The Effects of Low-Flow Ischemia on K+Fluxes in Isolated Rat Hearts Assessed by87Rb NMR

This study investigated whether Na+/K+ATPase is inhibited and KATPchannels activated during low flow ischemia (LFI) by monitoring Rb+uptake and efflux from rat hearts using87Rb NMR. In the uptake experiments, isolated Langendorff perfused hearts were exposed to Rb+-containing Krebs-Henseleit buffer (2.14 m +3.76 m K+) for 60 min. When Rb+uptake started the flow of perfusate was decreased from 10 to 1 ml/min/g wet weight for 44 min and then returned to normal. The rate of Rb+uptake and its equilibrium level decreased to 40 and 65% of the control (no ischemia) levels, respectively. Phosphocreatine and cytoplasmic [ATP]/[ADP] measured by31P NMR decreased by half, intracellular pH (pHi) decreased to 6.8±0.1, and Pi increased two-fold. In wash-out experiments the hearts were pre-loaded with Rb+for 30 min following which Rb+wash-out was initiated. Four minutes later, flow was either decreased in the absence or presence of 10μ 2,4-dinitrophenol (DNP), or 0.1 m DNP was infused at normal flow for 20 min. LFI resulted in biphasic Rb+efflux; during the initial phase, which lasted 8 min, the rate constant (k×103/min) did not differ from control (43±3). The efflux was slightly inhibited by 5μ glibenclamide (36±6) or 100μ 5-hydroxydecanoic acid (32±4). In the second phasekdecreased to half its initial value (18±2). More significant changes in energy state caused by LFI+10μ DNP had no effect on the efflux kinetics. Similar changes in energy state induced by 0.1 m DNP at normal flow were associated with activation of Rb+efflux (71±5). DNP-stimulated Rb+efflux was inhibited by acidosis (pHipHe=6.7) produced with 5 m morpholinoethane sulphonic acid (53±5) and by 100μ adenosine (58±7). We suggest that accumulation of ischemic products such as H+and adenosine decreases activation of KATPchannels in rat hearts.