Relationship between cell volume and cation content in thick ascending limb of rat kidney

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Abstract
We examined the relationship between the cell volume and cation concentration ([Nai] and [Ki]) of isolated segments of rat medullary thick ascending limb (MAL) after incubation at 30°C in various isotonic solutions. When the tubules were incubated in a normal NaCl solution containing 5 mmol/l K+, addition of 1 mmol/l of ouabain increased [Nai] and decreased [Ki] but did not change the total ([Nai]+[Ki]) concentration (about 90 mEq/l) or tubular volume. After incubation in various K+-free solutions, the tubules were almost fully K+-depleted; their volume per unit of length was similar in the three solutions, although the choline Cl-treated tubules had a very low sodium content compared to the NaCl-and Na2SO4-treated tubules (8 vs. 97 and 95 mEq/l respectively). Ouabain altered neither volume nor [Nai] of tubules incubated in choline Cl or Na2SO4 solution. Transfer of tubules from K+-free Na2SO4 or K+-free choline Cl solution into K+-free NaCl solution resulted in an increase in [Nai] (by 29 and 97 mEq/l respectively) without much increase in tubular volume. A marked swelling of the tubules was only observed when the K+-free NaCl solution contained also ouabain. Under this condition, [Nai] was comparable to the Na+ concentration of the incubation medium. After washing and incubation in a normal NaCl solution containing K+, the swollen tubules recovered their initial volume and restored Na+ and K+ concentration gradients across the cell membranes. The ([Nai]+[Ki]) concentration centration measured in the tubules preincubated in choline Cl solution was always smaller than that of the tubules preincubated either in NaCl or Na2SO4 solutions, an observation suggesting that choline ions enter rat MAL cells. Barium (3 mmol/l) prevented tubular swelling. This inhibition corresponded to a smaller increase in [Nai] than that observed in control tubules. Furosemide or bumetanide (even at 0.1 mmol/l) did not alter the increases in tubular volume and in Na+ content induced by ouabain. The data provide additional evidence that the isoosmotic swelling of MAL cells requires an almost full inhibition of Na+-pump activity and involves coupled net fluxes of Na+ and Cl− ions.

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