Figure 6.

Ca²⁺ sequestration into mitochondria mimics the IP₃ effect on the rate of spontaneous Ca²⁺ spiking. Purinergic stimulation of cardiomyocytes induces Ca²⁺ depletion of the sarcoplasmic reticulum and mitochondrial Ca²⁺ uptake. (A) A Fluo3-loaded cell was superfused with 50 ng/ml cyclosporin A (CPA). A similar effect of CPA was observed in four other cells. (B) A Fluo3-loaded cell was superfused with a Ca²⁺- and Na⁺-free solution to stop Ca²⁺ oscillations and to prevent the activity of the Na⁺/Ca²⁺ exchanger. Caffeine (10 mM) added to the Ca²⁺- and Na⁺-free solution was applied to the cell. After washing out caffeine, the same protocol was repeated and triggered Ca²⁺ release of a similar amplitude. Similar results were obtained in seven cells. (C) Top, ATP (20 μM) was applied to the cell. Caffeine was then added in Ca²⁺- and Na⁺-free solution. After washout, the same protocol was repeated in the absence of the purine. Bottom, similar results were observed in at least nine cells and gathered in the bar graph (Ca²⁺ content of the SR was estimated from the ratio of the magnitude of the Ca²⁺ peak triggered by caffeine to the magnitude of the averaged Ca²⁺ transients). **, significantly different (p ≤ 0.01).