Ischemic blockade of the cardiac Na+/H+-exchanger: a simultaneous 23Na and 31P study

intracellular pH (pHi), high energetic phosphates (HEP's) and postischemic contractile recovery in isolated rat hearts, using three different NHE-blockers: EIPA, HOE642 and EMD96785. Methods Isolated rat hearts were perfused according to Langendorff at a constant pressure of 73.5 mmHg at 37°C with a modified KrebsHenseleit buffer (pH 7.4) with glucose as substrate and were paced at 5 Hz. Left ventricular developed pressure (LVDP) and end diastolic pressure (EDP) were measured with an intraventricular balloon. [Na+]i, pHi and HEP's were measured using 23Na and 31 PN MR spectroscopy, respectively. 23Na and 31P were measured simultaneously at frequencies of 105.9 and 162.0 MHz, respectively, on a Bruker Avance DRX400 spectrometer equipped with a dual tuned probe and two digital receivers. 23Na spectra were acquired by adding 288 FID's using 90° pulses and a 210 ms interpulse delay. 31P spectra were acquired by adding 24 FID's using 90° pulses and a 2.5 s interpulse delay. 31 Pa nd23Na were both collected with a time resolution of 1 minute. To quantify PCr and ATP five 31P spectra were added. To discriminate between intra- and extracellular Na+, the shift reagent TmDOTP5- (3.5 mM) was added to the perfusate, necessitating a lower free Ca2+ concentration (0.85 mM). NHE-blockers were administered in a concentration of 3 µM during 5 minutes immediately prior to 30 minutes of global ischemia and 30 minutes of drug-free reperfusion. Data are presented as mean ± SEM, n=6 for all groups. Results Na+ overload after 30 min of ischemia was effectively reduced by