Comparison of renal actions of urodilatin and atrial natriuretic peptide

A 32-amino acid atrial natriuretic peptide (ANP)-like peptide, putatively synthesized by the kidney, has recently been isolated from human urine. This peptide, urodilatin (Uro), is structurally similar to the 28-amino acid ANP, suggesting that they might have similar actions on renal fluid and electrolyte excretion. The purpose of this study was to characterize the direct renal actions of low doses of Uro infusion and to compare them with the effects of equimolar intrarenal infusions of either ANP or the 24-amino acid atriopeptin III (AP III). Synthetic Uro was infused into the renal artery of pentobarbital sodium-anesthetized mongrel dogs (n = 8) at 0.14, 0.28, and 1.43 pmol · kg^-1 · min^-1 while renal perfusion pressure was servo-controlled at 100 mmHg. Uro infusion at 1.43 pmol · kg^-1 · min^-1 increased sodium excretion from an average control of 57.4 ± 10.1 to 159.0 ± 24.4 μeq/min. Uro infusion at the highest dose also increased potassium excretion (28.0 ± 4.5 vs. 40.4 ± 7.4 μeq/min), chloride excretion (56 ± 11 vs. 155 ± 22 μeq/min), and urine volume (0.54 ± 0.12 vs. 1.22 ± 0.25 ml/min). Fractional lithium excretion, a marker for proximal tubular sodium reabsorption, was not altered by Uro infusion, nor were urinary guanosine 3',5'-cyclic monophosphate excretion, glomerular filtration rate, or effective renal plasma flow changed. Equimolar infusions of these low doses of either α-human ANP (n = 6) or AP III (n = 8) had no effect on any of the measured variables. Thus, within the range of doses used in this study, Uro was a more effective natriuretic and diuretic agent than either ANP or AP III. Furthermore, the increase in renal fluid and electrolyte excretion with no changes in either renal hemodynamics or fractional lithium excretion supports the hypothesis that Uro inhibits tubular sodium reabsorption at a site beyond the proximal tubule.