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Amide transport channels across toad urinary bladder.

S D Levine, R E Worthington

    The Journal of Membrane Biology
    |February 17, 1976
    PubMed
    Summary
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    Urea and amides cross toad bladders via vasopressin-insensitive channels, not carriers. This facilitated transport is independent of water flow and involves selective channels.

    Area of Science:

    • Physiology
    • Membrane Transport
    • Renal Physiology

    Background:

    • Urea and small amides cross the toad urinary bladder via a vasopressin-insensitive pathway.
    • This transport is independent of osmotic water flow and exhibits characteristics of facilitated transport, including saturation and mutual inhibition.

    Purpose of the Study:

    • To distinguish between movement through fixed selective membrane channels and movement via a mobile carrier for amide transport.
    • To investigate the mechanism of urea and amide transport across the toad urinary bladder.

    Main Methods:

    • Determined mucosal to serosal (M to S) and serosal to mucosal (S to M) permeabilities of labeled amides in paired bladders.
    • Used unlabeled methylurea as a potent inhibitor of amide movement, added to either the mucosal or serosal bath.

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  • Eliminated osmotic water flow by adding ethylene glycol and ethanol.

    • Main Results:

    • Co-transport of labeled amides was demonstrated, indicating acceleration of labeled amide flow in the direction of net unlabeled amide flow.
    • Counter-transport could not be demonstrated.
    • Methylurea did not alter water permeability or transmembrane electrical resistance.

    Conclusions:

    • The demonstration of co-transport is consistent with the presence of vasopressin-insensitive, amide-selective channels.
    • The findings support a channel-mediated transport mechanism rather than a mobile carrier system for urea and amide transport across the toad urinary bladder.