Inhibition of mitochondrial calcium uptake rather than efflux impedes calcium release by inositol-1,4,5-trisphosphate-sensitive receptors

Mitochondria modulate cellular Ca2+ signals by accumulating the ion via a uniporter and releasing it via Na+- or H+-exchange. In smooth muscle, inhibition of mitochondrial Ca2+ uptake inhibits Ca2+ release from the sarcoplasmic reticulum (SR) via inositol-1,4,5-trisphosphate-sensitive receptors (IP3R). At least two mechanisms may explain this effect. First, localised uptake of Ca2+ by mitochondria may prevent negative feedback by cytosolic Ca2+ on IP3R activity, or secondly localised provision of Ca2+ by mitochondrial efflux may maintain IP3R function or SR Ca2+ content. To distinguish between these possibilities the role of mitochondrial Ca2+ efflux on IP3R function was examined. IP3 was liberated in freshly isolated single colonic smooth muscle cells and mitochondrial Na+–Ca2+ exchanger inhibited with CGP-37157 (10 μM). Mitochondria accumulated Ca2+ during IP3-evoked [Ca2+]c rises and released the ion back to the cytosol (within ∼15 s) when mitochondrial Ca2+ efflux was active. When mitochondrial Ca2+ efflux was inhibited by CGP-37157, an extensive and sustained loading of mitochondria with Ca2+ occurred after IP3-evoked Ca2+ release. IP3-evoked [Ca2+]c rises were initially unaffected, then only slowly inhibited by CGP-37157. IP3R activity was required for inhibition to occur; incubation with CGP-37157 for the same duration without IP3 release did not inhibit IP3R. CGP-37157 directly inhibited voltage-gated Ca2+ channel activity, however SR Ca2+ content was unaltered by the drug. Thus, the gradual decline of IP3R function that followed mitochondrial Na+–Ca2+ exchanger inhibition resulted from a gradual overload of mitochondria with Ca2+, leading to a reduced capacity for Ca2+ uptake. Localised uptake of Ca2+ by mitochondria, rather than mitochondrial Ca2+ efflux, appears critical for maintaining IP3R activity.