External divalent cations are known to play an important role in the functi
on 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 t
o external Ca2+ ions. Myocytes were isolated by collagenase digestion of at
rial appendages taken from patients undergoing coronary artery-bypass surge
ry. Currents were recorded from single isolated myocytes at 37 degrees C us
ing the whole cell patch-clamp technique. With 0.5 mM external Ca2+, voltag
e pulses positive to -20 mV (holding potential = -60 mV) activated outward
currents which very rapidly reached a peak (I-peak) and subsequently inacti
vated (tau = 7.5 +/- 0.7 msec at +60 mV) to a sustained level, demonstratin
g the contribution of both rapidly inactivating transient (I-tol) and non-i
nactivating sustained (I-so) outward currents. The I-tol component of I-pea
k, but not I-so, showed voltage-dependent inactivation using 100 msec prepu
lses (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%. Rem
oval of external Ca2+ had several effects: (i) I-peak was reduced in a mann
er consistent with an similar to 13 mV shift to negative voltages in the vo
ltage-dependent inactivation of I-tol. (ii) I-so was increased over the ent
ire voltage range and this was associated with an increase in a noninactiva
ting 4-AP-sensitive current. (iii) In 79% cells (11/14), a slowly inactivat
ing component was revealed such that the time-dependent inactivation was de
scribed 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. Remova
l of external Ca2+ was associated with an additional component to the volta
ge-dependent inactivation of I-peak and I-so, (V-1/2 = -20.5 +/-1.5 mV). Th
e slowly inactivating component was seen only in the absence of external Ca
2+ ions and was insensitive to 4-AP (2 mM). Experiments with Cs+-rich pipet
te solutions suggested that the Ca2+-sensitive currents were carried predom
inantly by K+ ions. External Ca2+ ions are important to voltage-gated K+ ch
annel function in human atrial myocytes and removal of external Ca2+ ions a
ffects I-tol and 4-AP-sensitive indistinct ways. (C) 2000 Academic Press.