A suction pipette whole-cell voltage-clamp technique was used to record membrane currents and potentials of isolated ventricular myocytes from rabbit hearts. 2. Transient outward current (Ito) was activated by voltage steps positive to -20 mV, increasing in amplitude with further depolarization to reach a maximum around +70 mV. The current attained its peak within 10 ms and then it inactivated for 100-200 ms. 3. A large portion of Ito still remained after the calcium current (ICa) was blocked when depolarizing pulses were applied at a frequency of 0.1 Hz or less. Therefore, this current component is referred to as calcium-insensitive Ito or It. 4. It showed voltage- and time-dependent inactivation similar to that observed in Purkinje fibres and other cardiac preparations. 5. The reversal potential of It depended on external K+ concentration, [K+]o, with a slope of 32 mV per 10-fold change in the presence of a normal [Na+]o (143 mM), while the slope was 48 mV per 10-fold change in low [Na+]o (1.0 mM). 6. It was completely inhibited by 2-4 mM-4-aminopyridine. Ito in the presence of ICa was also partially blocked by 4-aminopyridine and the remainder was abolished by 5 mM-caffeine. 7. The calcium-insensitive and caffeine-sensitive Ito differed in their decay rates as well as in their recovery time courses. The former was predominantly available at a slow pulsing rate, while the latter increased its amplitude with high-frequency depolarization. 8. The caffeine-sensitive Ito was inhibited by a blockade of ICa, by replacing Ca2+ with Sr2+, by external application of ryanodine and by internal application of EGTA. This indicates that the current is calcium-sensitive and is dependent on increased myoplasmic Ca2+ through Ca2+ influx via the sarcolemma and Ca2+ release from the sarcoplasmic reticulum. The current is therefore designated as IK, Ca. 9. The physiological functions of IK, Ca and It are indicated by their contribution to ventricular repolarization at fast and slow heart rates, respectively.
摘要
采用吸力移液器全细胞电压钳技术记录兔心脏分离心室肌细胞的膜电流和电位。2. 短暂外向电流（Ito）由正向至 -20 mV 的电压阶跃激活，随着进一步去极化幅度增加，在 +70 mV 左右达到最大值。电流在 10 ms 内达到峰值，然后在 100 - 200 ms 内失活。3. 当以 0.1 Hz 或更低频率施加去极化脉冲时，钙电流（ICa）被阻断后，很大一部分 Ito 仍然存在。因此，该电流成分被称为钙不敏感 Ito 或 It。4. It 表现出与浦肯野纤维和其他心脏标本中观察到的类似的电压和时间依赖性失活。5. It 的反转电位取决于细胞外钾离子浓度[K+]o，在正常[Na+]o（143 mM）存在的情况下，每 10 倍变化的斜率为 32 mV，而在低[Na+]o（1.0 mM）时，斜率为 48 mV 每 10 倍变化。6. It 被 2 - 4 mM 的 4 - 氨基吡啶完全抑制。在 ICa 存在的情况下，Ito 也被 4 - 氨基吡啶部分阻断，其余部分被 5 mM 的咖啡因消除。7. 钙不敏感和咖啡因敏感的 Ito 在衰减速率和恢复时间进程方面有所不同。前者在缓慢脉冲频率下主要存在，而后者在高频去极化时幅度增加。8. 咖啡因敏感的 Ito 被 ICa 的阻断、用 Sr2+替代 Ca2+、细胞外应用 Ryanodine 和细胞内应用 EGTA 所抑制。这表明该电流对钙敏感，并且依赖于通过肌膜钙内流和肌浆网钙释放导致的肌浆钙增加。因此，该电流被命名为 IK,Ca。9. IK,Ca 和 It 的生理功能分别通过它们对快速和慢速心率下心室复极化的贡献来体现。
