Pacemaking in interstitial cells of Cajal depends upon calcium handling by endoplasmic reticulum and mitochondria

1. Pacemaker cells, known as interstitial cells of Cajal (ICC), generate electrical rhythmicity in the gastrointestinal tract. Pacemaker currents in ICC result from the activation of a voltage-independent, non-selective cation conductance, but the timing mechanism responsible for periodic activation of the pacemaker current is unknown. 2. Previous studies suggest that pacemaking in ICC is dependent upon metabolic activity and Ca²⁺ release from intracellular stores. We tested the hypothesis that mitochondrial Ca²⁺ handling may underlie the dependence of gastrointestinal pacemaking on oxidative metabolism. 3. Pacemaker currents occurred spontaneously in cultured ICC and were associated with mitochondrial Ca²⁺ transients. 4. Inhibition of the electrochemical gradient across the inner mitochondrial membrane blocked Ca²⁺ uptake and pacemaker currents in cultured ICC and blocked slow wave activity in intact gastrointestinal muscles from mouse, dog and guinea-pig. 5. Pacemaker currents and rhythmic mitochondrial Ca²⁺ uptake in ICC were also blocked by inhibitors of IP₃-dependent release of Ca²⁺ from the endoplasmic reticulum and by inhibitors of endoplasmic reticulum Ca²⁺ reuptake. 6. Our data suggest that integrated Ca²⁺ handling by endoplasmic reticulum and mitochondria is a prerequisite of electrical pacemaking in the gastrointestinal tract.