Estimation of outward currents in isolated human atrial myocytes using inactivation time course analysis

The aim was to investigate outward currents in single, isolated, human, atrial myocytes and to determine the relative contribution of individual current components to the total outward current. Currents were recorded using the whole-cell patch-clamp technique at 36–37°C. Individual outward current components were estimated from recordings of total outward current using a mathematical procedure based on the inactivation time course of the respective currents. This method allows estimation of outward currents without the use of drugs or conditioning voltage-clamp protocols to suppress individual current components. A rapidly activating and partially inactivating total outward current was recorded when myocytes were voltage clamped at potentials positive to –20 mV (peak current density 24.0±0.97 pA/pF at +40 mV; n=107 cells, 33 patients). This total outward current comprised three overlapping currents: a rapidly inactivating, transient, outward current (Ito1) a slowly and partially inactivating current (ultrarapid delayed rectifier, IKur) and a third current component which most probably reflects a non selective cation current (not characterized). The average current densities at +40 mV were 8.92±0.44 pA/pF for Ito1 and 15.1±0.72 pA/pF for IKur (n=107 cells). Recovery from inactivation was bi-exponential for both currents and was faster for Ito1. A slowly activating delayed rectifier current (IK) was not found. The current densities of peak Ito1 and IKur varied strongly between individual myocytes, even in those from the same patient. The ratio IKur/Ito1 was 0.5–6.9 with a mean of 1.98±0.11 (n=107 cells), suggesting that IKur is the main repolarizing current. The amplitudes of the total outward current, Ito1 and IKur, and the ratio of the latter two were independent of patient age (16–87 years).