Ultrarapid Delayed Rectifier K+ Current in H9c2 Rat Ventricular Cell Line: Biophysical Property and Molecular Identity
Ultrarapid delayed rectifier K<sup>+</sup> currents (I<sub>Kur</sub>s) contribute importantly to cardiac repolarization. However, understanding of I<sub>Kur</sub>s has been hampered by the difficulty of dissecting them from overlapping currents in primary cells. We found with whole-cell patch-clamp recordings that H9c2 cells, a rat ventricular cell line, under 50% confluence (single myoblasts) express exclusively I<sub>Kur</sub>-like current which activates rapidly upon depolarization and partially inactivates during 150-ms pulses. The H9c2 I<sub>Kur</sub> activates within the same voltage range as native cardiac I<sub>Kur</sub>s, with a half-activation voltage of -13 mV. The H9c2 I<sub>Kur</sub> can be completely blocked by tetraethylammonium and 4-aminopyridine. Reversal potential (-79 mV) and envelope-of tail analyses indicate that the H9c2 I<sub>Kur</sub> is carried by a single population of K<sup>+</sup> channels. H9c2 I<sub>Kur</sub> was increased by β-adrenoceptor-PKA and decreased by α<sub>1</sub>-adrenoceptor-PKC activation by isoproterenol and phenylephrine, respectively. Immunocytochemistry was performed with antibodies against 11 different K<sup>+</sup> channels. Positive immuno-stain-ing of the cytoplasmic membrane of H9c2 cells was seen only with Kv3.1b antibody. Antisense oligodeoxynucleotides directed against Kv3.1b subunit sequence significantly inhibited the H9c2 I<sub>Kur</sub>. We conclude that the H9c2 cells at the myoblast stage express mainly I<sub>Kur</sub> and Kv3.1b may be a molecular component of the H9c2 I<sub>Kur</sub>, and H9c2 cells provide a suitable system for studying I<sub>Kur</sub>.