Prolonged periods of
hypoxia are deleterious to higher brain functions and increase the likelihood of developing
dementias. Here, we have used fluorimetric techniques to investigate the effects of chronic
hypoxia (2.5% O(2), 24 h) on Ca(2+) stores in type I cortical astrocytes, because such stores are crucial to various astrocyte functions, including Ca(2+)-dependent modulation of neuronal activity. Rises of [Ca(2+)](i) evoked by exposure of astrocytes to
bradykinin were enhanced following chronic
hypoxia, as were transient increases in [Ca(2+)](i) recorded in Ca(2+)-free perfusate. The enhanced responses were due partly to impaired plasmalemmal Na(+)/Ca(2+) exchange following chronic
hypoxia. More importantly, chronic
hypoxia increased the Ca(2+) content of mitochondria (as determined by exposing cells to mitochondrial inhibitors), such that they were unable to act as Ca(2+)
buffers following
bradykinin-evoked Ca(2+) release from the endoplasmic reticulum. Hypoxic enhancement of mitochondrial Ca(2+) content was also observed in confocal images of cells loaded with the mitochondrial Ca(2+)
indicator,
Rhod-2. Confocal imaging of cells loaded with
tetramethylrhodamine ethyl
ester, an
indicator of mitochondrial membrane potential, indicated that mitochondria were hyperpolarized in astrocytes following chronic
hypoxia. Our findings indicate that
hypoxia disturbs Ca(2+) signaling in type I astrocytes, primarily by causing mitochondrial Ca(2+) overload.