We determined the site of kininogenase secretion in the nephron by performing stop-flow studies in dogs. Kininogenase activity, inulin, sodium, and potassium were measured in the same fractions. The highest kininogenase concentration was found in the fractions with the lowest sodium concentration corresponding to urine samples that must have been trapped in the distal nephron. Kininogenases in these urine fractions were 5-17 times higher than in fractions from the proximal nephron or from the free-flow sample. The ratio of kininogenase increase to inulin increase was always higher than 3.0, thus indicating that the increase in kininogenase concentration was mainly due to secretion and not to water reabsorption. When a peak of potassium was present, it occurred 1-2 ml before the peak of kininogenases. We concluded that kininogenases are secreted into the urine at the level of the distal nephron by either the tubule itself or by a specialized structure located at this part of the nephron, such as the macula densa.

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