The effects of low-flow ischemia on K+ fluxes in isolated rat hearts assessed by 87Rb NMR.

This study investigated whether Na+/K+ ATPase is inhibited and KATP channels activated during low flow ischemia (LFI) by monitoring Rb+ uptake and efflux from rat hearts using 87Rb NMR. In the uptake experiments, isolated Langendorff perfused hearts were exposed to Rb+-containing Krebs-Henseleit buffer (2.14 m m+3.76 m 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 by 31P 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 10 microm 2,4-dinitrophenol (DNP), or 0.1 m 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 (kx10(3)/min) did not differ from control (43+/-3). The efflux was slightly inhibited by 5 microm glibenclamide (36+/-6) or 100 microm 5-hydroxydecanoic acid (32+/-4). In the second phase k decreased to half its initial value (18+/-2). More significant changes in energy state caused by LFI+10 microm DNP had no effect on the efflux kinetics. Similar changes in energy state induced by 0.1 m m DNP at normal flow were associated with activation of Rb+ efflux (71+/-5). DNP-stimulated Rb+ efflux was inhibited by acidosis (pHi approximately pHe = 6.7) produced with 5 m m morpholinoethane sulphonic acid (53+/-5) and by 100 microm adenosine (58+/-7). We suggest that accumulation of ischemic products such as H+and adenosine decreases activation of KATP channels in rat hearts.