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

Abstract 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 + i during reperfusion using 23 Na 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, 250 μ mol/l ouabain and/or 3 μ mol/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 + i increased 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± s.e.m. ; 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 + i started 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 + i dropped 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 + i upon 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 + i was faster than in control hearts, corroborating this finding.