The whole-cell Ca release-activated Ca current, I , is regulated by the mitochondrial Ca uniporter channel and is independent of extracellular and cytosolic Na.

Key Points: Ca entry through Ca  release-activated Ca  channels activates numerous cellular responses. Under physiological conditions of weak intracellular Ca buffering, mitochondrial Ca uptake regulates CRAC channel activity. Knockdown of the mitochondrial Ca uniporter channel prevented the development of I in weak buffer but not when strong buffer was used instead. Removal of either extracellular or intra-pipette Na had no effect on the selectivity, kinetics, amplitude, rectification or reversal potential of whole-cell CRAC current. Knockdown of the mitochondrial Na -Ca exchanger did not prevent the development of I in strong or weak Ca buffer. Whole cell CRAC current is Ca -selective. Mitochondrial Ca channels, and not Na -dependent transport, regulate CRAC channels under physiological conditions.

Abstract: Ca entry through store-operated Ca release-activated Ca (CRAC) channels plays a central role in activation of a range of cellular responses over broad spatial and temporal bandwidths. Mitochondria, through their ability to take up cytosolic Ca , are important regulators of CRAC channel activity under physiological conditions of weak intracellular Ca buffering. The mitochondrial Ca transporter(s) that regulates CRAC channels is unclear and could involve the 40 kDa mitochondrial Ca uniporter (MCU) channel or the Na -Ca -Li exchanger (NCLX). Here, we have investigated the involvement of these mitochondrial Ca transporters in supporting the CRAC current (I ) under a range of conditions in RBL mast cells. Knockdown of the MCU channel impaired the activation of I under physiological conditions of weak intracellular Ca buffering. In strong Ca buffer, knockdown of the MCU channel did not inhibit I development demonstrating that mitochondria regulate CRAC channels under physiological conditions by buffering of cytosolic Ca via the MCU channel. Surprisingly, manipulations that altered extracellular Na , cytosolic Na or both failed to inhibit the development of I in either strong or weak intracellular Ca buffer. Knockdown of NCLX also did not affect I . Prolonged removal of external Na also had no significant effect on store-operated Ca entry, on cytosolic Ca oscillations generated by receptor stimulation or on CRAC channel-driven gene expression. In the RBL mast cell, Ca flux through the MCU but not NCLX is indispensable for activation of I .