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 < 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 almost-equal-to 63-mu-M respectively. 5. To estimate the percentage of activation of current through K+(ATP) channels (I(K, 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 I(K, ATP) was fully activated by metabolic inhibitors. Activation of 0.41 +/- 0.07% of maximal I(K, 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 I(K, 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 Mg(i)2+ and 0, 10, 100 and 300-mu-M-ADPi in inside-out membrane patches excised from guinea-pig ventricular myocytes. The K(d) ([ATP]i causing half-maximal block of K+(ATP) channels) and the Hill coefficient (the steepness of the dose-response curve to ATP) were calculated for each [ADP]i From these results it was predicted that I(K, ATP) would rapidly be activated to > 0.4% of maximal at the typical values of [ATP]i and free [ADP]i estimated during early ischaemia and hypoxia. 7. We conclude that due to the high density of K(ATP)+ channels in ventricular myocytes, activation of < 0.5% is sufficient to account for the observed increase in unidirectional K+ efflux and APD shortening during early hypoxia, as well as the magnitude of extracellular K+ accumulation during myocardial ischaemia. Furthermore, the shift in the sensitivity of K+(ATP) channels to [ATP]i caused by increases in free [ADP]i during hypoxia and ischaemia is adequate to account for this degree of activation of I(K, ATP) before a major decline in [ATP]i has occurred.
