In eukaryotic cells, hormones and neurotransmitters that engage the phosphoinositide pathway evoke a biphasic increase in intracellular free Ca(2+) concentration: an initial transient release of Ca(2+) from intracellular stores is followed by a sustained phase of Ca(2+) influx. This influx is generally store dependent. Most attention has focused on the link between the endoplasmic reticulum and store-operated Ca(2+) channels in the plasma membrane. Here, we describe that respiring mitochondria are also essential for the activation of macroscopic store-operated Ca(2+) currents under physiological conditions of weak intracellular Ca(2+) buffering. We further show that Ca(2+)-dependent slow inactivation of Ca(2+) influx, a widespread but poorly understood phenomenon, is regulated by mitochondrial buffering of cytosolic Ca(2+). Thus, by enabling macroscopic store-operated Ca(2+) current to activate, and then by controlling its extent and duration, mitochondria play a crucial role in all stages of store-operated Ca(2+) influx. Store-operated Ca(2+) entry reflects a dynamic interplay between endoplasmic reticulum, mitochondria and plasma membrane.
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