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 Ca(2+) entry were studied in cultured rat brown adipocytes. Cytoplasmic Ca(2+) or Mg(2+) and mitochondrial membrane potential were measured by fluorometry. The sustained component of rises in cytoplasmic Ca(2+) concentration ([Ca(2+)](i)) produced by thapsigargin was abolished by removing extracellular Ca(2+), depressed by depleting extracellular Na(+), and enhanced by raising extracellular pH. FCCP, dinitrophenol, and rotenone caused bi- or triphasic rises in [Ca(2+)](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 Ca(2+)-free EGTA solution, and an Na(+)-free solution. The FCCP-induced second phase paralleling mitochondrial repolarization was partially blocked by removing extracellular Ca(2+) and fully blocked by oligomycin but not by thapsigargin or an Na(+)-deficient solution, was accompanied by a rise in cytoplasmic Mg(2+) concentration, and was summated with a high pH-induced rise in [Ca(2+)](i), whereas the extracellular Ca(2+)-independent component was blocked by U-73122 and cyclopiazonic acid. The FCCP-induced third phase was blocked by removing Ca(2+) but not by thapsigargin, depressed by decreasing Na(+), and enhanced by raising pH. Cyclopiazonic acid-evoked rises in [Ca(2+)](i) in a Ca(2+)-free solution were depressed after FCCP actions. Thus mitochondrial uncoupling causes Ca(2+) release, activating Ca(2+) release from the ER and store-operated Ca(2+) entry, and directly elicits a novel plasmalemmal Ca(2+) entry, whereas Ca(2+) release from the ER activates Ca(2+) accumulation in, or release from, mitochondria, indicating bidirectional mitochondria-ER couplings in rat brown adipocytes.