Degnan K J, Zadunaisky J A
J Membr Biol. 1980 Aug 7;55(3):175-85. doi: 10.1007/BF01869459.
The unidirectional Na+, Cl-, and urea fluxes across isolated opercular epithelia from seawater-adapted Fundulus heteroclitus were measured under different experimental conditions. The mean Na+, Cl0, and urea permeabilities were 9.30 x 10(-6) cm . sec-1, 1.24 x 10(-6) cm . sec-1, and 5.05 x 10(-7) cm . sec-1, respectively. The responses of the unidirectional Na+ fluxes and the Cl- influx (mucosa to serosa) to voltage clamping were characteristic of passively moving ions traversing only one rate-limiting barrier. The Na+ conductance varied linearly with, and comprised and mean 54% of, the total tissue ionic conductance. The Cl- influx and the urea fluxes were independent of the tissue conductance. Triaminopyrimidine (TAP) reduced the Na+ fluxes and tissue conductance over 70%, while having no effect on the Cl- influx of urea fluxes. Mucosal Na+ substitution reduced the Na+ permeability 60% and the tissue conductance 76%, but had no effect on the Cl- influx or the urea fluxes. Both the Na+ and Cl- influxes were unaffected by respective serosal substitutions, indicating the lack of any Na+/Na+ and Cl-/Cl- exchange diffusion. The results suggest that the unidirectional Na+ fluxes are simple passive fluxes proceeding extracelluarly (i.e., movement through a cation-selective paracellular shunt). This pathway is dependent on mucosal (external) Na+, independent of serosal (internal) Na+, and may be distinct from the transepithelial Cl- and urea pathways.U
在不同实验条件下，测量了适应海水的河口鳉鱼分离鳃上皮细胞上单向的钠离子、氯离子和尿素通量。钠离子、氯离子和尿素的平均渗透率分别为9.30×10⁻⁶厘米·秒⁻¹、1.24×10⁻⁶厘米·秒⁻¹和5.05×10⁻⁷厘米·秒⁻¹。单向钠离子通量和氯离子内流（从黏膜到浆膜）对电压钳制的反应是仅通过一个限速屏障被动移动离子的特征。钠离子电导率与总组织离子电导率呈线性变化，平均占总组织离子电导率的54%。氯离子内流和尿素通量与组织电导率无关。三氨基嘧啶（TAP）使钠离子通量和组织电导率降低超过70%，而对氯离子内流或尿素通量没有影响。黏膜钠离子替代使钠离子渗透率降低60%，组织电导率降低76%，但对氯离子内流或尿素通量没有影响。钠离子和氯离子内流均不受各自浆膜替代的影响，表明不存在任何钠离子/钠离子和氯离子/氯离子交换扩散。结果表明，单向钠离子通量是细胞外进行的简单被动通量（即通过阳离子选择性细胞旁通道移动）。该途径依赖于黏膜（外部）钠离子，独立于浆膜（内部）钠离子，并且可能与跨上皮氯离子和尿素途径不同。
