Calcium signalling in sarcoplasmic reticulum, cytoplasm and mitochondria during activation of rabbit aorta myocytes

This study investigated the relationship between cytoplasmic, mitochondrial , and sarcoplasmic reticulum (SR) [Ca2+] in rabbit aorta smooth muscle cell s, following cell activation. Smooth muscle cells were loaded with the Ca2-sensitive fluorescent indicator Mag-Fura-2-AM, and then either permeabiliz ed by exposure to saponin, or dialyzed with a patch pipette in the whole-ce ll configuration to remove cytoplasmic indicator. When the intracellular so lution contained millimolar EGTA or BAPTA, activation of SR Ca2+ release th rough IP3 or ryanodine receptors induced a decrease in the [Ca2+] reported by Mag-Fura-2. However, when EGTA was present at less than or equal to 100 mu M, the same stimuli caused an increase in the [Ca2+] reported by Mag-Fur a-2. The increase in [Ca2+] caused by phenylephrine or caffeine was delayed , and prolonged, with respect to the cytoplasmic Ca2+ transient. Evidence i s presented that this Mag-Fura-2 signal reflected a rise in mitochondrial [ Ca2+]. Agents that inhibit mitochondrial function, such as FCCP or cyanide in combination with oligomycin B, converted the increase in organelle Mag-F ura-e fluorescence to a decrease, while also prolonging the cytoplasmic Ca2 + transient. There was considerable similarity between the localization of Mag-Fura-2 fluorescence and the mitochondria-selective indicator tetramethy lrhodamine ethyl ester. Thus, we propose that there is close functional int egration between the SR and mitochondria in aorta smooth muscle cells, with mitochondria taking up Ca2+ from the cytoplasm following cell activation. (C) 2000 Harcourt Publishers Ltd.