The triple quantum filtered 23Na NMR method is applied here to measure the effects of
EIPA, a specific inhibitor of the Na+/H+
antiporter, on relative intracellular
sodium concentrations in isolated working hearts at baseline, during
ischemia, and at subsequent reperfusion. In analogy to the spectrophotometric isosbestic point, an approach is developed that defines a value of tau at which the effect of the relaxation times on the TQF signal intensities is minimized, and the signals are proportional to the
sodium concentration for both ischemic and working hearts.
EIPA at 1.5 microM significantly inhibited (P < 0.01) the influx of intracellular Na+ during 20 min of
ischemia at 36.2 degrees C in this rat heart model. In parallel 31P NMR studies,
EIPA had no effect on either the development of
acidosis during
ischemia or on the recovery of pHi during reperfusion despite its profound effect on intracellular Na+ influx. Thus, under our conditions the Na+/H+
antiporter did not play a critical role in the maintenance of intracellular pH.
EIPA treatment resulted in improved recovery (P < 0.005) of mechanical function after 20 min of
ischemia. [
ATP] was higher in treated hearts during
ischemia and reperfusion.