Wasserstrom J A, Vites A M
Department of Medicine, Division of Cardiology, Feinberg Cardiovascular Research Institute, Northwestern University Medical School, Chicago, Illinois, 60611, USA.
Am J Physiol. 1999 Aug;277(2):H488-98. doi: 10.1152/ajpheart.1999.277.2.H488.
The effects of Cd(2+) (20 microM) and different bath temperatures were used to study the contributions of two separate triggering mechanisms, L-type Ca(2+) current (I(Ca)) and reverse mode Na(+)/Ca(2+) exchange, to excitation-contraction (E-C) coupling in cat ventricular myocytes. Ionic currents and cell shortening were studied with patch pipettes filled with K(+)-containing internal solution and discontinuous ("switch") voltage clamp. Superfusion with Cd(2+) blocked cell shortening that closely mirrored the block of I(Ca); the voltage dependence of Cd(2+)-induced reduction in contraction was bell-shaped, displaying minima at test potentials below -10 mV and above +50 mV and a maximum at about +20 mV. Cd(2+)-insensitive cell shortening was blocked by ryanodine (10 microM) and Ni(2+) (4-5 mM). When an action potential was used as the command waveform for the voltage clamp (action potential clamp), Cd(2+) reduced contraction to approximately 60 +/- 7% of control cell shortening (n = 7). The remaining contraction was blocked by ryanodine and Ni(2+). Superfusion with nifedipine (10 microM) caused nearly identical effects to Cd(2+). The voltage dependence of contraction was sigmoidal at temperatures above 34 degrees C but bell-shaped below 30 degrees C. When Cd(2+) was added to superfusate, contraction was abolished at 25 degrees C (to 6 +/- 3% of control) but reduced only modestly at 34 degrees C (to 65 +/- 13% of control, test potential +10 mV, n = 4, P < 0.01). These results indicate that 1) there is a component of contraction that is sensitive to I(Ca) antagonists, and the block is equivalent with either organic or inorganic antagonists; 2) the contribution of Na(+)/Ca(2+) exchange to triggering of contraction under our experimental conditions is fairly linear throughout the entire voltage range tested; 3) the contribution of I(Ca) is superimposed on this background component contributed by the Na(+)/Ca(2+) exchanger; and 4) triggering via the exchanger is temperature-dependent, providing a major contribution at physiological temperatures but failing at temperatures below 30 degrees C in a nearly all-or-none fashion.
采用镉离子（Cd²⁺，20微摩尔）和不同的浴温，研究两种独立的触发机制，即L型钙电流（I(Ca)）和反向模式钠/钙交换，对猫心室肌细胞兴奋-收缩（E-C）偶联的作用。使用充满含钾内液的膜片钳吸管和间断（“切换”）电压钳技术研究离子电流和细胞缩短情况。用Cd²⁺灌流可阻断细胞缩短，这与I(Ca)的阻断密切相关；Cd²⁺诱导的收缩减少的电压依赖性呈钟形，在测试电位低于-10毫伏和高于+50毫伏时显示最小值，在约+20毫伏时显示最大值。对Cd²⁺不敏感的细胞缩短可被ryanodine（10微摩尔）和镍离子（Ni²⁺，4 - 5毫摩尔）阻断。当动作电位用作电压钳的指令波形（动作电位钳）时，Cd²⁺将收缩减少至对照细胞缩短的约60±7%（n = 7）。剩余的收缩可被ryanodine和Ni²⁺阻断。用硝苯地平（10微摩尔）灌流产生的效应与Cd²⁺几乎相同。在温度高于34℃时收缩的电压依赖性呈S形，但在低于30℃时呈钟形。当向灌流液中加入Cd²⁺时，在25℃时收缩被消除（降至对照的6±3%），但在34℃时仅适度降低（降至对照的65±13%，测试电位+10毫伏，n = 4，P < 0.01）。这些结果表明：1）存在对I(Ca)拮抗剂敏感的收缩成分，且有机或无机拮抗剂的阻断效果相当；2）在我们的实验条件下，钠/钙交换对收缩触发的贡献在整个测试电压范围内相当线性；3）I(Ca)的贡献叠加在由钠/钙交换体贡献的这一背景成分上；4）通过交换体的触发是温度依赖性的，在生理温度下起主要作用，但在低于30℃的温度下几乎以全或无的方式失效。
