Gr. Monteith et Mp. Blaustein, Heterogeneity of mitochondrial matrix free Ca2+: resolution of Ca2+ dynamics in individual mitochondria in situ, AM J P-CELL, 45(5), 1999, pp. C1193-C1204
The role of mitochondria in Ca2+ homeostasis is controversial. We employed
the Ca2+-sensitive dye rhod 2 with novel, high temporal and spatial resolut
ion imaging to evaluate changes in the matrix free Ca2+ concentration of in
dividual mitochondria ([Ca2+](m)) in agonist-stimulated, primary cultured a
ortic myocytes. Stimulation with 10 mu M serotonin (5-HT) evoked modest cyt
osolic Ca2+ transients [cytosolic free Ca2+ concentration ([Ca2+](cyt)) <50
0 nM; measured with fura 2] and triggered contractions in short-term cultur
ed myocytes. However, 5-HT triggered a large mitochondrial rhod 2 signal (i
ndicating pronounced elevation of [Ca2+](m)) in only 4% of cells. This reve
aled heterogeneity in the responses of individual mitochondria, all of whic
h stained with MitoTracker Green FM. In contrast, stimulation with 100 mu M
ATP evoked large cytosolic Ca2+ transients (>1,000 nM) and induced pronoun
ced, reversible elevation of [Ca2+](m) (measured as rhod 2 fluorescence) in
60% of cells. This mitochondrial Ca2+ uptake usually lagged behind the cyt
osolic Ca2+ transient peak by 3-5 s, and [Ca2+], declined more slowly than
did bulk [Ca2+](cyt). The uptake delay may prevent mitochondria from interf
ering with rapid signaling events while enhancing the mitochondrial respons
e to large, long-duration elevations of [Ca2+](cyt). The responses of arter
ial myocytes to modest physiological stimulation do not, however, depend on
such marked changes in [Ca2+](m).