Effects of hyper- and hypothyroidism on the basal levels of angiotensin I and kinetic parameters of renin-angiotensin system in male rats.

The basal levels of angiotensin I and kinetic parameters of renin-angiotensin system were studied under control, hyper- and hypothyroidism conditions. The serum levels of triiodothyronine (T3) and thyroxine (T4) and plasma angiotensin II have been measured radioimmunoassay. Hyperthyroidism was induced by 5.5 micrograms/200 g of T3 or 100 micrograms/200 g of T4 administration for 12 days, and hypothyroidism by propylthiouracil (PTU) administration of 1 mg/200 g for 12 days. Basal levels of angiotensin I and plasma renin activity (PRA) increased after T3 injection, were not altered by T4 and decreased after PTU administration. T3 and T4 induced an increase in plasma renin concentration (PRC), while PTU induced a decrease in PRC. Plasma renin substrate (PRS) decreased in hyperthyroid rats and was unchanged by experimentally induced hypothyroidism. A good correlation between T3 serum levels and PRA was found, but there was no such correlation between T4 serum levels and PRA.