Smith Robert M, Baibakov Boris, Lambert Nevin A, Vogel Steven S
Institute of Molecular Medicine and Genetics, Medical College of Georgia, Augusta, GA 30912-2630, USA.
Traffic. 2002 Jun;3(6):397-406. doi: 10.1034/j.1600-0854.2002.30603.x.
Cell function can be modulated by the insertion and removal of ion channels from the cell surface. The mechanism used to keep channels quiescent prior to delivery to the cell surface is not known. In eggs, cortical vesicle exocytosis inserts voltage-gated calcium channels into the cell surface. Calcium influx through these channels triggers compensatory endocytosis. Secretory vesicles contain high concentrations of calcium and hydrogen ions. We propose that lumenal hydrogen ions inhibit vesicular calcium channel gating prior to exocytosis, discharge of lumenal protons upon vesicle-plasma membrane fusion enables calcium channel gating. Consistent with this hypothesis we find that cortical vesicle lumens are acidic, and exocytosis releases lumenal hydrogen ions. Acidic extracellular pH reversibly blocks endocytosis, and the windows of opportunity for inhibition with a calcium-channel blocker or hydrogen ions are indistinguishable. Calcium ionophore treatment circumvents the low pH block, suggesting that calcium influx, or an upstream step, is obstructed. Inhibition of calcium influx by preventing membrane depolarization is unlikely, as elevation of the extracellular potassium concentration failed to overcome the pH block, and low extracellular pH was found to depolarize the membrane potential. We conclude that low pH inhibits endocytosis at a step between membrane depolarization and calcium influx.
细胞功能可通过离子通道在细胞表面的插入和移除来调节。在将通道递送至细胞表面之前用于使其保持静止的机制尚不清楚。在卵细胞中，皮质囊泡胞吐作用将电压门控钙通道插入细胞表面。通过这些通道的钙内流触发代偿性内吞作用。分泌性囊泡含有高浓度的钙和氢离子。我们提出，管腔内的氢离子在胞吐作用之前抑制囊泡钙通道的门控，囊泡与质膜融合时管腔内质子的释放使钙通道能够门控。与该假设一致，我们发现皮质囊泡腔呈酸性，并且胞吐作用会释放管腔内的氢离子。酸性细胞外pH可逆地阻断内吞作用，并且用钙通道阻滞剂或氢离子进行抑制的机会窗口难以区分。钙离子载体处理可规避低pH阻断，表明钙内流或上游步骤受到阻碍。通过防止膜去极化来抑制钙内流不太可能，因为细胞外钾浓度的升高未能克服pH阻断，并且发现低细胞外pH会使膜电位去极化。我们得出结论，低pH在膜去极化和钙内流之间的某个步骤抑制内吞作用。
