Maintenance theophylline therapy in children: Effect on urinary calcium, phosphate and cyclic AMP excretion

Eleven children, five females and six males, aged 3 to 15 years, were included in the study. They had suffered from chronic asthma for periods of 1 to 8 years and had been on long-term therapy with theophylline for 3 to 12 months. Theophylline was taken orally as Theo-Dur (Key Pharmaceuticals) in two daily doses of 7 to 14 mg/kg per dose. Serum levels were monitored bimonthly and were maintained at 10 to 20 μg/ml. Each child served as his own control. Pretreatment values were taken for serum calcium, serum phosphorus, serum creatinine, serum cyclic AMP, urinary calcium and phosphate, urinary creatinine and urinary cyclic AMP. During the study period, the patients were given a constant calcium diet of 800 mg/m² per day for 7 days. On the seventh day of the diet, urine was collected for 24 hours, and blood was drawn at 9 o'clock on the next morning. Plasma and urinary cyclic AMP were determined by a competitive protein-binding assay, (4) and nephrogenous cyclic AMP was calculated according to Broadus et al. (5). The intra- and interassay coefficients of variation were 4.6% and 7.1%, respectively.