1. We have previously shown that the hypertrophy of the kidney induced by a high protein diet consists of a preferential hypertrophy of the thick ascending limb (TAL) of Henle's loop. This might be related to an increase in the active salt transport by this segment. Sodium, potassium-dependent adenosine triphosphatase (Na+,K+-ATPase) activity was measured in TAL from kidneys of rats fed either a low (LP) or a high (HP) protein diet for several weeks.

2. Enzymatic activity was measured by microdensitometry, after appropriate cytochemical reaction, for an adenosine 5′-triphosphate (ATP) concentration of 0–66 mmol/l. Both activity per unit tubular length and mean activity per unit tissue volume were recorded. A calibration was designed to convert usual microdensitometry units (extinction) into conventional biochemical units (mol of product formed).

3. For non-limiting substrate concentrations, the Na+,K+-ATPase activity, expressed per unit length of tubule on the sections, was 50% higher in HP than in LP rats, an increase proportional to that of the simultaneously measured tubule diameter. When expressed per unit tubular volume, Na+,K+-ATPase activity was similar in both groups of rats. The dissociation constant for ATP was also similar in both groups.

4. Results show that a high protein diet induces an increase in Na+,K+-ATPase activity in TAL, thus enabling an enhanced NaCl transport in this segment. This increase in transport capacity is not due to an increase in the density of enzymatic units but to an increase in their number, in relation to the hypertrophy of the TAL.

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