ATP-sensitive K+ channels and cellular K+ loss in hypoxic and ischaemic mammalian ventricle

1. The contribution of ATP-sensitive K+ (K+ATP) channels to the rapid increase in cellular K+ efflux and shortening of action potential duration (APD) during early myocardial ischaemia and hypoxia remains controversial, because for the first 10 min of ischaemia or hypoxia in intact hearts cytosolic [ATP] remains about two orders of magnitude greater than the [ATP] causing half-maximal blockade of K+ATP channels in excised membrane patches. The purpose of this study was to investigate this apparent discrepancy. 2. During substrate-free hypoxia, total, diastolic and systolic unidirectional K+ efflux rates increased by 43, 26 and 103% respectively after 8.3 min in isolated arterially perfused rabbit interventricular septa loaded with 42K+. APD shortened by 39%. From the Goldman-Hodgkin-Katz equation, the relative increases in systolic and diastolic K+ efflux rates were consistent with activation of a voltage-independent K+ conductance. 3. During total global ischaemia, [K+]o measured with intramyocardial valinomycin K(+)-sensitive electrodes increased at a maximal rate of 0.68 mM min-1, which could be explained by a less than 26% increase in unidirectional K+ efflux rate (assuming no change in K+ influx), less than the increase during hypoxia. APD shortened by 23% over 10 min. 4. During hypoxia and ischaemia, cytosolic [ATP] decreased by about one-third from 6.8 +/- 0.5 to 4.3 +/- 0.3 and 4.6 +/- 0.4 mM respectively, and free cytosolic [ADP] increased from 15 to 95 and approximately 63 microM respectively. 5. To estimate the percentage of activation of current through K+ATP channels (IK,ATP) necessary to double the systolic K+ efflux rate (comparable to the increase during hypoxia), K+ efflux during a single simulated action potential was measured by blocking non-K+ currents under control conditions and after IK,ATP was fully activated by metabolic inhibitors. Activation of 0.41 +/- 0.07% of maximal IK,ATP was sufficient to double the systolic K+ efflux rate. The equivalent amount of constant hyperpolarizing current also shortened the APD in the isolated myocytes by 41 +/- 5%, compared to the 39% APD shortening observed during hypoxia in the intact heart. 6. The degree of activation of IK,ATP expected to occur during hypoxia and ischaemia was estimated by characterizing the ATP sensitivity of K+ATP channels in the presence of 2 mM-free Mgi2+ and 0, 10, 100 and 300 microM-ADPi in inside-out membrane patches excised from guinea-pig ventricular myocytes.