Agonist-induced mitochondrial Ca2+ transients in smooth muscle

We investigated the role of mitochondria (MT) in calcium signaling in a culture of rat aortic smooth muscle cells. We used targeted aequorin to selectively measure [Ca2+] in this organelle. Our results reveal that smooth muscle cell stimulation with agonists causes a large, transient increase in mitochondrial [Ca2+] ([Ca2+]m). This large transient can be blocked with inhibitors of the sarco-endoplasmic reticulum Ca2+-ATPase, suggesting a close relationship between the sarcoplasmic reticulum (SR) and the mitochondria. FCCP completely abolished the response to agonists, and targeted mitochondrial GFP revealed a vast tubular network of MT in these cells. When added before stimulation with ATP, IP3 inhibitors partially blocked the ATP-induced rise in mitochondrial Ca2+ release. The role of the Na+/Ca2+ exchanger (NCX) was examined by removing extracellular Na+. This procedure prevented the decrease in the [Ca2+]m transient normally seen on removal of extracellular Ca2+. We propose a functional linkage of MT and SR dependent on a narrow junctional space between the two organelles in which Ca2+ diffusion is restricted. Approximately half of the mitochondria appear to be associated with the superficial SR, which communicates with the extracellular space via NCX.