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

Abstract

The aim was to improve the measurement of both the time course and amplitude of anoxia-induced KATP-channel current (IKATP) in isolated heart cells to specify the role of these channels in the time course of K+ accumulation in the ischemic myocardium. Ionic currents in isolated ventricular heart cells of the mouse were measured with a patch clamp technique under normoxic conditions (atmospheric pO2), during wash-out of oxygen, and under anoxic conditions (pO2 < 0.2 mmHg). During the measurement, the actual pO2 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 pO2 and can be evaluated best at pO2 values near 0 mmHg. Out of 28 cells, 23 cells started to develop IKATP at pO2 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 IKATP between 0.4 and 1.8 mmHg. With respect to the time course, 18 out of 27 cells started to develop IKATP within the first minute after pO2 had decreased to values below 0.2 mmHg. The amplitude of IKATP induced by anoxia and various metabolic inhibitors was large, 29 +/- 12 and 48 +/- 21 nA (+40 mV), respectively. The anoxia-induced IKATP was significantly smaller than IKATP induced by metabolic inhibitors. During the pulses of 50 ms duration to +40 mV, the amplitude of IKATP decayed and, after clamping back to -80 mV, IKATP generated large tail currents. This suggests a notable change in the concentration gradient of K+ ions in the time range of tens of milliseconds. The results in isolated myocytes indicate that KATP 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.