Renal tubular function in cyclosporine-treated patients

Renal tubular function was studied in 32 consecutive patients treated with cyclosporine for autoimmune uveitis. Cyclosporine dosage was modified to maintain the serum creatinine level at less than 2 mg/dl. No evidence of aminoaciduria, glucosuria, hypokalemia, hypophosphatemia, or hypouricemia indicative of the Fanconi syndrome was detected during three to six months of observation. The renal tubular reabsorption of phosphate, determined by renal threshold phosphate concentration (TmPO4/GFR), remained unchanged. An early decrease in the serum magnesium level was accompanied by a decrease in daily urinary magnesium excretion. Concurrent with the development of a mild reduction of the glomerular filtration rate, the fractional excretion of magnesium was slightly higher and that of calcium was lower than pretreatment values. There were modest elevations of serum uric acid and potassium values. Furthermore, lysozymuria was not observed. Thus, the alterations in tubular functions observed in our cyclosporine-treated patients are comparable to those associated with mild chronic renal insufficiency of various causes and are not specific manifestations of cyclosporine tubular toxicity.