Fomina A F, Nowycky M C
Department of Neurobiology and Anatomy, Medical College of Pennsylvania Hahnemann University, Philadelphia, Pennsylvania 19129, USA.
J Neurosci. 1999 May 15;19(10):3711-22. doi: 10.1523/JNEUROSCI.19-10-03711.1999.
Exocytosis in excitable cells is strongly coupled to Ca2+ entry through voltage-gated channels but can be evoked by activation of membrane receptors that release Ca2+ from inositol 1,4, 5-trisphosphate-sensitive internal stores. In many cell types, depletion of Ca2+ stores activates Ca2+ influx across the plasma membrane, a process known as capacitative or store-operated Ca2+ entry. This influx is mediated by a number of voltage-independent, Ca2+-selective currents. In addition to replenishing Ca2+ stores, these currents are hypothesized to play an important role in agonist-evoked secretion in nonexcitable cells, although this has not been confirmed experimentally. The existence and physiological function of such currents in excitable cells is not known. Using the capacitance detection technique to monitor exocytosis, we provide direct experimental evidence that a similar mechanism exists in bovine adrenal chromaffin cells. Depletion of intracellular Ca2+ stores with thapsigargin, a SERCA pump inhibitor, or with BAPTA, an exogenous Ca2+ chelator, activates a small-amplitude, voltage-independent current that is carried by Ca2+ and Na+ ions. Ca2+ entry through this pathway is sufficient to stimulate exocytosis at negative membrane potentials. In addition, depolarization-evoked exocytosis is markedly facilitated on activation of the current. These data suggest that excitable cells possess a store-operated Ca2+ influx mechanism that may both directly trigger exocytosis and modulate excitation-secretion coupling.
可兴奋细胞中的胞吐作用与通过电压门控通道的Ca2+内流紧密偶联,但也可由激活从肌醇1,4,5-三磷酸敏感的内部储存库释放Ca2+的膜受体引发。在许多细胞类型中,Ca2+储存库的耗竭会激活跨质膜的Ca2+内流,这一过程称为容量性或储存库操纵性Ca2+内流。这种内流由多种电压非依赖性、Ca2+选择性电流介导。除了补充Ca2+储存库外,尽管尚未通过实验证实,但据推测这些电流在非可兴奋细胞的激动剂诱发分泌中起重要作用。在可兴奋细胞中这种电流的存在及其生理功能尚不清楚。我们使用电容检测技术监测胞吐作用,提供了直接的实验证据表明牛肾上腺嗜铬细胞中存在类似的机制。用毒胡萝卜素(一种SERCA泵抑制剂)或BAPTA(一种外源性Ca2+螯合剂)耗尽细胞内Ca2+储存库,会激活一种由Ca2+和Na+离子携带的小幅度、电压非依赖性电流。通过该途径的Ca2+内流足以在负膜电位下刺激胞吐作用。此外,在电流激活时,去极化诱发的胞吐作用会明显增强。这些数据表明可兴奋细胞具有一种储存库操纵性Ca2+内流机制,该机制可能既直接触发胞吐作用,又调节兴奋-分泌偶联。