Sodium exchange during hypoxia and on reoxygenation in the isolated rabbit heart

Sodium exchange was examined during and after a period of hypoxic and substrate free perfusion in the isolated but arterially perfused interventricular septum of the rabbit heart. Temperature was 35 degrees C and muscles were stimulated at a mean rate of 90 beats/min. The uptake and washout of isotopes of sodium were followed during hypoxia and on reoxygenation. The extracellular space was estimated from the distribution volume of 51Cr-EDTA. During 45 mins hypoxia the intracellular sodium increased from 9.6 +/- 1.1 to 22.6 +/- 1.9 mmol/kg wet tissue. At this time maximal contracture had developed and recovery of mechanical function on reoxygenation was small. No increased efflux of sodium could be detected on reoxygenation. A net efflux of sodium could be detected when conditions were chosen to stimulate sodium--calcium exchange. These experiments do not support the hypothesis that the previously reported influx of calcium on reoxygenation is directly linked to a net loss of sodium.