Species dependence of mitochondrial calcium transients during excitation-contraction coupling in isolated cardiomyocytes

Whether mitochondrial Ca2+ transport is rapid enough to respond to changes in cytosolic [Ca2+] ([Ca2+](c)) which occur during excitation-contraction coupling in the heart is controversial; different results were obtained with different techniques and different species. In this study mitochondrial [Ca2+] ([Ca2+](m)) was measured in indo-1/AM-loaded myocytes from rat and guinea-pig hearts where the cytosolic indo-1 had been removed by extended incubation of cells at 37 degrees C ("heat treatment"). The mitochondrial origin of the remaining fluorescence was confirmed by sensitivity of the indo-1 signal to ruthenium red. In resting rat myocytes, [Ca2+](m) was lower than [Ca2+](c), whereas in guinea-pig cells [Ca2+](m) was higher than [Ca2+](c). Upon electrical stimulation of cells, no change occurred in [Ca2+](m) in rat myocytes. However, in guinea-pig cells mitochondrial Ca2+ transients were clearly visible with a mean indo-1 ratio amplitude of 0.153 +/- 0.2 (n = 20), compared with 0.306 +/- 0.02 (n = 25), p < 0.001, prior to heat treatment. These observations suggest significant differences in mitochondrial Ca2+ transport in cardiomyocytes from different species. (C) 1999 Academic Press.