Anoxia generates rapid and massive opening of KATP channels in ventricular cardiac myocytes

The aim was to improve the measurement of both the time course and amplitude of anoxia-induced K-ATP-channel current (I-KATP) in isolated heart cells to specify the role of these channels in the time course of K+ accumulation in the ischemic myocardium. Methods: Ionic currents in isolated ventricular heart cells of the mouse were measured with a patch clamp technique under normoxic conditions (atmospheric pO(2)), during wash-out of oxygen, and under anoxic conditions (pO(2)<0.2 mmHg). During the measurement, the actual pO(2) in the close proximity of the cell was determined with an optical technique by exciting Pd-meso-tetra(4-carboxyphenyl)porphin with light flashes of 508-570 nm and evaluating the quenching kinetics of the emitted phosphorescence signal at 630-700 nm. These quenching kinetics steeply depend on pO(2) and can be evaluated best at pO(2) values near 0 mmHg. Results: Out of 28 cells, 23 cells started to develop I-KATP at pO(2) values between 0 and 0.4 mmHg, i.e. in the range of the level of half maximum activity of the cytochrome oxidase. The remaining five cells developed I-KATP between 0.4 and 1.8 mmHg. With respect to the time course, 18 out of 27 cells started to develop I-KATP within the first minute after pO(2) had decreased to values below 0.2 mmHg. The amplitude of I-KATP induced by anoxia and various metabolic inhibitors was large, 29+/-12 and 48+/-21 nA (+40 mV), respectively. The anoxia-induced I-KATP was significantly smaller than I-KATP induced by metabolic inhibitors. During the pulses of 50 ms duration to +40 mV, the amplitude of I-KATP decayed and, after clamping back to -80 mV, I-KATP generated large tail currents. This suggests a notable change in the concentration gradient of K+ ions in the time range of tens of milliseconds. Conclusions: The results in isolated myocytes indicate that K-ATP channels open sufficiently rapidly after starting anoxia and generate sufficiently large conductance at maintained anoxia to explain both the time course and magnitude of the ischemic K+ accumulation if an appropriate counter-ion flux is available. (C) 1999 Elsevier Science B.V. All rights reserved.