Effects of Components of Ischemia on the Kv4.3 Current Stably Expressed in Chinese Hamster Ovary Cells

We investigated the effects of three components of ischemia: external acidosis (pH=6.0), extracellular hyperkalemia ([K+]=20 mmol/l), and resting membrane depolarization to −60 mV, on Kv4.3 current stably expressed in Chinese Hamster Ovary cells. We used single electrode whole cell patch clamp techniques to study changes in the current elicited. External acidosis caused a positive shift in the steady state activation curve from −13.4±2.1 mV to −3.3±1.5 mV (n=8, P=0.004) and the steady state inactivation curve from −56.5±0.4 mV to −46.7±0.5 mV (n=14, P<0.0001). Acidosis also caused an acceleration of recovery from inactivation with the t1/2 decreasing from 306 ms (95% CI 287–327 ms) to 194 ms (95% CI 182–207 ms), (n=14, P<0.05). Hyperkalemia did not affect any of these parameters. Combined acidosis and hyperkalemia produced effects similar to those seen with acidosis. Changing the holding potential from −90 mV to −60 mV with test potentials of +5 and +85 mV decreased the peak currents by 34.1% and 32.4% respectively (n=14). However, in the presence of external acidosis the decrease in peak currents induced by changing the holding potential was less marked. In acidotic bath the peak current at −60 mV was reduced by only 13.6% at a test potential of +5 mV and 12.3% at a test potential of +85 mV (n=14). Taken together our data suggest that the membrane depolarization and changes in pH which occur under ischemic conditions would be accompanied by relative preservation of Kv4.3 currents and provide a molecular basis for the observation of preserved epicardial Ito and epicardial action potential duration (APD) shortening in ischemia.