Shigematsu S, Kiyosue T, Sato T, Arita M
Oita Medical University, Japan.
Basic Res Cardiol. 1997 Jun;92(3):123-8. doi: 10.1007/BF00788629.
Rate-dependent alterations of action potential duration (APD) in rat ventricular myocytes were investigated. Action potentials of the isolated myocytes were recorded with patch electrodes containing EGTA (11 mM), and showed a marked rate-dependent prolongation in the APD (0.2-5 Hz). This prolongation was significantly inhibited in the presence of 4-aminopyridine (4-AP), a blocker of the transient outward K+ current (Ito). Thus, the rate-dependent decrease in Ito may underlie the change in APD. In contrast, the action potentials recorded from rat ventricular papillary muscles with conventional microelectrodes did not show rate-dependent alterations in the APD, i.e., the APD remained practically unaltered at the frequency range of 0.2-5 Hz. These results suggest that the rate-dependent prolongation of APD (due to rate-dependent blockade of Ito) becomes evident when the intracellular Ca2+ was chelated by the internal application of EGTA via patch pipette. We speculate that the rate-dependent prolongation of APD (via decreases in Ito) is masked in the ventricular papillary muscles, probably due to rate-dependent decreases in the inward current (e.g., electrogenic Na(+)-Ca2+ exchange current) that is regulated by the intracellular calcium.
研究了大鼠心室肌细胞动作电位时程（APD）的频率依赖性改变。用含有乙二醇双四乙酸（EGTA，11 mM）的膜片电极记录分离的心肌细胞动作电位，结果显示在0.2 - 5 Hz频率范围内，APD呈现显著的频率依赖性延长。4 - 氨基吡啶（4 - AP）是瞬时外向钾电流（Ito）的阻滞剂，在其存在下，这种延长受到显著抑制。因此，Ito的频率依赖性降低可能是APD变化的基础。相反，用传统微电极记录的大鼠心室乳头肌动作电位在0.2 - 5 Hz频率范围内未显示出APD的频率依赖性改变，即APD在该频率范围内基本保持不变。这些结果表明，当通过膜片移液管内向施加EGTA螯合细胞内Ca2+时，APD的频率依赖性延长（由于Ito的频率依赖性阻断）变得明显。我们推测，在心室乳头肌中，APD的频率依赖性延长（通过Ito的降低）可能被掩盖，这可能是由于受细胞内钙调节的内向电流（例如，生电性Na(+)-Ca2+交换电流）的频率依赖性降低所致。
