Effects of theophylline on renal function in premature infants.

Xanthines are frequently being used in the management of premature babies. Studies in adult subjects have demonstrated a diuretic effect of aminophylline due to the inhibition of solute reabsorption in various segments of the nephron. We evaluated the effects of aminophylline on the developing kidney. Nineteen premature infants, with a mean +/- SD gestational age of 31.1 +/- 2.8 weeks and mean birth weight of 1481 +/- 454 g were studied at mean age of 4.5 +/- 4.0 days before and after a 20-minute loading infusion of 6 mg/kg aminophylline, followed by maintenance therapy at a dose of 2 mg/kg every 12 hours. A marked diuresis occurred immediately after the loading dose, the ratio of urinary output to water intake increased from 0.58 +/- 0.36 to 1.19 +/- 0.65. Concomitantly, the fractional excretion of sodium increased from 2.7 +/- 2.6% to 5.7 +/- 4.4% and that of potassium rose from 21 +/- 19% to 31 +/- 21%. Urinary calcium and uric acid excretion were also enhanced: calcium to creatinine ratio rose from 0.31 +/- 0.29 to 0.60 +/- 0.54 and uric acid to creatinine ratio increased from 2.5 +/- 1.5 to 3.8 +/- 2.0. Tubular reabsorption of phosphorus (TRP) was not affected. Most of the effects were no longer evident after 24 hours, despite continuing aminophylline maintenance therapy. In premature infants the aminophylline loading dose, but not maintenance therapy, affected renal functions. Because heart rate, blood pressure, and creatinine clearance did not change, it appears that aminophylline acts directly on tubular reabsorptive functions.