Local renin-angiotensin system contributes to hyperthyroidism-induced cardiac hypertrophy

We have reported previously that thyroid hormone activates the circulating and tissue renin-angiotensin systems without involving the sympathetic nervous system, which contributes to cardiac hypertrophy in hyperthyroidism. This study examined whether the circulating or tissue renin- angiotensin system plays the principal role in hyperthyroidism-induced cardiac hypertrophy. The circulating renin-angiotensin system in Sprague- Dawley rats was fixed by chronic angiotensin II infusion (40 ng/min, 28 days) via mini-osmotic pumps. Daily J.p. injection of thyroxine (0 · 1 mg/kg per day, 28 days) was used to mimic hyperthyroidism. Serum free tri- iodothyronine, plasma renin activity, plasma angiotensin II, cardiac renin and cardiac angiotensin II were measured with RIAs. The cardiac expression of renin mRNA was evaluated by semiquantitative reverse transcriptase-polymerase chain reaction. Plasma renin activity and plasma angiotensin II were kept constant in the angiotensin II and angiotensin II+thyroxine groups (0 · 12 ± 0 · 03 and 0 · 15 ±0 · 03 μg/h per liter, 126 ± 5 and 130 ± ng/l respectively) (means ± S.E.M.). Despite stabilization of the circulating renin-angiotensin system, thyroid hormone induced cardiac hypertrophy (5 · 0 ± 0 · 5 vs 3 · 5 ± 0 · 1 mg/g) in conjunction with the increases in cardiac expression of renin mRNA, cardiac renin and cardiac angiotensin II (74 ± 2 vs 48 ± 2%, 6 · 5 ± 0 · 8 vs 3 · 8 ± 0 · 4 ng/h per g, 231 ± 30 vs 149 ± 2 pg/g respectively). These results indicate that the local renin-angiotensin system plays the primary role in the development of hyperthyroidism-induced cardiac hypertrophy.