Korn S J, Horn R
Neurosciences Department, Roche Institute of Molecular Biology, Nutley, New Jersey 07110.
J Gen Physiol. 1991 Nov;98(5):893-907. doi: 10.1085/jgp.98.5.893.
The effect of extracellular pH (pHo) on the duration of calcium-dependent chloride currents (ICl(Ca] was studied in voltage clamped AtT-20 pituitary cells. ICl(Ca) was activated by Ca2+ influx through plasma membrane Ca2+ channels, which were opened by step depolarization to voltages between -20 and +60 mV. Increasing pHo from 7.3 to 8.0 reversibly prolonged ICl(Ca) tail currents in perforated patch recordings from cells bathed in both Na(+)-containing and Na(+)-free solutions. This prolongation was prevented in standard whole cell recordings when the pipette solution contained 0.5 mM EGTA. The effects of raised pHo were not due to alteration of intracellular pH, since tail current prolongation still occurred when intracellular pH was buffered at 7.3 with 80 mM HEPES. The prolongation of ICl(Ca) at pHo 8 could not be accounted for by a direct action on Ca2+ channels, since tail currents were prolonged when pHo was changed rapidly during the tail current, after all Ca2+ channels were closed. The effects of increasing pHo on ICl(Ca) also could not be explained by a direct action on Cl- channels, since changing to pHo 8 did not prolong Cl- tail currents when intracellular Ca2+ concentration [( Ca2+]i) was fixed by EGTA in whole cell recordings. Raising pHo did, however, prolong depolarization-evoked [Ca2+]i transients, measured directly with the Ca2+ indicator dye, fura-2. Taken together, these data demonstrate the presence of a Na(+)-independent, pHo-sensitive mechanism for reduction of [Ca2+]i after influx through Ca2+ channels. This mechanism is associated with the plasma membrane, and is active on a time scale that is relevant to the duration of single action potentials in these cells. We suggest that this mechanism is the plasma membrane Ca2+ ATPase.
在电压钳制的AtT - 20垂体细胞中研究了细胞外pH值（pHo）对钙依赖性氯电流（ICl(Ca)）持续时间的影响。ICl(Ca)由通过质膜钙通道的Ca2+内流激活，这些通道通过逐步去极化至 - 20至 + 60 mV之间的电压而打开。在含Na(+)和无Na(+)溶液中孵育的细胞的穿孔膜片钳记录中，将pHo从7.3提高到8.0可使ICl(Ca)尾电流可逆地延长。当移液管溶液含有0.5 mM EGTA时，在标准全细胞记录中这种延长被阻止。升高pHo的作用并非由于细胞内pH值的改变，因为当用80 mM HEPES将细胞内pH值缓冲在7.3时，尾电流仍会延长。pHo为8时ICl(Ca)的延长不能用对Ca2+通道的直接作用来解释，因为在所有Ca2+通道关闭后，在尾电流期间快速改变pHo时尾电流会延长。升高pHo对ICl(Ca)的影响也不能用对Cl-通道的直接作用来解释，因为在全细胞记录中当细胞内Ca2+浓度[(Ca2+]i)由EGTA固定时，将pHo变为8不会延长Cl-尾电流。然而，提高pHo确实会延长用Ca2+指示剂染料fura - 2直接测量的去极化诱发的[(Ca2+]i)瞬变。综上所述，这些数据表明存在一种不依赖Na(+)、对pHo敏感的机制，用于在Ca2+通过通道内流后降低[(Ca2+]i)。这种机制与质膜相关，并且在与这些细胞中单个动作电位持续时间相关的时间尺度上起作用。我们认为这种机制是质膜钙ATP酶。
