Observation of two inorganic phosphate NMR resonances in the perfused hypothermic rat heart.

The effect of hypothermia on isolated perfused rat hearts was studied with 31P NMR. Hearts were continuously perfused with phosphate-free Krebs-Henseleit buffer while the perfusate temperature was adjusted. Perfusate pH was kept at 7.40 +/- 0.02 throughout the experiments. Using the chemical shift difference between PCr and Pi the intracellular pH was estimated. At 36, 20, and 10 degreesC a cytosolic alkalinization at a pH of 7.05 +/- 0.04, 7.21 +/- 0.05, and 7.40 +/- 0.03 was observed, respectively. At 10 degreesC two Pi resonances were observed with a separation of 0.25 ppm. This resonance corresponded to a Pi resonance of a cellular compartment with a local pH of 7.78 +/- 0.06, likely mitochondrial. This additional resonance disappeared upon warming of the hearts back to 36 degreesC.