Amide transport channels across toad urinary bladder

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Abstract
Urea and other small amides cross the toad urinary bladder by a vasopressinsensitive pathway which is independent of somotic 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 thought a fixed selective membrane channel and (2) movement via a mobile carrier. The former wold be characterized by co-transport (acceleration of labele amide flow in the direction of net flow in the opposite direction). Mucosal to serosal (M→S) and serosal to mucosal (S→M) permeabilities of labeled amides were determined in paired bladers. 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 to the opposite bat. Co-transport of labeled methylurea and, to a lesser degree, acetamide and urea with unlabeled methylurea was observed. Co-transport of the nonamides 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 channcels rather than a mobile carrier.