Endothelial mitochondria regulate the intracellular Ca2+ response to fluid shear stress

Shear stress is known to stimulate an intracellular free calcium concentration ([Ca 2+ ] i ) response in vascular endothelial cells (ECs). [Ca 2+ ] i is a key second messenger for signaling that leads to vasodilation and EC survival. Although it is accepted that the shear-induced [Ca 2+ ] i response is, in part, due to Ca 2+ release from the endoplasmic reticulum (ER), the role of mitochondria (second largest Ca 2+ store) is unknown. We hypothesized that the mitochondria play a role in regulating [Ca 2+ ] i in sheared ECs. Cultured ECs, loaded with a Ca 2+ -sensitive fluorophore, were exposed to physiological levels of shear stress. Shear stress elicited [Ca 2+ ] i transients in a percentage of cells with a fraction of them displaying oscillations. Peak magnitudes, percentage of oscillating ECs, and oscillation frequencies depended on the shear level. [Ca 2+ ] i transients/oscillations were present when experiments were conducted in Ca 2+ -free solution (plus lanthanum) but absent when ECs were treated with a phospholipase C inhibitor, suggesting that the ER inositol 1,4,5-trisphosphate receptor is responsible for the [Ca 2+ ] i response. Either a mitochondrial uncoupler or an electron transport chain inhibitor, but not a mitochondrial ATP synthase inhibitor, prevented the occurrence of transients and especially inhibited the oscillations. Knockdown of the mitochondrial Ca 2+ uniporter also inhibited the shear-induced [Ca 2+ ] i transients/oscillations compared with controls. Hence, EC mitochondria, through Ca 2+ uptake/release, regulate the temporal profile of shear-induced ER Ca 2+ release. [Ca 2+ ] i oscillation frequencies detected were within the range for activation of mechanoresponsive kinases and transcription factors, suggesting that dysfunctional EC mitochondria may contribute to cardiovascular disease by deregulating the shear-induced [Ca 2+ ] i response.