Chase H S, Al-Awqati Q
J Gen Physiol. 1983 May;81(5):643-65. doi: 10.1085/jgp.81.5.643.
Regulation of the sodium permeability of the luminal membrane is the major mechanism by which the net rate of sodium transport across tight epithelia is varied. Previous evidence has suggested that the permeability of the luminal membrane might be regulated by changes in intracellular sodium or calcium activities. To test this directly, we isolated a fraction of the plasma membrane from the toad urinary bladder, which contains a fast, amiloride-sensitive sodium flux with characteristics similar to those of the native luminal membrane. Using a flow-quench apparatus to measure the initial rate of sodium efflux from these vesicles in the millisecond time range, we have demonstrated that the isotope exchange permeability of these vesicles is very sensitive to calcium. Calcium reduces the sodium permeability, and the half-maximal inhibitory concentration is 0.5 microM, well within the range of calcium activity found in cells. Also, the permeability of the luminal membrane vesicles is little affected by the ambient sodium concentration. These results, when taken together with studies on whole tissue, suggest that cell calcium may be an important regulator of transepithelial sodium transport by its effect on luminal sodium permeability. The effect of cell sodium on permeability may be mediated by calcium rather than by sodium itself.
调节管腔膜对钠的通透性是改变钠跨紧密上皮细胞转运净速率的主要机制。先前的证据表明，管腔膜的通透性可能受细胞内钠或钙活性变化的调节。为了直接验证这一点，我们从蟾蜍膀胱中分离出一部分质膜，其具有快速、对氨氯地平敏感的钠通量，其特性与天然管腔膜相似。使用流动猝灭装置在毫秒时间范围内测量这些囊泡中钠流出的初始速率，我们已经证明这些囊泡的同位素交换通透性对钙非常敏感。钙降低钠通透性，半数最大抑制浓度为0.5微摩尔，这在细胞中发现的钙活性范围内。此外，管腔膜囊泡的通透性几乎不受环境钠浓度的影响。这些结果与对整个组织的研究结果结合起来表明，细胞钙可能通过其对管腔钠通透性的影响而成为跨上皮钠转运的重要调节因子。细胞钠对通透性的影响可能是由钙介导的，而不是由钠本身介导的。
