Carvounis C P, Levine S D, Franki N, Hays R M
J Membr Biol. 1979 Sep;49(3):269-81. doi: 10.1007/BF01871122.
Urea and water transport across the toad bladder can be separately activated by low concentrations of vasopressin or 8 Br-cAMP. Employing this method of selective activation, we have determined the reflection coefficient (sigma) of urea and other small molecules under circumstances in which the bladder was transporting urea or water. An osmotic method for the determination of sigma was used, in which the ability of a given solute to retard water efflux from the bladder was compared to that of raffinose (sigma = 1.0) or water (sigma = 0). When urea transport was activated (low concentration of vasopressin), sigma for urea and other solutes was low, (sigma urea, 0.08--0.39; sigma acetamide, 0.55; sigma ethylene glycol, 0.60). When water transport was activated (0.1 mM 8 Br-cAMP) sigma urea approached 1.0 sigma urea also approached 1.0 at high vasopressin concentrations. In a separate series of studies, sigma urea was determined in the presence of 2 x 10(-5) M KMnO4 in the luminal bathing medium. Under these conditions, when urea transport is selectively blocked, sigma urea rose from a value of 0.12 to 0.89. Thus, permanganate appears to "close" the urea transport channel. These findings indicate that the luminal membrane channels for water and solutes differ significantly in their dimensions. The solute channels, limited in number, have relatively large radii. They carry a small fraction (approximately 10%) of total water flow. The water transport channels, on the other hand, have small radii, approximately the size of a water molecule, and exclude solutes as small as urea.
低浓度的血管加压素或8-溴环磷酸腺苷（8 Br-cAMP）可分别激活蟾蜍膀胱对尿素和水的转运。采用这种选择性激活方法，我们测定了在膀胱转运尿素或水的情况下尿素及其他小分子的反射系数（σ）。使用了一种渗透压法来测定σ，即将给定溶质阻止水从膀胱流出的能力与棉子糖（σ = 1.0）或水（σ = 0）的能力进行比较。当尿素转运被激活时（低浓度血管加压素），尿素和其他溶质的σ较低（σ尿素，0.08 - 0.39；σ乙酰胺，0.55；σ乙二醇，0.60）。当水转运被激活时（0.1 mM 8 Br-cAMP），σ尿素接近1.0，在高浓度血管加压素时σ尿素也接近1.0。在另一系列研究中，在管腔灌流介质中存在2×10⁻⁵ M高锰酸钾的情况下测定σ尿素。在这些条件下，当尿素转运被选择性阻断时，σ尿素从0.12升至0.89。因此，高锰酸盐似乎会“关闭”尿素转运通道。这些发现表明，管腔膜上的水和溶质通道在尺寸上有显著差异。溶质通道数量有限，半径相对较大。它们承载的水流量占总水流量的一小部分（约10%）。另一方面，水转运通道半径较小，大约是水分子的大小，并且会排斥像尿素这样小的溶质。
