A long lasting Ca2+ -activated outward current in guinea-pig atrial myocytes

Among other characteristics, the steady-state current-voltage relationship of patch-clamped single atrial myocytes from guinea-pig hearts is defined by an outward current hump in the potential region –15 to +40mV. This hump was reversibly suppressed by Co²⁺ (3 mM) or nitrendipine (5 M) and enhanced by Bay K 8644 (5 M). The maintained outward current component suppressed by Co²⁺ extended between –15.2±1.9 mV and +39.5 ±1.7 mV (mean±SEM of 14 cells) and has an amplitude of 95.7±9.4 pA at +10 mV. In isochronal I-V curves, the hump was already visible at 400 ms with essentially the same amplitude as at 1500 ms. The Co²⁺ -sensitive outward current underlying the hump was poorly time-dependent during 1.5 s voltage pulses but slowly relaxed upon repolarization. Tail currents reversed near the K⁺ equilibrium potential under our experimental conditions. The current hump of the steady-state I-V curve was also abolished by caffeine (10 mM) or ryanodine (3 M), both drugs that interfere with sarcoplasmic reticulum function. Apamin (1 M) or quinine (100 M) but not TEA (5–50 mM) markedly reduced its amplitude. However, at similar concentrations as required to inhibit the hump, both apamin and quinine appeared to be poorly specific for Ca²⁺ -activated K⁺ currents in heart cells since they also inhibited the L-Type Ca²⁺ current. It is concluded that a long lasting Ca²⁺ -activated outward current, probably mainly carried by K⁺ ions but not sensitive to TEA, exists in atrial myocytes which is responsible for the current hump of the background I-V curve.