The Effect of Vasopressin on Phosphate Transport in the Proximal Tubule of the Dog

Recollection micropuncture study was performed in 11 thyroparathyroidectomized dogs during antidiuresis to determine the effect of continuous vasopressin infusion at 50 mU/kg/h on proximal tubule phosphate and sodium transport. The animals were divided into two groups according to changes in mean arterial blood pressure. In the first group (five dogs) with increased blood pressure and glomerular filtration rate (GFR), mean proximal tubule fluid-to-plasma inulin ratio fell significantly from 1.69 to 1.53, whereas it remained unchanged at 1.60 in the second group (six dogs) with no change in blood pressure. In contrast, mean proximal tubule fluid-to-plasma ultrafilterable phosphate ratio increased consistently in both groups, regardless of blood pressure changes. Since natriuresis as well as phosphaturia were observed in all animals, the sodium effect of vasopressin in the distal nephron must be mainly responsible for the natriuresis. It was concluded that vasopressin, when given in the doses employed, inhibits phosphate transport in the proximal tubule and sodium reabsorption in the distal nephron. An additional effect on proximal tubule sodium reabsorption appears to be related to the rise in blood pressure and GFR secondary to vasopressin administration.