Effect of antidiuretic hormone on water transport

Membrane lipids, and particularly cholesterol, are instrumental not only in the control of diffusion across biological membranes but also in the determination of the activity of membrane-bound enzymes, their modulation by hormones and other agents, and the determination of membrane fluidity (for original references, see [4,6]). It is generally accepted that incorporation of cholesterol in a lipid bilayer membrane tends to decrease significantly the permeability of these membranes to water. Movement of water across these membranes occurs primarily by dissolution in the membrane matrix. The decrease in the rate of water transport as a result of cholesterol incorporation is due mainly to a decrease in membrane fluidity. As a general rule, it is found that the presence of cholesterol in membranes or the incorporation of cholesterol into dispersions composed of phosphatidylserine or ganglioside lead to a decrease in the fluidity of the hydrocarbon chains of lipid membranes which are in the liquid-crystalline state [4,20]. 

In contrast to lipid bilayer membranes, it has been found [4] that the permeability coefficient of the human red-cell membrane to water did not change when the free cholesterol content in the membrane was varied from 0.84 to 1.87 mg/ml cells. Furthermore, the permeability of the human red-cell membrane to sulfate and some nonelectrolytes remained constant when membrane cholesterol was partially removed (for review, see [6]). These results, however, should not be taken as evidence that water transport in human red cells is independent of membrane cholesterol, since this degree of variation may be insufficient to produce alteration. In fact, extensive depletion of membrane cholesterol induces a marked increase in nonelectrolyte permeability. The effect of membrane cholesterol on the transport of water is also found in other membrane systems. For example, the polyene antibiotic. Amphotericin B, which interacts specifically with sterol-containing membranes, increases the permeability of the mucosal but not the serosal membrane of toad bladder to water and other solutes [32]. It is possible that membrane cholesterol only effects the movements through the lipid bilayer pathway. This may explain the findings that the permeabiUty coefficient of the human red cell membrane to water 

One of the first parameters which comes to mind among those factors which may affect membrane permeability properties in any system is pH. A study of pH effects may give some information about the role of ionizable protein groups in membrane structure on its permeability. It has been shown that the hydraulic water conductivity (Lp) is independent of pH in the range from 6 to 8 in human red cells, init l cell 

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