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 P-31 NMR. Hearts were continuously perfused with phosphate-free Krebs-Hense leit 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 P-i the intracellular pH was estimated. At 36, 2 0, and 10 degrees C 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 degrees C two P-i resonances were observed with a separation of 0.25 ppm. This resona nce corresponded to a P-i resonance of a cellular compartment with a local pH of 7.78 +/- 0.06, likely mitochondrial. This additional resonance disapp eared upon warming of the hearts back to 36 degrees C.