Effects of Theophylline on the Serum Calcium of Rats After Parathyroidectomy and Administration of Parathyroid Hormone

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Abstract
In rats parathyroidectomized for 4 days, a single injection of theophylline (120 mg/kg) caused a rapid, marked and relatively long-lasting elevation in serum Ca. Theophylline also retarded the fall in serum Ca which follows acute parathyroidectomy by cautery or by surgical thyroparathyroidectomy. Theophylline had no detectable effect on the serum Ca of intact rats. Bilateral nephrectomy did not alter the effect of theophylline on the serum Ca of parathyroidectomized rats, suggesting that the drug does not depend on a renal action for its hypercalcemic effect. Simultaneous administration of parathyroid extract and theophylline to parathyroidectomized rats resulted in a serum Ca level which was lower than that caused by the extract or drug alone. Since theophylline administration alters the metabolism of cyclic AMP in many tissues, these results suggest that cyclic AMP may be involved in the action of parathyroid hormone and, therefore, in the regulation of blood Ca.