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A rise in [K+]o, by depolarizing the resting membrane potential and partially inactivating the inward Na+ current (INa), is believed to play a critical role in slowing conduction during myocardial ischemia. In multicellular ventricular preparations, elevation of [K+]o has been suggested to decrease Vmax to a greater extent than expected from membrane depolarization alone. The mechanism of this voltage-independent effect of [K+]o is currently unknown, and its significance in single cardiac cells has not been determined. We have examined the voltage-independent effects of elevated [K+]o on INa and the action potential upstroke in isolated rabbit atrial and ventricular myocytes under voltage- and current-clamp conditions. Superfusate [K+] was varied from 5 mmol/L to 14 or 24 mmol/L, whereas [Na+] was maintained at 150 mmol/L. In cultured atrial cells and excised outside-out patches from freshly isolated atrial and ventricular cells, the amplitude and kinetics of INa were unchanged by elevation of [K+]o. In atrial cells, action potentials elicited from a holding potential of -70 mV had a similar Vmax (114.9 +/- 5.7 versus 112.2 +/- 4.8 V/s, mean +/- SEM, n = 6) and action potential amplitude (115.0 +/- 2.4 versus 113.4 +/- 3.9 mV) in 5 and 24 mmol/L [K+]o. In contrast, in ventricular cells at a holding potential of -70 mV, increasing [K+]o fro 5 to 14 mmol/L decreased Vmax from 161.8 +/- 18.0 to 55.3 +/- 5.0 V/s (n = 7, P < .001) and action potential amplitude from 128.1 +/- 1.3 to 86.6 +/- 5.4 mV (P < .001). This voltage-independent decrease in Vmax and action potential amplitude induced by elevated [K+]o was abolished in the presence of 1 mmol/L Ba2+, suggesting that it is attributable to an increased background K+ conductance. We conclude that elevation of [K+]o to levels expected during ischemia causes a marked voltage-independent depression of Vmax in ventricular cells, which may, in turn, contribute to the slowing of myocardial conduction characteristic of early ischemia.