Post-ischemic 31P NMR determination of myocardial intracellular pH in vivo using ATP peak.

Abstract

31P NMR allows non-invasive measurement of intracellular pH, which drops during tissue hypoxia or ischemia. Determination is usually based on the chemical shift between the inorganic phosphate (P(i)) and phosphocreatine (PCr) peaks. During reperfusion, P(i) is taken up to form PCr and ATP, and in our model at least (an isolated, working rat heart perfused with an erythrocyte suspension), the level of P(i) reduces well below the pre-ischemic level, making pH determination difficult. The chemical shifts of the three ATP peaks also depend on pH, and the level of ATP remains high during reperfusion, so these might be used to determine pH. The results of one experiment are presented in detail, showing the time course of high energy phosphate levels before, during and after a 32 min ischemic insult, and close agreement between the pH determinations from the Pi and gamma-ATP peaks can be seen. The formula used to calculate pH from the ATP peak was: pH (ATP) = 0.59 delta 2-5.0 delta + 15.9 where delta is the shift in ppm between PCr and gamma-ATP. All pH readings by both methods from a series of seven experiments were compared and a 1:1 agreement demonstrated (correlation coefficient 0.63, p < 0.0001). Although the ATP shifts also depend on magnesium complexation which we have ignored, this appears to be justifiable within the errors of the method; the good agreement between the results of the two methods, and the ability to determine pH during reperfusion suggest that calculation of intracellular pH from the chemical shift of gamma-ATP is a useful technique.