Tubular reabsorption rates as related to elevated glomerular filtration in diabetic children.

To study renal tubular reabsorption and tubulo-glomerular balance in diabetic children, glomerular filtration rate (GFR) and tubular reabsorption rates of sodium, glucose, ultrafilterable calcium, and phosphate were measured during fasting in 26 ambulatory type I (insulin-dependent) diabetic children without clinical signs of microangiopathy (age 7-14 yr; duration of diabetes 3-14 yr). Similar measurements were made in 28 healthy school children (age 8-14 yr). Mean GFR in the diabetic children was significantly higher than in the normal children (138 versus 109 ml/min/1.73 m2, P less than 0.01). Mean tubular reabsorption rates of sodium, glucose, and calcium were significantly increased in the diabetic subjects (P less than 0.001). In contrast, tubular reabsorption rate of phosphate in the diabetic subjects was not enhanced. The renal threshold concentration of phosphate (TmPO4/GFR) was suppressed in the diabetic compared with the healthy subjects (1.23 versus 1.73 mmol/L, P less than 0.001). TmPO4/GFR was unrelated to circulating parathyroid and growth hormone concentrations but correlated inversely with the reabsorption rate of glucose (r = -0.53, P less than 0.01). Sodium reabsorption was closely correlated to GFR in both diabetic (r = 0.99, P less than 0.0001) and healthy subjects (r = 1.00, P less than 0.0001), and both groups showed identical regression lines. The tubular glucose reabsorption rate was independent of GFR in the diabetics. Tubular calcium and phosphate reabsorptions correlated equally well with sodium reabsorption and with GFR in the diabetic and healthy subjects (P less than 0.001). The maximal reabsorption of phosphate relative to GFR was lowered in the diabetic children.(ABSTRACT TRUNCATED AT 250 WORDS)