Ortho-substituted polychlorinated biphenyls alter calcium regulation by a ryanodine receptor-mediated mechanism: structural specificity toward skeletal- and cardiac-type microsomal calcium release channels

We investigated a novel molecular mechanism by which polychlorinated biphenyls (PCBs) alter microsomal Ca2+ transport with sarcoplasmic reticulum (SR) membranes isolated from skeletal and cardiac muscles. Aroclors with an intermediate weight percent of chlorine enhance by >6-fold the binding of 1 nM[3H]ryanodine to its conformationally sensitive site on the SR Ca2+ -release channel [i.e., ryanodine receptor (RyR)] with high potency (EC50=1.4 microM), whereas Aroclors with either high or low chlorine composition show little activity. Structure-activity studies with selected pentachlorobiphenyl congeners reveal a stringent structural requirement for chlorine substitution at the ortho-positions, with 2,2',3,5',6-pentachlorobiphenyl having the highest potency toward skeletal and cardiac isoforms of RyR (EC50=330 nM and 2 microM, respectively). In contrast, 3,3',4,4',5-pentachlorobiphenyl does not enhance ryanodine binding, suggesting that noncoplanarity of the biphenyl rings is required for channel activation. However, 2,2',4,6,6'-pentachlorobiphenyl is significantly less active toward RyR, suggesting that some degree of rotation about the biphenyl bond is required. 2,2',3,5',6-Pentachlorobiphenyl induces a dose-dependent release of Ca2+ from actively loaded SR vesicles with a maximum rate of 1.2 micromol mg-1 min-1 (EC50=1 microM), whereas 3,3',4,4',5-pentachlorobiphenyl (< / = microM) does not alter Ca2+ transport. The mechanism of PCB-induced channel activation involves a significant decrease in the inhibitory potency of Ca2+ and Mg2+ (20-fold and 100-fold, respectively). Neither 2,2',3,5',6- nor 3,3',4,4',5-pentachlorobiphenyl (< / = 10 microM) alters the activity of the skeletal isoform of sarcoplasmic/endoplasmic reticulum Ca2+ -ATPase or the cardiac isoform of sarcoplasmic/endoplasmic reticulum Ca2+ -ATPase, and PCB-induced Ca2+ release can be fully blocked by either microM ryanodine or ruthenium red. These results are the first to demonstrate a selective ryanodine receptor-mediated mechanism by which ortho-substituted PCBs alter microsomal Ca2+ transport and may have toxicological relevance.