Tissue pH in human kidney transplants during hypothermic ischemia

Ex vivo NMR spectroscopy was used to investigate pH in 67 human kidney transplants. 1H and 31P spectra were recorded at 1.5 T during regular hypothermic storage in histidine-tryptophane-α-ketoglutarate (HTK) solution. Estimations of cytosolic pH from chemical shift differences between inorganic phosphate and phosphodiesters and of extracellular pH from the ε1 and δ2 protons of histidine were based upon systematic titration studies. The possibility to predict acute tubular necrosis (ATN) by measuring pH was compared to results obtained with peak area ratios of phosphomonoesters (PME) and Pi and of the γ-phosphorus of nucleoside 5′-triphosphate (γ-NTP) and Pi. Cytosolic pH was 6.86 ± 0.10 in kidneys showing immediate post-transplant function and 6.84 ± 0.10 in those with ATN. Time-dependent studies demonstrated a monoexponential pH decay (velocity constant: 0.14 ± 0.07 h−1). Extracellular pH varied between 7.40 and 7.15. Grafts with immediate function showed higher PME/Pi (2.24 ± 0.57 vs. 1.77 ± 0.50, p < 0.05) and γ-NTP/Pi (0.33 ± 0.16 vs. 0.16 ± 0.08, p < 0.001). Intra- and extracellular pH can be monitored non-invasively during hypothermic transplant storage. The pH gradient between both compartments provides quantitative information about the buffer capacity of the preservation medium. Acidification is not a primary cause of ATN during regular HTK storage. The total nucleotide pool is a determinant of the reversibility of ischemic injury.