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 ([Ca ²⁺]_i). A subset of rat dorsal root ganglion neurons in culture exhibited regenerative CICR when sensitized by caffeine. [Ca ²⁺]_i oscillated 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 Ca ²⁺ 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⁺]_i slowed 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.