Effects of phenylephrine and isoproterenol on the activity of cyclic AMP-dependent protein kinase of hypothyroid rat tissues.

The present study was designed to examine the properties of cyclic AMP-dependent protein kinase from normal and hypothyroid rat tissues. The activity of the liver enzyme decreased in hypothyroid rats and this reduced activity may be due to the decrease in synthesis of enzyme protein. The enzyme activity from the kidney of hypothyroid rats increased by 35% in the absence of cyclic AMP. Further, the enzyme from the kidney of hypothyroid animals responded to phenylephrine and isoproterenol. In the heart, the basal enzyme activity was the same in both groups, but the enzyme from the heart of hypothyroid animals did not respond to isoproterenol. When hypothyroid rats were treated with 3,3',5-triiodothyronine, the enzyme activities from those rat tissues showed essentially the same level as those of the corresponding normal ones. These results suggest that thyroid hormone regulates adrenergic agonists-mediated responses both at the sites of adrenoceptors and that of protein phosphorylation. Furthermore, the dependence on thyroid hormone varies according to the tissue.