2005_pset2_ans - 7.06 Problem Set#2 Spring 2005 1 You are a...

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1 7.06 Problem Set #2, Spring 2005 1. You are a renal physiologist interested in how water is reabsorbed back into the blood from tubules within the kidneys. You decide to concentrate on the collecting duct, the last section of the kidney tubules where water reabsorption can occur before the urine collects in the ureters and exits the kidneys. As a research tool, you are using a specific type of MDCK cells (Madin-Darby canine kidney cells) that are known to resemble a specific type of cell in the collecting duct. When grown on a porous membrane filter coated with collagen and other components of the basal lamina, MDCK cells form a polarized epithelium. (See Lodish et. al. Textbook, page 204; Figure 6-6.) This experimental setup allows you to manipulate the culture medium on both sides of the cell monolayer and thus study the transport of ions and small molecules across it. For this problem, assume that the transport of water across the epithelial cell layer can be measured as a change in volume in the two separate reservoirs. a. The MDCK cells that you are using express a number of pumps and channels, including: the Na+/K+ ATPase, an Na+ channel that is sensitive to the drug amiloride, and K+ channels. The Na+/K+ pump is located in the basolateral membrane (which, in the kidney, faces away from the tubular fluid). In the experimental set-up in Figure 6-6 this is the surface facing the membrane on which the cells are cultured. The amiloride- sensitive Na+ channel is found in the apical membrane (facing the tubular fluid in the kidney). The potassium channels are found in both the basolateral and apical membranes. From this information, suggest a mechanism by which water is reabsorbed from the tubular fluid across the cell layer of the collecting duct. (Assume for this problem that negatively charged Cl- ions can traverse the epithelium via the paracellular pathway.) Na+ enters through the amiloride-sensitive Na+ channel in the apical membrane via facilitated transport, down its concentration gradient. The Na+/K+ ATPase pumps Na+ out across the basolateral membrane, keeping intracellular Na+ low. Movement of Na+ ions across the tubular epithelium sets up a negative potential inside the tubular lumen. This is dissipated in part by potassium ions (pumped in through the basolateral membrane) entering the tubular lumen and in part by Cl - that follows Na across the epithelium via the paracellular pathway. Net movement of ions across the tubular epithelium from the tubular fluid to the blood, in turn, creates an osmotic force that drives the reabsorption of water from the kidneys into the blood.
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2 b. To test your model from part a ), you obtain a new cell line of MDCK cells known to be deficient in the formation of tight junctions between cells. As you expected, a monolayer of these cells is unable to transport water. Describe two effects that a deficiency of tight junctions would have that could explain why these cells cannot transport water. Without tight junctions, ions are free to move across the cell layer between the cells (by the
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2005_pset2_ans - 7.06 Problem Set#2 Spring 2005 1 You are a...

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