Renin-angiotensin system in hypothyroid rats: effects of potassium iodide and triiodo-L-thyronine.

Kinetic studies of the renin-angiotensin system (RAS) were carried out by measuring plasma renin activity (PRA), plasma renin concentration (PRC) and plasma renin substrate (PRS). Changes in this system were studied during hypothyroidism, after administration of propylthiouracil (PTU), and in thyroidectomized rats. A significant decrease in PRA and PRC was observed in those animals previously treated with PTU. However, a significant increase in PRC, and a decrease in PRS, were found in T animals, but no changes in PRA were observed. In these animals, after daily administration of potassium iodide for 1 week (T + KI), no changes in RAS were observed in comparison with T rats. Nevertheless, administration of daily doses of triiodo-L-thyronine (T + T3) induced a significant increase in PRA, leaving PRC unaltered. In this case the changes in PRA were related to the increase in PRS after T3 treatment. These results suggest that two different mechanisms were involved in renin release, one activated in T rats and the other in pharmacological hypothyroidism.