Continuous determination of extracellular space and changes of K+, Na+, Ca2+, and H+ during global ischaemia in isolated rat hearts.

Isolated rat hearts were perfused according to the Langendorff technique and prepared to allow the measurement of left ventricular pressure (LVP), the first derivative of LVP (dP/dt), coronary perfusion pressure (CPP) and heart rate (HR). The hearts were perfused with a modified Krebs-Henseleit solution (KHS; control group) or KHS containing tetramethylammonium-chloride (TMA; 100 microM), which did not influence heart haemodynamics or extracellular potassium accumulation during global ischaemia (GI). TMA was used as a marker to determine changes in the size of the extracellular space (ECS) during 60 min of GI. Extracellular concentrations of K+, Na+, Ca2+, H+ and TMA were measured using double-barreled polyvinyl chloride (PVC) mini-electrodes. Relative changes in the ECS size and net cation movements were calculated from the extracellular TMA and cation concentrations. After 60 min of GI ECS decreased by 74% due to a water shift into the intracellular space (ICS). Within 10 s after the onset of GI extracellular potassium concentration increased in a typical triphasic pattern. A biphasic net efflux of K+ was maximal 2 and 15 min after the onset of ischaemia interrupted by a maximal uptake after 7 min, probably due to the stimulation of the Na+/K+ ATPase. The changes in extracellular sodium and calcium concentrations were biphasic; showing an initial increase occurring approximately during the first 20 min after the onset of GI followed by a decrease. Despite the initial increase of extracellular Na+ and Ca2+ maximal net influx of Na+ and Ca2+ were calculated after 2 and 19 min of GI. Accumulation of H+ in the extracellular space occurred mainly during the first 20 min of GI. It is concluded that, (1) TMA is a useful marker for continuous measurement of changes in the size of the ECS during GI; (2) Changes in extracellular ion concentration are influenced markedly by a water shift from the ECS into the ICS; (3) Stimulation of the Na+/K+ pump causes a transient net uptake of K+ and reduces the net influx of Na+; (4) Ca2+ uptake is decreased transiently probably by increased Ca2(+)-ATPase activity; (5) Energy dependent protective mechanisms for the maintainance of intracellular ionic homeostasis are exhausted after approximately 15 to 20 min of GI in isolated working rat hearts.