum (SR) and brain endoplasmic reticulum. The mechanism underlying the actions of PCBs on Ca²⁺ transport is further elucidated with skeletal SR enriched in Ry₁R. Disruption of the association between immunophilin FKBP12 and Ry₁R with FK 506 or rapamycin completely eliminates PCB 95-enhanced binding of [³H]ryanodine (IC₅₀ ∼ 35 μm) to Ry₁R and PCB 95-induced release of Ca²⁺ from actively loaded SR vesicles (IC₅₀ ∼ 11 μm), demonstrating a FKBP12-dependent mechanism. FK 506 selectively eliminates PCB 95-induced Ca²⁺ release from SR because Ry₁R maintains responsiveness to caffeine and Ca²⁺. PCB 95 and FK 506 are used to examine the relationship between ryanodine-sensitive Ca²⁺ channels and ryanodine-insensitive Ca²⁺ leak pathways present in SR vesicles. Micromolar ryanodine completely blocks ryanodine-sensitive Ca²⁺ efflux but neither eliminates the ryanodine-insensitive Ca²⁺ leak unmasked by thapsigargin nor enhances the loading capacity of SR vesicles. PCB 95 alone enhances thapsigargin evoked Ca²⁺release and therefore diminishes the loading capacity of SR vesicles. However, in the presence of micromolar ryanodine, PCB 95 dose-dependently eliminates the Ca²⁺ leak unmasked by thapsigargin and significantly enhances the loading capacity of SR vesicles. The actions of PCB 95 on SR-loading capacity are additive with those of FK 506. Structural specificity for these novel actions are further demonstrated with coplanar PCB 126, which is inactive toward Ry₁R and lacks the ability to alter the Ca²⁺leak pathway. The results reveal that FKBP12 relates ryanodine-insensitive Ca²⁺ “leak” and ryanodine-sensitive Ca²⁺ channel efflux pathways of SR by modulating distinct conformations Ry₁R complexes. Noncoplanar PCBs, like PCB 95, alter SR Ca²⁺ buffering by an FKBP12-mediated mechanism. An immunophilin-based mechanism could account for the toxic actions attributed to certain noncoplanar PCB congeners.