Abstract

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 μ m), 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 +, Ca 2+, 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 Ca 2+ maximal net influx of Na + and Ca 2+ 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 concentrations 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) Ca 2+ uptake is decreased transiently probably by increased Ca 2+-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.

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