Levine S D, Worthington R E
J Membr Biol. 1976 Feb 17;26(1):91-107. doi: 10.1007/BF01868868.
Urea and other small amides cross the toad urinary bladder by a vasopressinsensitive pathway which is independent of osmotic water flow. Amide transport has characteristics of facilitated transport: saturation, mutual inhibition between amides, and selective depression by agents such as phloretin. The present studies were designed to distinguish among several types of transport including (1) movement through a fixed selective membrane channel and (2) movement via a mobile carrier. The former would be characterized by co-transport (acceleration of labeled amide flow in the direction of net flow of unlabeled amide), the latter by counter-transport (acceleration of labeled amide flow in the opposite direction). Mucosal to serosal (M leads to S) and serosal to mucosal (S leads to M) permeabilities of labeled amides were determined in paired bladders. Unlabeled methylurea, a particularly potent inhibitor of amide movement, was added to either the M or S bath, while osmotic water flow was eliminated by addition of ethylene glycol and ethanol could not be demonstrated. Methylurea did not alter water permeability or transmembrane electrical resistance. The demonstration of co-transport is consistent with the presence of ADH-sensitive amide-selective channels rather than a mobile carrier.
尿素和其他小分子酰胺通过一条对血管加压素不敏感的途径穿过蟾蜍膀胱，该途径独立于渗透水流。酰胺转运具有易化转运的特征：饱和性、酰胺之间的相互抑制以及诸如根皮素等试剂的选择性抑制。本研究旨在区分几种转运类型，包括（1）通过固定的选择性膜通道的移动以及（2）通过移动载体的移动。前者的特征是共转运（标记酰胺流在未标记酰胺净流方向上加速），后者的特征是逆向转运（标记酰胺流在相反方向上加速）。在成对的膀胱中测定标记酰胺从黏膜到浆膜（M至S）和从浆膜到黏膜（S至M）的通透性。将未标记的甲基脲（一种特别有效的酰胺移动抑制剂）添加到M或S浴中，同时通过添加乙二醇消除渗透水流，且未观察到乙醇的作用。甲基脲未改变水通透性或跨膜电阻。共转运的证明与存在抗利尿激素敏感的酰胺选择性通道而非移动载体一致。
