Bidirectional Ca2+ coupling of mitochondria with the endoplasmic reticulum and regulation of multimodal Ca2+ entries in rat brown adipocytes

How the endoplasmic reticulum ( ER) and mitochondria communicate with each other and how they regulate plasmalemmal Ca2+ entry were studied in cultured rat brown adipocytes. Cytoplasmic Ca2+ or Mg2+ and mitochondrial membrane potential were measured by fluorometry. The sustained component of rises in cytoplasmic Ca2+ concentration ([Ca2(+)](i)) produced by thapsigargin was abolished by removing extracellular Ca2+, depressed by depleting extracellular Na+, and enhanced by raising extracellular pH. FCCP, dinitrophenol, and rotenone caused bi- or triphasic rises in [Ca2+](i), in which the first phase was accompanied by mitochondrial depolarization. The FCCP-induced first phase was partially inhibited by oligomycin but not by ruthenium red, cyclosporine A, U- 73122, a Ca2+- free EGTA solution, and an Na+-free solution. The FCCP-induced second phase paralleling mitochondrial repolarization was partially blocked by removing extracellular Ca2+ and fully blocked by oligomycin but not by thapsigargin or an Na+-deficient solution, was accompanied by a rise in cytoplasmic Mg2+ concentration, and was summated with a high pH- induced rise in [Ca2+](i), whereas the extracellular Ca2+-independent component was blocked by U- 73122 and cyclopiazonic acid. The FCCP- induced third phase was blocked by removing Ca2+ but not by thapsigargin, depressed by decreasing Na+, and enhanced by raising pH. Cyclopiazonic acid- evoked rises in [Ca2+](i) in a Ca2+- free solution were depressed after FCCP actions. Thus mitochondrial uncoupling causes Ca2+ release, activating Ca2+ release from the ER and store- operated Ca2+ entry, and directly elicits a novel plasmalemmal Ca2+ entry, whereas Ca2+ release from the ER activates Ca2+ accumulation in, or release from, mitochondria, indicating bidirectional mitochondria- ER couplings in rat brown adipocytes.