Amide transport channels across toad urinary bladder.

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.