31P NMR kinetics study of cardiac metabolism under mild hypoxia

The effects of mild hypoxia on the metabolic and mechanical functions of isovolumic perfused rat hearts have been studied. 31P NMR has been used to follow the metabolite concentrations as well as the intracellular pH. Additionally, the energy transfer through the creatine kinase reaction was estimated by the magnetization-transfer technique. The needs of myofibrillar energy and of mitochondrial ATP production have been assessed through mechanical activity and oxygen-consumption rate. It has been observed that mild hypoxia simultaneously impairs contractile and metabolic functions. The aerobic ATP production is maintained under these conditions while anaerobic energy metabolism seems accelerated. The accumulation of some metabolites (ADP and P(i)) and the decrease of creatine kinase forward flux (Vfor) tend, however, to prove that ATP availability for myofibrils is lowered. The large aerobic energy production observed must therefore be explained by an energy wastage in the mitochondria. In spite of normal ATP concentration, a contractile dysfunction is observed and can be explained by the P(i) accumulation, which is known to impair the use of the myofibrillar ATP. Another hypothesis supported by the magnetization-transfer experiments is the poor ATP availability resulting from the ATP wastage in the mitochondria and from the inefficient energy transport by the PCr-Cr shuttle.