Mitochondrial Modulation of Ca2+-Induced Ca2+-Release in Rat Sensory Neurons

Ca2+-induced Ca2+-release (CICR) from ryanodine-sensitive Ca2+stores provides a mechanism to amplify and propagate a transient increase in intracellular calcium concentration ([Ca2+]i). A subset of rat dorsal root ganglion neurons in culture exhibited regenerative CICR when sensitized by caffeine. [Ca2+]ioscillated in the maintained presence of 5 mM caffeine and 25 mM K+. Here, CICR oscillations were used to study the complex interplay between Ca2+regulatory mechanisms at the cellular level. Oscillations depended on Ca2+uptake and release from the endoplasmic reticulum (ER) and Ca2+influx across the plasma membrane because cyclopiazonic acid, ryanodine, and removal of extracellular Ca2+terminated oscillations. Increasing caffeine concentration decreased the threshold for action potential-evoked CICR and increased oscillation frequency. Mitochondria regulated CICR by providing ATP and buffering [Ca2+]i. Treatment with the ATP synthase inhibitor, oligomycin B, decreased oscillation frequency. When ATP concentration was held constant by recording in the whole cell patch-clamp configuration, oligomycin no longer affected oscillation frequency. Aerobically derived ATP modulated CICR by regulating the rate of Ca2+sequestration by the ER Ca2+pump. Neither CICR threshold nor Ca2+clearance by the plasma membrane Ca2+pump were affected by inhibition of aerobic metabolism. Uncoupling electron transport with carbonyl cyanide p-trifluoromethoxy-phenyl-hydrazone or inhibiting mitochondrial Na+/Ca2+exchange with CGP37157 revealed that mitochondrial buffering of [Ca2+]islowed oscillation frequency, decreased spike amplitude, and increased spike width. These findings illustrate the interdependence of energy metabolism and Ca2+signaling that results from the complex interaction between the mitochondrion and the ER in sensory neurons.