High Proton Flux through Membranes Containing Antidiuretic Hormone Water Channels
| Title: | High Proton Flux through Membranes Containing Antidiuretic Hormone Water Channels |
|---|---|
| Authors: | Harris, H. William; Kikeri, Deepak; Janoshazi, Agnes; Solomon, A. K.; Zeidel, Mark L. |
| Publisher: | American Journal of Physiology |
| Date Published: | August 01, 1990 |
| Reference Number: | 279 |
Antidiuretic hormone (ADH) stimulation of toad urinary bladder granular cells causes simultaneous increases in transepithelial water and H+ permeabilities (PF and PH+, respectively), suggesting that ADH-elicited water channels inserted into granular cell apical membranes might be permeable to both water and H+. We have previously used self-quenching fluorophores entrapped within endocytic vesicles selectively retrieved from water-permeable apical membranes to measure vesicle PF. The membranes of these vesicles possess an extremely high PF such that our measurements provide only minimum estimates of vesicle PF and have limited our ability to quantitate the properties of ADH water channels. We therefore quantitated vesicle PH+ using similar rapid mixing techniques. Vesicle PH+ was 5.1 +/- 0.5 x 10(-3) cm/s. Activation energy of this process was 3.6 +/- 0.6 kcal/mol, indicative of H+ flux through an aqueous channel. The mercurial reagent, para-chloromercuribenzenesulfonate (PCMBS), which inhibits ADH-stimulated transepithelial PF in intact bladders by 50-60%, inhibited vesicle PH+ by 55%. N-Ethylmaleimide and phloretin, which do not alter ADH-stimulated PF, did not affect vesicle PH+. We conclude that membranes containing ADH water channels possess substantial PH+ that likely reflects proton flux through water channels. The apparent high PH+ of the ADH water channel may have important implications for intracellular trafficking of these water channels in ADH-responsive epithelial cells.
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.
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A great deal of research shows that ADH increases Pf through the mediation of water channels (WCs). WCs are intrinsic membrane proteins that act as channels through which water can flow. There are different types of them located in different regions of plants and animals. Some WCs are regulated by hormones; others are not. The WC located in the toad granular cell is regulated by ADH. Here, WCs sit inside relatively large tubular sacs (vesicles) called aggrephores that sit inside the cell beneath the cell's apical membrane.
ADH stimulation causes these aggrephores to travel to and fuse with the apical membrane. Upon fusion, the WCs are inserted into the membrane. Once there, they form channels through which water can flow. When AVP absents itself from the granular cell, the WCs are removed from the apical membrane and transported back in vesicles into the cell's interior. When this occurs the apical membrane reverts to its normal low level of Pf.
Because granular cells simultaneously increase their apical membrane Pf and Ph+ upon ADH stimulation, Harris, et al., speculated that the ADH-stimulated WC might serve as a channel for both water and H+ to cross the apical membrane. To test their hypothesis, they traced the movement of H+ across apical membrane by marking it with self-quenching flourophores absorbed within the vesicles that retrieved the WCs from the apical membrane and brought them back inside the cell.
The authors found that in addition to having a remarkably high degree of Pf, the membranes of these vesicles possessed a high degree of Ph+. Further, the amount of energy needed to activate this high degree of Ph+ was low, and the addition of a specific mercury compound inhibited the permeability level. The last two properties - low activation energy and inhibition by mercury - are properties of WCs. Taken together, this data indicates that membranes containing ADH WCs possess substantial Ph+ and that the H+ very likely crosses the apical membrane through the WC.