Calcium signal transmission between ryanodine receptors and mitochondria

Control of energy metabolism by increases of mitochondrial matrix [Ca²⁺] ([Ca²⁺](m)) may represent a fundamental mechanism to meet the ATP demand imposed by heart contractions, but the machinery underlying propagation of [Ca²⁺] signals from ryanodine receptor Ca²⁺ release channels (RyR) to the mitochondria remains elusive. Using permeabilized cardiac (H9c2) cells we investigated the cytosolic [Ca²⁺] ([Ca²⁺](c)) and [Ca²⁺](m) signals elicited by activation of RyR. Caffeine, Ca²⁺, and ryanodine evoked [Ca²⁺](c) spikes that often appeared as frequency- modulated [Ca²⁺](c) oscillations in these permeabilized cells. Rapid increases in [Ca²⁺](m) and activation of the Ca²⁺-sensitive mitochondrial dehydrogenases were synchronized to the rising phase of the [Ca²⁺](c) spikes. The RyR-mediated elevations of global [Ca²⁺](c) were in the submicromolar range, but the rate of [Ca²⁺](m) increases was as large as it was in the presence of 30 μM global [Ca²⁺](c). Furthermore, RyR-dependent increases of [Ca²⁺](m) were relatively insensitive to buffering of [Ca²⁺](c) by EGTA. Therefore, RyR-driven rises of [Ca²⁺](m) appear to result from large and rapid increases of perimitochondrial [Ca²⁺]. The falling phase of [Ca²⁺](c) spikes was followed by a rapid decay of [Ca²⁺](m). CGP37157 slowed down relaxation of [Ca²⁺](m) spikes, whereas cyclosporin A had no effect, suggesting that activation of the mitochondrial Ca²⁺ exchangers accounts for rapid reversal of the [Ca²⁺](m) response with little contribution from the permeability transition pore. Thus, rapid activation of Ca²⁺ uptake sites and Ca²⁺ exchangers evoked by RyR- mediated local [Ca²⁺](c) signals allow mitochondria to respond rapidly to single [Ca²⁺](c) spikes in cardiac cells.