Both Na+-K+ATPase and Na+-H+Exchanger are Immediately Active upon Post-ischemic Reperfusion in Isolated Rat Hearts

Limited time resolution has hampered proper evaluation of changes in intracellular Na+(Na+i) in whole hearts upon post-ischemic reperfusion. In isolated rat hearts perfused at 37°C, we studied the contribution of the Na+-K+ATPase and the Na+-H+exchanger to control of Na+iduring reperfusion using23Na NMR and the shift reagent Tm(DOTP)5−with a time resolution of 5 s. To assess activities of the Na+-K+ATPase and the Na+-H+exchanger, 250μmol/l ouabain and/or 3μmol/l EIPA, respectively, was added to the perfusate during the first 5 min of reperfusion, following 20 min of ischemia. When used, ouabain was also present for 2 min prior to ischemia. Na+iincreased during ouabain perfusion prior to ischemia (132±5 and 133±4% of the pre-ischemic control value after 2 min, in ouabain and ouabain+EIPA hearts, respectively; mean± ;n=6 per group) resulting in higher end-ischemic values in ouabain and ouabain+EIPA hearts (249±9 and 267±17% of the pre-ischemic control value, respectively) than in control and EIPA hearts (207±21 and 199±10% of the pre-ischemic control value, respectively). In ouabain, hearts Na+istarted to rise directly upon reperfusion and amounted to 117±6% of the end-ischemic value after 60 s of reperfusion. In control hearts, however, Na+idropped immediately and was 87±5% of the end-ischemic value after 60 s, indicating that the Na+-K+ATPase resumed function directly upon reperfusion. The initial steep increase of Na+iupon reperfusion in ouabain hearts, which diminished after ≈40 s to the rate of increase observed during ischemia, was absent in ouabain+EIPA hearts. This indicates the existence, although masked by Na+-K+ATPase activity, of a Na+-H+exchange mediated Na+influx upon reperfusion. If only EIPA was present during reperfusion the initial decrease in Na+iwas faster than in control hearts, corroborating this finding.