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Department of Physiology, University of Maryland School of Medicine, Baltimore, Maryland 21201
The role of mitochondria in
Ca2+ homeostasis is controversial.
We employed the Ca2+-sensitive dye
rhod 2 with novel, high temporal and spatial resolution imaging to
evaluate changes in the matrix free
Ca2+ concentration of individual
mitochondria
([Ca2+]m)
in agonist-stimulated, primary cultured aortic myocytes. Stimulation with 10 µM serotonin (5-HT) evoked modest cytosolic
Ca2+ transients
[cytosolic free
Ca2+ concentration
([Ca2+]cyt)
<500 nM; measured with fura 2] and triggered contractions in
short-term cultured myocytes. However, 5-HT triggered a large mitochondrial rhod 2 signal (indicating pronounced elevation of [Ca2+]m)
in only 4% of cells. This revealed heterogeneity in the responses of
individual mitochondria, all of which stained with MitoTracker Green
FM. In contrast, stimulation with 100 µM ATP evoked large cytosolic
Ca2+ transients (>1,000 nM) and
induced pronounced, reversible elevation of
[Ca2+]m
(measured as rhod 2 fluorescence) in 60% of cells. This mitochondrial Ca2+ uptake usually lagged behind
the cytosolic Ca2+ transient peak
by 3-5 s, and
[Ca2+]m
declined more slowly than did bulk
[Ca2+]cyt.
The uptake delay may prevent mitochondria from interfering with rapid
signaling events while enhancing the mitochondrial response to large,
long-duration elevations of
[Ca2+]cyt.
The responses of arterial myocytes to modest physiological stimulation
do not, however, depend on such marked changes in [Ca2+]m.
mitochondria; rhod 2; vascular smooth muscle; fluorescence digital imaging
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