Intracellular Calcium Homeostasis in the Heart

胞质Ca^<2+>浓度的调节是维持细胞功能的重要过程。在心肌细胞中，Ca^<2+>的内流主要通过电压门控Ca^<2+>通道进行，外流主要通过Na^+-Ca^<2+>交换器进行。即动作电位的去极化打开l型Ca^<2+>通道，触发肌肉收缩。在舒张期，Na^+-Ca^<2+>交换体利用Na^+的电化学驱动力泵出Ca^<2+>。在各种病理条件下，Ca^<2+>的泵出受到干扰，使细胞超载Ca^<2+>。为了研究Na^+-Ca^<2+>交换器的功能，我们测量了Na^+-Ca^<2+>交换器产生的膜电流（Na^+-Ca^<2+>交换电流），同时测量了细胞内游离Ca^<2+>。用胶原酶处理豚鼠肝细胞，获得单心室肌细胞，用全细胞膜片钳技术测定其膜电流。Ca^<2+>-指示剂，Indo-1通过全细胞膜片钳电极加载到细胞内。结果发现，膜的去极化使胞外Na^+-Ca^<2+>交换电流的振幅增大，同时胞内Ca^<2+>浓度升高。Na^+-Ca^<2+>交换电流的积分与细胞内Ca^<2+>浓度的增加有良好的对应关系。在单独的一系列实验中，使用光敏化合物Nitr-5可瞬间提高胞质Ca^<2+>浓度。这种笼状Ca^<2+>的闪光光解激活了向内Na^+-Ca^<2+>的单指数时间交换电流。这一发现与Na^+-Ca^<2+>交换模型一致，Na^+输运过程中存在电致步骤。在常规交换器模型的基础上，我们得出在26‰/ C -80 mv下，换手率至少为125 s^<-1>，场地密度大于2000个/ mu m^2。少

The regulation of the cytosolic Ca^<2+> concentration is a essential process to maintain the cellular function in every type of cells. In the cardiac myocytes the influx of Ca^<2+> is conducted mainly through the Voltage-gated Ca^<2+> channel, and the efflux via the Na^+-Ca^<2+> exchanger. Namely, the depolarization of the action potential opens the L-type Ca^<2+> channels, which triggers the contraction of the muscle. During the diastole, the Na^+-Ca^<2+> exchange pump out the Ca^<2+> using the electrochemical driving force for Na^+. Under various types of pathological conditions, pumping out of Ca^<2+> is disturbed, so that the cell become overloaded with Ca^<2+>. To investigate the function of the Na^+-Ca^<2+> exchanger, we measured the membrane current, which is produced by the Na^+-Ca^<2+> exchanger (the Na^+-Ca^<2+> exchange current), simultaneously with the measurement of the intracellular free Ca^<2+>. The single ventricular myocytes were obtained by treating the guinea-pig hea … More rt with collagenase, and its membrane currents were measured using the whole cell patch clamp technique. The Ca^<2+>-indicator, Indo-1 was loaded within the cell through the whole cell patch clamp electrode. It was found that the depolarization of the membrane increased the amplitude of the outward Na^+-Ca^<2+> exchange current, and simultaneously the intracellular Ca^<2+> concentration increased. The integration of the Na^+-Ca^<2+> exchange current well corresponded to the increase of the intracellular Ca^<2+> concentration. In a separate series of experiments, the cytosolic Ca^<2+> concentration was raised instantaneously using the photolabile compound, Nitr-5. The flash photolysis of this caged Ca^<2+> activated the inward Na^+-Ca^<2+> exchange current in a single exponential time course. This finding is consistent with an Na^+-Ca^<2+> exchange model having the electrogenic step in the Na^+ transporting step. Based on the conventional exchanger model, we conclude that the turnover rate is at least 125 s^<-1> at 26 ﾟC -80 mv, and the site density is more than 2000/ mu m^2. Less