Diamond J M
Fed Proc. 1978 Oct;37(12):2639-43.
Epithelia may be classified as "tight" or "leaky," depending on whether there is a significant pathway for transepithelial ion permeation via the junctions and bypassing the cells. The resistance of this paracellular channel may depend partly on structures visible in the electron microscope, partly on wall charge. Permeability determinations in the leaky junctions of gallbladder epithelium, using many different organic cations, suggest that the critical barriers barriers to ion permeation are 5--8 A in radius and bind cations by up to four strongly proton-accepting oxygens. The apical cell membrane of tight epithelia contains a Na+-selective channel that is blocked by amiloride and Ca2+, subject to negative feedback control by the Na+ pump in the basolateral membrane, and somehow promoted by aldosterone. To determine the permeabilities of these two channels (the junctional channel of leaky epithelia, and the Na+ channel of tight epithelia) to water and nonelectrolytes remains a major unsolved problem.
上皮组织可分为“紧密型”或“渗漏型”，这取决于是否存在通过连接部并绕过细胞进行跨上皮离子渗透的重要途径。这种细胞旁通道的阻力可能部分取决于电子显微镜下可见的结构，部分取决于壁电荷。使用多种不同有机阳离子对胆囊上皮渗漏连接部的通透性测定表明，离子渗透的关键屏障半径为5 - 8埃，可通过多达四个强质子接受性氧原子结合阳离子。紧密上皮组织的顶端细胞膜含有一种钠选择性通道，该通道被氨氯吡脒和钙离子阻断，受基底外侧膜中钠泵的负反馈控制，并以某种方式受醛固酮促进。确定这两种通道（渗漏上皮组织的连接通道和紧密上皮组织的钠通道）对水和非电解质的通透性仍然是一个主要未解决的问题。
