Reversal by theophylline of some changes characteristically accompanying hypothyroidism in rats

Chronic dietary administration of theophylline (0.625, 1.25 or 2.50 g/kg food) for 7--9 weeks prevented the decrease in resting oxygen consumption, heart rate and systolic blood pressure characteristically observed in rats treated simultaneously with either of the antithyroid drugs, aminotriazole (ATZ; 0.25 g/kg food) or propylthiouracil (1.0 g/kg food). However, theophylline failed to restore both the food intake and growth rate of hypothyroid rats to that of euthyroid controls. Measurements of 131I uptake by the thyroid gland, thyroid acinar cell height and protein-bound iodine failed to reveal an effect of theophylline in either control or hypothyroid animals although theophylline increased thyroid to body weight ratio in both groups. Thus, the restoration of rate of oxygen consumption, heart rate and systolic blood pressure of hypothyroid rats to that of euthyroid controls was apparently not exerted through an increase in thyroid activity. The reduction in rate of oxygen consumption, heart rate and systolic blood pressure, generally associated with lack of thyroid hormone, appear more likely to be associated with reduced responsiveness to catecholamines in the hypothyroid rats. The results suggest that an increase in the half-life of cAMP by administration of theophylline to hypothyroid rats returned these functions to the level of euthyroid controls.