Metabolic effects of aminophylline in weanling rats.

Methylxanthines are commonly used to treat asthma and apnea in infants and children. A physiological effect of these compounds is an elevation of the metabolic rate, but the mechanism of this increase is unclear. We have investigated the hypothesis that this elevation of metabolic rate in young animals is in part due to increased physical activity. Metabolic rate and spontaneous physical activity of weanling Fischer 344 rats were measured before and during 2 days of aminophylline administration. Our results show increased metabolic rate measured over 23 h, concomitant with increased activity during treatment. Resting metabolic rate was not elevated. The results suggest that increased physical activity plays a major role in increasing metabolic rate during aminophylline treatment. Metabolic rate returned to control levels on the 2nd day of treatment despite a smaller but still significant elevation in physical activity. This suggests development of tolerance to the effects of aminophylline on activity together with adaptation to the metabolic effects of this drug.