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 μM), 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 the skeletal and cardiac isoforms of RyR (EC50 = 330 nM and 2 μM, 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 μmol mg-1 min-1 (EC50 = 1 μM), whereas 3,3′,4,4′,5-pentachlorobiphenyl (≤10 μM) 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 μM) alters the activity of the skeletal isoform of sarcoplasmic/ endoplasmic reticulum Ca2+-ATPase or the cardiac isoform of sarcoplasmic/endoplasmic reticulum Ca2+-ATPase, and 7CB-induced Ca2+ release can be fully blocked by either μM 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.
|Original language||English (US)|
|Number of pages||12|
|State||Published - Apr 1996|
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