External divalent cations are known to play an important role in the function of voltage-gated ion channels. The purpose of this study was to examine the sensitivity of the voltage-gated K+ currents of human atrial myocytes to external Ca2+ ions. Myocytes were isolated by collagenase digestion of atrial appendages taken from patients undergoing coronary artery-bypass surgery. Currents were recorded from single isolated myocytes at 37 degrees C using the whole cell patch-clamp technique. With 0.5 mM external Ca2+, voltage pulses positive to -20 mV (holding potential = -60 mV) activated outward currents which very rapidly reached a peak (I-peak) and subsequently inactivated (tau = 7.5 +/- 0.7 msec at +60 mV) to a sustained level, demonstrating the contribution of both rapidly inactivating transient (I-tol) and non-inactivating sustained (I-so) outward currents. The I-tol component of I-peak, but not I-so, showed voltage-dependent inactivation using 100 msec prepulses (V-1/2 = -35.2 +/- 0.5 mV). The K+ channel blocker, 4-aminopyridine (4-AP, 2 mM), inhibited I-tol by -76% and reduced I-so by similar to 33%. Removal of external Ca2+ had several effects: (i) I-peak was reduced in a manner consistent with an similar to 13 mV shift to negative voltages in the voltage-dependent inactivation of I-tol. (ii) I-so was increased over the entire voltage range and this was associated with an increase in a noninactivating 4-AP-sensitive current. (iii) In 79% cells (11/14), a slowly inactivating component was revealed such that the time-dependent inactivation was described by a double exponential time course (tau(1) = 7.0 +/- 0.7, tau(2) = 90 +/- 21 msec at +60 mV) with no effect on the fast time constant. Removal of external Ca2+ was associated with an additional component to the voltage-dependent inactivation of I-peak and I-so, (V-1/2 = -20.5 +/-1.5 mV). The slowly inactivating component was seen only in the absence of external Ca2+ ions and was insensitive to 4-AP (2 mM). Experiments with Cs+-rich pipette solutions suggested that the Ca2+-sensitive currents were carried predominantly by K+ ions. External Ca2+ ions are important to voltage-gated K+ channel function in human atrial myocytes and removal of external Ca2+ ions affects I-tol and 4-AP-sensitive indistinct ways.