Diuresis was induced in 40 Wistar rats by the intraperitoneal injection of water. Of these animals, 20 were given, during the diuresis, a subcutaneous injection of 50 mU of Pitressin Tannate in oil. The extravascular protein in the renal medulla was estimated by injecting Evans Blue as a protein label, washing out the medullary vessels with saline and estimating the Evans Blue remaining in the tissues by a spectrophotometric method. It was found that after Evans Blue had been allowed to circulate for 2 min, almost twice as much extravascular dye was found in the diuretic rats as in those which had been treated with ADH. It is suggested that the extravascular protein hinders the removal of interstitial water by the vasa recta during diuresis, while during antidiuresis the oncotic pressure within the vessels causes removal of water from the medulla by means of the ascending vasa recta.
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Williams et al. (1971) studied this question.