Apical Membrane Vesicles of ADH-stimulated Toad Bladder are Highly Water Permeable

Title: Apical Membrane Vesicles of ADH-stimulated Toad Bladder are Highly Water Permeable
Authors: Harris, H. William; Handler, Joseph S.; Blumenthal, Robert
Publisher: American Journal of Physiology
Date Published: February 01, 1990
Reference Number: 280
Antidiuretic hormone (ADH) stimulation of the toad urinary bladder causes intracellular vesicles called aggrephores to fuse with the apical plasma membrane of granular cells. Aggrephore membranes contain particle aggregates. Particle aggregates are believed to be water channels that cause large increases in the water permeability (PF) of the granular cell apical membrane. Removal of ADH causes the retrieval of particle aggregate-containing apical membrane via endocytosis and a decline in PF. We have previously shown that fluid phase markers are sequestered in these particle aggregate-containing vesicles during retrieval of the apical membrane and that these vesicles can be recovered in cell homogenates. We have now loaded these vesicles with the self-quenching fluorophore carboxyfluorescein (CF) to measure and compare their PF with that of CF-loaded resealed human erythrocyte ghosts. The membranes of these retrieved vesicles have a very high water permeability. The minimum PF of 99% of these vesicles is 4.5 X 10(-2) cm/s. This PF is comparable with that of erythrocyte ghosts (5.4 X 10(-2) cm/s) measured under identical conditions. We conclude that these vesicles are highly permeable to water, and this is consistent with their postulated function of retrieving water channels that have been inserted into the apical membrane in response to ADH.

This translation by the NDI Foundation is to assist the lay reader. To provide a clear, accessible interpretation of the original article, we eliminated or simplified some technical detail and complicated scientific language. We concentrated our translation on those aspects of the article dealing directly with NDI. The NDI Foundation thanks the researchers for their work toward understanding and more effectively treating this disorder.
© Copyright NDI Foundation 2007 (JC)

The antidiuretic hormone (ADH) instigates a molecular sequence which allows the toad urinary bladder to maintain body water balance in its body. Granular cells line the tissue of the toad bladder. The apical membranes of the granular cells do not allow much water to flow through them. Another way of saying this is that they have a low water permeability (Pf). Within the granular cell are little sacs (vesicles) called aggrephores. These contain particle aggregates: membrane proteins that act as channels through which water can flow. These particle aggregates are also known as water channels (WCs). One model for how the apical membrane Pf is increased so that body water balance is maintained is called the shuttle hypothesis.

According to the shuttle hypothesis, when ADH is present in the granular cell, it induces the aggrephores to travel from the interior of the cell to the apical membrane and fuse with it. Once fused, the water channels that are inside it are inserted into the apical membrane. The apical membrane now is filled with water channels so water can now flow through it much more easily and in greater volume than before. Another way of saying this is that the water channels dramatically increase the apical membrane's Pf.

When ADH absents itself from the granular cell, the water channels are retrieved from the apical membranes, and the apical membrane Pf returns to its normal low state. The water channels are taken back into the cell in the aggrephores and in aggregate rich sections of the apical membrane.

Harris, et al., wanted to measure the Pf of these aggrephores after they returned from the apical membrane to the interior of the cell. They did this by loading the aggrephores with self-quenching flurophore carboxyfluorescein (CF). Quenching is any type of interference which reduces the intensity of fluorescence. By adding the CF to the aggrephores and adding an anti-CF antibody to quench all the CF outside the aggrephores, the authors were able to track the CF in the aggrephores.

They discovered that the retrieved aggrephores had an extremely high Pf, comparable to the Pf of red blood cells (minus their hemoglobin) that they measured under identical circumstances. This high Pf strongly supports the idea that they are filled with particle aggregates (i.e., water channels), thus supporting the shuttle hypothesis described above.