[Renal handling of 3,5,3'-triiodothyronine (T3) in the dog studied by stopflow analysis]

In order to investigate the renal handling of 3,5,3'-triiodothyronine (T3), we studied the clearance of T3 (CT3) in dogs and the site of tubular secretion and reabsorption of T3 in dog kidney using the stopflow technique (Malvin et al.). Four female dogs, weighing between 12.2 and 17.8 kg, were used for CT3 measurement. Fourteen anesthetized dogs, weighing between 7.8 and 17.5 kg, were used for the stopflow study. After the catheter was inserted into the left ureter, 15% mannitol solution and isotonic saline containing both 0.2% PSP and 0.5% creatinine or 0.1% inulin, were infused and then 10-30 micrograms/kg of T3 or 100 micrograms/kg of T4 was injected as a bolus. When the urine flow reached a stable state of at least 5 ml/min about one-hr after T3 or T4 injection, the ureteral catheter was clamped shut for 10 min. After the release of the clamp, 20 fractions of urine, 1 ml each, were collected sequentially. The changes in pH and PSP concentrations were used as indices of urine from the distal and proximal tubules, respectively. Urinary T3 was determined by RIA. CT3 was obtained by calculating the ratio of the 24-hr urinary T3 excretion to the serum free T3 concentration. CT3, 51.9 +/- 12.3 ml/min, was greater than the clearance of creatinine (Ccr), 23.8 +/- 4.7 ml/min, suggesting that T3 is secreted at the tubules in dogs. Almost immediately after the release of the clamped ureter, the concentration of urinary T3, corrected with excreted urinary creatinine or inulin, was increased, reaching the maximum value at No. 2 or 3 fraction. This maximum urinary T3 value was followed by decreased concentrations of urinary T3, reaching the minimum around No. 13--15 fraction. The fraction with the highest urinary T3 concentration was close to the one with the lowest pH, and the fraction with the lowest urinary T3 concentration was close to the one with the highest PSP concentration. These data suggest that T3 might be reabsorbed or metabolized at the level of the proximal tubules and secreted into the urine at the level of the distal tubules.