Schafer J A, Patlak C S, Andreoli T E
J Gen Physiol. 1974 Aug;64(2):201-27.
This paper reports a theoretical analysis of osmotic transients and an experimental evaluation both of rapid time resolution of lumen to bath osmosis and of bidirectional steady-state osmosis in isolated rabbit cortical collecting tubules exposed to antidiuretic hormone (ADH). For the case of a membrane in series with unstirred layers, there may be considerable differences between initial and steady-state osmotic flows (i.e., the osmotic transient phenomenon), because the solute concentrations at the interfaces between membrane and unstirred layers may vary with time. A numerical solution of the equation of continuity provided a means for computing these time-dependent values, and, accordingly, the variation of osmotic flow with time for a given set of parameters including: P(f) (cm s(-1)), the osmotic water permeability coefficient, the bulk phase solute concentrations, the unstirred layer thickness on either side of the membrane, and the fractional areas available for volume flow in the unstirred layers. The analyses provide a quantitative frame of reference for evaluating osmotic transients observed in epithelia in series with asymmetrical unstirred layers and indicate that, for such epithelia, P(f) determinations from steady-state osmotic flows may result in gross underestimates of osmotic water permeability. In earlier studies, we suggested that the discrepancy between the ADH-dependent values of P(f) and P(DDw) (cm s(-1), diffusional water permeability coefficient) was the consequence of cellular constraints to diffusion. In the present experiments, no transients were detectable 20-30 s after initiating ADH-dependent lumen to bath osmosis; and steady-state ADH-dependent osmotic flows from bath to lumen and lumen to bath were linear and symmetrical. An evaluation of these data in terms of the analytical model indicates: First, cellular constraints to diffusion in cortical collecting tubules could be rationalized in terms of a 25-fold reduction in the area of the cell layer available for water transport, possibly due in part to transcellular shunting of osmotic flow; and second, such cellular constraints resulted in relatively small, approximately 15%, underestimates of P(f).
本文报道了对渗透瞬变的理论分析以及对暴露于抗利尿激素（ADH）的离体兔皮质集合管中管腔到浴液渗透的快速时间分辨率和双向稳态渗透的实验评估。对于与未搅拌层串联的膜的情况，初始和稳态渗透流之间可能存在相当大的差异（即渗透瞬变现象），因为膜与未搅拌层之间界面处的溶质浓度可能随时间变化。连续性方程的数值解提供了一种计算这些随时间变化的值的方法，因此，对于给定的一组参数，包括：P(f)（厘米·秒⁻¹），渗透水渗透系数、主体相溶质浓度、膜两侧未搅拌层的厚度以及未搅拌层中可用于体积流的分数面积，可计算出渗透流随时间的变化。这些分析为评估在与不对称未搅拌层串联的上皮细胞中观察到的渗透瞬变提供了一个定量参考框架，并表明，对于此类上皮细胞，根据稳态渗透流确定P(f)可能会导致对渗透水渗透性的严重低估。在早期研究中，我们认为P(f)与P(DDw)（厘米·秒⁻¹，扩散水渗透系数）的ADH依赖性值之间的差异是细胞对扩散的限制的结果。在本实验中，在启动ADH依赖性管腔到浴液渗透后20 - 30秒内未检测到瞬变；并且稳态ADH依赖性从浴液到管腔和从管腔到浴液的渗透流是线性且对称的。根据分析模型对这些数据的评估表明：第一，皮质集合管中细胞对扩散的限制可以通过可用于水运输的细胞层面积减少25倍来解释，这可能部分归因于渗透流的跨细胞分流；第二，这种细胞限制导致对P(f)的低估相对较小，约为15%。
