Effect of thyroid hormone on cyclic AMP phosphodiesterase in rat kidney

The effect of thyroid hormone on the activity of low Km cyclic AMP) phosphodiesterase of the particulate fraction from the kidney was investigated. This enzyme activity increased in the cortex from hypothyroid rats and reverted to the normal level after the subcutaneous administration of L-3,5,3'-triiodothyronine (T3) daily for 3 days. However, this hormone did not affect the enzymes from the normal rats. In vitro experiments revealed that T3 as well as various known phosphodiesterase inhibitors similarly inhibited the activity of low Km cyclic AMP phosphodiesterase from both normal and hypothyroid rat kidneys. The results of the present study suggest that the effect of thyroid hormone on cyclic AMP phosphodiesterase was not direct but was mediated by a modulation of the rate of protein synthesis and an unknown mechanism in the kidney.