Terrar D A, White E
University Department of Pharmacology, Oxford, U.K.
Proc R Soc Lond B Biol Sci. 1989 Nov 22;238(1291):171-88. doi: 10.1098/rspb.1989.0074.
Action potentials were recorded from single cells isolated from guinea-pig ventricular muscle. Contraction was measured with an optical technique. Tail currents thought to be activated by cytosolic calcium were recorded when action potentials were interrupted by application of a voltage-clamp. A family of tail currents was recorded by interrupting the action potential at various times after the upstroke. The envelope of tail current amplitudes was taken as an index of changes in cytosolic calcium. Consistent with this interpretation, tail currents were negligible following intracellular loading with the calcium chelator BAPTA to suppress calcium transients. The cytosolic calcium transient estimated from the envelope of tails reached a peak approximately 50 ms after the upstroke of the action potential, and fell close to diastolic levels before repolarization was complete; 10 mM caffeine delayed the time to peak contraction, and caused a prolongation of the cytosolic calcium transient estimated from the envelope of tail currents. Caffeine also induced the appearance of a distinct late plateau phase of the action potential. Intracellular BAPTA suppressed the late plateau, contraction and tail currents in cells exposed to caffeine. Exposure to caffeine increased the time constant for decay of tail currents (from approximately 25 to 70 ms). When action potentials were greatly abbreviated by interruption with a voltage-clamp, a progressive decline occurred in the subsequent three contractions and tail currents. There was a progressive reversal of these effects over four responses when the full action potential duration was restored. None of these effects was observed in cells exposed to caffeine. Calcium-activated tail currents appear to be a useful qualitative index of changes in cytosolic calcium. The observations are consistent with the suggestion that cytosolic calcium is reduced during the plateau by a combination of calcium extrusion through Na-Ca exchange and calcium uptake into caffeine-sensitive stores. It also appears that reduction of stores loading during abbreviated action potentials reduces subsequent contraction in cells not exposed to caffeine.
从豚鼠心室肌分离的单个细胞记录动作电位。用光学技术测量收缩。当通过施加电压钳中断动作电位时，记录被认为由胞质钙激活的尾电流。通过在动作电位上升后不同时间中断动作电位来记录一系列尾电流。尾电流幅度的包络被用作胞质钙变化的指标。与这种解释一致，在用钙螯合剂BAPTA进行细胞内加载以抑制钙瞬变后，尾电流可忽略不计。从尾电流包络估计的胞质钙瞬变在动作电位上升后约50毫秒达到峰值，并在复极化完成前降至接近舒张水平；10 mM咖啡因延迟了收缩峰值时间，并导致从尾电流包络估计的胞质钙瞬变延长。咖啡因还诱导了动作电位明显的晚期平台期的出现。细胞内BAPTA抑制了暴露于咖啡因的细胞中的晚期平台期、收缩和尾电流。暴露于咖啡因会增加尾电流衰减的时间常数（从约25毫秒增加到70毫秒）。当通过电压钳中断使动作电位大大缩短时，随后的三次收缩和尾电流会逐渐下降。当恢复完整的动作电位持续时间时，在四个反应中这些效应会逐渐逆转。在暴露于咖啡因的细胞中未观察到这些效应。钙激活的尾电流似乎是胞质钙变化的有用定性指标。这些观察结果与以下观点一致，即平台期胞质钙通过钠钙交换的钙外排和钙摄取到咖啡因敏感储存库的组合而降低。还似乎在缩短的动作电位期间储存库加载的减少会降低未暴露于咖啡因的细胞中的后续收缩。
