Sharma Vinod, Tung Leslie
Department of Biomedical Engineering, Johns Hopkins University, 720 Rutland Avenue, Baltimore, MD 21205, USA.
Pflugers Arch. 2004 Dec;449(3):248-56. doi: 10.1007/s00424-004-1335-9.
An exhaustive characterization of how an isolated cardiac cell responds to applied electric fields could serve as an important groundwork for understanding responses of more complex higher order systems. Field stimulation of single cardiac cells during the early plateau of the action potential results in a nonuniform change in transmembrane potential (Vm) across the cell length that is more heavily weighted in the negative direction. These negatively shifted Vm responses are not replicated theoretically using present day membrane models. The goal of this study was to explore the membrane currents involved in the field responses during the plateau by selectively blocking various ion channels. Enzymatically isolated single guinea pig cells were stimulated with uniform field S1-S2 pulses, and the transmembrane potential responses were optically recorded from several sites along the cell length to assess the drug effect. We used nine different pharmacological agents to manipulate the conductance of major cardiac ion channels of which only barium (Ba2+) altered the transmembrane potential responses. At 50 microM Ba2+, which specifically blocks inwardly rectifying current I(K1), the negative shift in Vm responses was accentuated. At 1 mM Ba2+ , which blocks both I(K1) and sustained plateau current I(Kp), the negative shift diminished. However, 1 mM Ba2+ also depolarized the cells, and depressed or completely eliminated the action potential. Based on these results we conclude that I(K1) contributes to field-induced responses during the plateau stimulation by passing a net inward current, which when blocked accentuates the negative shift in the Vm responses. A conclusive role of I(Kp) could not be demonstrated because of confounding changes in membrane potential. However, from our results it remains as the most viable candidate for the elusive current that contributes a net outward current to produce negatively weighted Vm responses during plateau stimulation and warrants further investigation.
详尽描述单个心脏细胞对施加电场的反应，可为理解更复杂的高阶系统的反应奠定重要基础。在动作电位的早期平台期对单个心脏细胞进行电场刺激，会导致跨细胞长度的跨膜电位（Vm）发生不均匀变化，且在负向方向上的权重更大。目前的膜模型在理论上无法复制这些负向偏移的Vm反应。本研究的目的是通过选择性阻断各种离子通道，探索平台期电场反应中涉及的膜电流。用均匀电场S1 - S2脉冲刺激酶解分离的单个豚鼠细胞，并沿细胞长度的几个位点光学记录跨膜电位反应，以评估药物作用。我们使用了九种不同的药理试剂来操纵主要心脏离子通道的电导，其中只有钡（Ba2+）改变了跨膜电位反应。在50 microM Ba2+时，它特异性阻断内向整流电流I(K1)，Vm反应的负向偏移加剧。在1 mM Ba2+时，它同时阻断I(K1)和持续平台电流I(Kp)，负向偏移减小。然而，1 mM Ba2+也使细胞去极化，并抑制或完全消除动作电位。基于这些结果，我们得出结论，I(K1)通过传递净内向电流，在平台期刺激期间对电场诱导的反应有贡献，当该电流被阻断时，会加剧Vm反应的负向偏移。由于膜电位的混杂变化，无法证明I(Kp)的决定性作用。然而，从我们的结果来看，它仍然是最有可能的候选者，即那种难以捉摸的电流，在平台期刺激期间贡献净外向电流以产生负权重的Vm反应，值得进一步研究。
