NaCl and Ca delivery at the bend of rat deep nephrons decreases during antidiuresis.

The effects of 1-desamino-8-D-arginine vasopressin (dDAVP) on superficial and juxtamedullary nephrons were investigated by micropuncture in diabetes insipidus (DI) Brattleboro rats chronically treated with the peptide. The rats, acutely deprived of endogenous calcitonin, parathyroid hormone (PTH), and glucagon [hormone-deprived (HD) rats], were examined either 4 days after cessation of dDAVP treatment (HDT, control diuretic rats) or when the treatment was continued until the micropuncture experiment, during which dDAVP was also given intravenously (HDT + dDAVP, experimental nondiuretic rats). In the presence of dDAVP, the reabsorption of Cl, Na, Mg, and Ca by the superficial loop of Henle was significantly increased, as previously observed in HD-untreated rats during acute infusion of dDAVP. The effects on the superficial distal tubule were also similar. The effects on K, however, were different both in the loop and in the distal tubule. At the bend of the juxtamedullary nephrons, the treatment alone (HDT rats) increased fractional delivery (FD%) of Na and Cl, whereas FD% of Mg, Ca, K, and P was unaltered. In HDT + dDAVP rats, FD% of H2O, Cl, Na, and Ca was significantly lower than in HDT rats, and FD% of K, Mg, and P did not differ significantly. In conclusion, in the presence of dDAVP, the FD% of H2O, Na, and Cl at the bend of the long-loop nephrons decreases, in accordance with our previous hypothesis that water removal along the rat descending limb increases outward NaCl diffusion along this segment.