EXTRACELLULAR VOLUME AND TRANSSARCOLEMMAL PROTON MOVEMENT DURING ISCHEMIA AND REPERFUSION - A P-31 NMR SPECTROSCOPIC STUDY OF THE ISOVOLUMIC RAT-HEART

We have measured, directly and simultaneously, changes in extracellula r volume and intra- and extracellular pH during ischemia in the isolat ed rat heart using P-31 NMR spectroscopy. Hearts were perfused with bu ffer containing 15 mm sodium phenylphosphonate at pH 7.4. Wash in and wash out experiments showed that phenylphosphonate entered only the ex tracellular (interstitial, vascular and chamber) space of the heart an d had no adverse effects on myocardial energetics, contractile functio n or coronary flow rate. Hearts were subjected to 28 min of total, glo bal ischemia, during which the phenylphosphonate resonance area in the P-31 NMR spectra decreased by 83%, indicating that extracellular flui d had moved rapidly from the heart to the bath surrounding the heart, partly as a result of vascular collapse. A separate, morphological stu dy confirmed that 95% of the vasculature had collapsed by 28 min ische mia. Intra- and extracellular pH were determined from the chemical shi fts of the P(i) and the phenylphosphonate resonances, respectively. In the pre-ischemic rat heart, intracellular pH was 7.15+/-0.03 and extr acellular pH was 7.39+/-0.03. By 4 min of ischemia, intra- and extrace llular pH were the same and decreased concomitantly throughout the rem ainder of ischemia to final values of 6.09+/-0.19 and 6.16+/-0.23, res pectively. On reperfusion, the extracellular volume and pH returned to pre-ischemic levels within 1 min, but restoration of intracellular pH took >2.5 min. Thus, a large volume of extracellular fluid moves out of the rat heart to the surrounding bath and the intra- and extracellu lar pH become the same during total, global ischemia.