Relationship Between Vasopressin-Sensitive Water Transport and Plasma Membrane Fluidity in Kidney Collecting Tubule

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Title: Relationship Between Vasopressin-Sensitive Water Transport and Plasma Membrane Fluidity in Kidney Collecting Tubule
Authors: Fushimi, MD, Kiyohide; Verkman, Alan S.
Publisher: American Journal of Physiology
Date Published: January 01, 1991
Reference Number: 211
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Abstract • Translation
The role of plasma membrane fluidity in the regulation of kidney tubule water permeability has been uncertain. We have used new methods to image the fluorescence anisotropy of fluidity-sensitive fluorophores (Fushimi, Dix, and Verkman. Biophys. J. 57: 241-254, 1990) to quantitate membrane fluidity in cells of the vasopressin-sensitive cortical collecting tubule (CCT) and water-impermeable cortical thick ascending limb (CTAL). Isolated tubule segments from rabbit kidney were perfused in vitro, and apical or basolateral plasma membranes were stained with trimethylammonium diphenylhexatriene (TMA-DPH). TMA-DPH anisotropy (r) was imaged quantitatively by an epifluorescence microscope equipped with rotatable polarizers; TMA-DPH nanosecond lifetime (tau) was measured by flash-lamp excitation and gated photomultiplier detection. In CCT, apical membrane r (0.254 +/- 0.003) was similar to basolateral r (0.252 +/- 0.005). Serosal vasopressin at a dose that increased water permeability greater than 10-fold (250 microU/ml) did not affect apical membrane r (delta r = 0.002 +/- 0.003; 7 tubules). A 0.002 change in r was less than that produced by a 2 degrees C temperature variation. In CTAL, apical membrane r was 0.249 +/- 0.002, similar to r from basolateral membrane of proximal tubule (0.24), but much less than that of proximal tubule apical membrane (0.29). These results establish methodology to quantitate fluidity in intact kidney tubule segments and provide the first measurements of plasma membrane fluidity in CTAL and CCT. Our results suggest that regulation of bulk membrane fluidity in CCT apical membrane is not a component of the hydrosmotic action of vasopressin and that low apical membrane fluidity is not responsible for the low water and NH3 permeabilities in CTAL.

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 nephron is the main working unit of the kidney. There are about a million of them in each kidney. Each nephron consists of a filter called a glomerulus and a little tube called a tubule. The tubule has different sections, each with a specific purpose. The section nearest the glomerulus is called the proximal tubule. The section furthest from the glomerulus is the distal tubule. The section in-between the proximal and distal tubule is called the loop of Henle. The loop of Henle has a thick and a thin ascending limb, and a thick and a thin descending limb.

The tubule looks almost like a tangled piece of spaghetti connected at one end to the glomerulus and at the other to the kidney collecting duct. Blood is filtered in the glomerulus, leaving body water, full of useful substances such as sugars and salts, to travel through the tubule. Almost 80% of the water (and much of the substances it carries as solutes within it) is reabsorbed into the kidney's inner tissue through the proximal tubule. Most of the remaining water is reabsorbed in the collecting duct. What remains is concentrated urine which is shunted through the ureter from the kidney to the bladder.

Much of the body water reabsorption takes place through the process of osmosis, which depends on unequal concentrations of solutions on either side of a semipermeable membrane. The loop of Henle generates the conditions by which the osmosis takes place in the nephron and the collecting duct. The different degrees of permeability in different segments of the nephron contributes to this process.

Since body water balance is critical to health, researchers have investigated the mechanisms of cell tissue permeability in the nephron and collecting duct. Measurements of the degree of membrane's water permeability are represented by water permeability coefficients (Pf).

There are different ways a membrane can allow water through it. One is called diffusional permeability. Here water spreads out and through the membrane itself, much like water will soak into a dry sponge. Another way is through the vehicle of a water channel, specialized proteins that insert themselves in membranes to let relatively great amounts of water pass through the membranes quickly.

A specific water channel is located in the principal cells of the kidney collecting duct. When signaled by a molecular sequence initiated by the hormone, vasopressin, these water channels insert themselves in the apex of the cell membrane (called the apical membrane). This makes the apical membrane much more water permeable than usual. And this allows body water to flow through the collecting duct cells to be reabsorbed in the kidney's inner tissue. Apical membrane water permeability is crucial to this process and it could not be achieved without the vasopressin-regulated water channels.

Some researchers have speculated that another factor called membrane fluidity also plays a role in the regulation of water permeability in the kidney tubule. However, this has been difficult to test because of the inability to directly measure membrane fluidity in intact membrane tissue.

Fushimi and Verkman developed a means to test and quantify membrane fluidity and they applied it to the intact membranes of the kidney collecting duct, the thick ascending limb of Henle (CTAL) and the proximal tubule. Finding the first fluidity measurements of these nephron segments enabled the researchers to examine the role of these membranes' fluidity in the regulation of water permeability. They could take steps towards discovering if membrane fluidity did or did not help regulate water permeability in these cells. Their test results suggest that it does not.

Vasopressin, applied in doses that increased the apical water permeability over ten-fold, did not alter apical membrane fluidity significantly. The authors concluded that membrane fluidity of the apical membrane does not play a role in the enhanced water permeability instigated by vasopressin.

The CTAL has extremely low water permeability. The authors found the membrane fluidity of the cortical section of the CTAL is not particularly low and is significantly higher than fluidity in the apical membrane of the proximal tubule, which has high water permeability. They conclude that the high water permeability of the membranes of the proximal tubule is due to water channels, not membrane fluidity. In short, membrane fluidity is not an important factor in the regulation of water permeability in those membranes.