Observation of a second phosphate pool in the perfused heart by 31P NMR; is this the mitochondrial phosphate?

We report here our observations on perfused rat hearts using 31P NMR which show that inorganic phosphate (Pi) is located in two regions of differing pH. From the chemical shifts of the Pi signals and their behaviour under various stimuli we identify one signal (pH 7.0) as coming from the cytosol and suggest that the other (pH 7.38) comes from the mitochondrial matrix space.