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

This study investigated whether Na+/K+ ATPase is inhibited and K-ATP channe ls activated during low flow ischemia (LFI) by monitoring Rb+ uptake and ef flux from rat hearts using Rb-87 NMR. In the uptake experiments, isolated L angendorff perfused hearts were exposed to Rb+-containing Krebs-Henseleit b uffer (2.14mM + 3.76 mM K+) for 60 min. When Rb+ uptake started the flow of perfusate was decreased from 10 to 1ml/min/g wet weight for 44 min and the n returned to normal. The rate of Rb+ uptake and its equilibrium level decr eased to 40 and 65% of the control (no ischemia) levels, respectively Phosp hocreatine and cytoplasmic [ATP]/[ADP] measured by P-31 NMR decreased by ha lf, intracellular pH (pHi) decreased to 6.8+/-0.1 and Pi increased two-ford . In wash-out experiments the hearts were pre-loaded with Rb+ far 30 min fo llowing which Rb+ wash-out was initiated. Four minutes later, flow was eith er decreased in the absence or presence of 10 mu M 2,4-dinitrophenol (DNP), or 0.1mM DNP was infused at normal flow for 20 min. LFI resulted in biphas ic Rb+ efflux; during the initial phase, which lasted 8 min, the rate const ant (k x 10(3)/min) did not differ from control (43 +/- 3). The efflux was slightly inhibited by 5 mu M glibenclamide (36 +/- 6) or 100 mu M 5-hydroxy decanoic acid (32 +/- 4). In the second phase k decreased to half its initi al value (18 +/- 2). More significant changes in energy state caused by LFI + 10 mu M DNP had no effect on the efflux kinetics. Similar changes in ene rgy state induced by 0.1mM DNP at normal flow were associated with activati on of Rb+ efflux (71 +/- 5). DNP-stimulated Rb+ efflux was inhibited by aci dosis (pHi similar to pHe = 6.7) produced with 5mM morpholinoethane sulphon ic acid (53 +/- 5) and by 100 mu M adenosine (58 +/- 7). We suggest that ac cumulation of ischemic products such as H+ and adenosine decreases activati on of K-ATP channels in rat hearts. (C) 1999 Academic Press.