Abstract
The properties of the outward K+ currents of H9c2, a clonal cell line derived from rat heart, at both the undifferentiated myoblast and differentiated myotube stages were characterized using the patch clamp technique under whole-cell clamp configuration. Subjecting myoblasts to depolarization steps activated a voltage-gated outward K+ current (IKV), which showed a rapid rise, a slow decay and steady-state inactivation by depolarization of the holding potential. The myoblastic IKV was suppressed by exogenous tetraethylammonium with a 50% inhibition concentration (IC50) of 1.9mM. Subjecting myotubes to depolarization steps generated other K+ currents with much slower activation rates followed by slow tail currents on repolarization. These slow currents were completely soppressed by superfusion with Ca2+-free Tyrode's solution and by intracellular application of 10mM EGTA, indicating that these were Ca2+-activated K+ currents (IKCa). High concentration of charybdotoxin (CTX, 100nM) and apamin (100nM) depressed the total IKCa by 57.0 and 43.0%, respectively, white simultaneous application of both toxins suppressed it completely. The expression of IKV and two distinct IKCa in the myotubes indicated that the differentiated H9c2 cell line exhibited outward-going K+ channel properties that were more similarto skeletal than cardiac muscle, suggesting that the gene expression of IKCa channels in these cells is regulated by a differentiation program.
