Differential regulation of voltage-activated potassium currents in cultured human atrial myocytes

To examine whether the two components of the voltage-activated outward K+ current, an initially rapidly inactivating component (I-to,I-1) and a slowly inactivating sustained component (I-sus), in human atrial myocytes are distinct currents differentially regulated, we studied their behavior during serum-induced growth of cultured myocytes. Currents were recorded in whole cell patch-clamped myocytes. After 1 wk of culture (day 8), membrane capacitance was twice the value in freshly dissociated myocytes (178.7 +/- 23 vs. 83.1 +/- 5.5 pF; P < 0.001). I-to,I-1 density did not differ from that in freshly dissociated myocytes (at +40 mV: 4.38 +/- 0.8 vs. 3.71 +/- 0.6 pA/pF), whereas that of I-sus was markedly increased (at +40 mV: 9.76 +/- 2 vs. 2.21 +/- 0.29 pA/pF; P < 0.001). After inactivation of I-to,I-1 by a prepulse, sustained depolarization elicited in cultured myocytes an I-sus with a density of 10.22 +/- 1.18 pA/pF and an apparent tail current reversal potential of -73.5 +/- 3.2 mV, indicating high K+ selectivity. I-sus was highly sensitive to 4-aminopyridine (55.4 +/- 4.4% inhibition in 50 mu M) and to D-600 (with a concentration inhibiting 50% of maximal response of 34.2 x 10(-6) M). Addition of 5-10 nM staurosporine at day 3 prevented cell growth and reduced I-to,I-1 density but not the increase in I-sus density, which was inhibited by 10 mu M staurosporine. Our results indicate that I-to,I-1 and I-sus are regulated independently during in vitro myocyte growth in human atrial myocytes and that the increase in I-sus density is not mediated by a protein kinase C-dependent pathway.