ACCELERATED COMMUNICATION Noncoplanar PCB 95 Alters Microsomal Calcium Transport by an Immunophilin FKBP12-Dependent Mechanism

Ortho-substituted polychlorinated biphenyls (PCBs) have been shown to alter microsomal Ca 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 Ca 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 [H]ryanodine (IC50 ; 35 mM) to Ry1R and PCB 95-induced release of Ca from actively loaded SR vesicles (IC50 ; 11 mM), demonstrating a FKBP12-dependent mechanism. FK 506 selectively eliminates PCB 95-induced Ca release from SR because Ry1R maintains responsiveness to caffeine and Ca. PCB 95 and FK 506 are used to examine the relationship between ryanodine-sensitive Ca channels and ryanodine-insensitive Ca leak pathways present in SR vesicles. Micromolar ryanodine completely blocks ryanodinesensitive Ca efflux but neither eliminates the ryanodine-insensitive Ca leak unmasked by thapsigargin nor enhances the loading capacity of SR vesicles. PCB 95 alone enhances thapsigargin evoked Ca release and therefore diminishes the loading capacity of SR vesicles. However, in the presence of micromolar ryanodine, PCB 95 dose-dependently eliminates the Ca 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 Ca 21 leak pathway. The results reveal that FKBP12 relates ryanodine-insensitive Ca “leak” and ryanodine-sensitive Ca channel efflux pathways of SR by modulating distinct conformations Ry1R complexes. Noncoplanar PCBs, like PCB 95, alter SR Ca buffering by an FKBP12-mediated mechanism. An immunophilin-based mechanism could account for the toxic actions attributed to certain noncoplanar PCB congeners.