Role of the renin-angiotensin system in the development thyroxine-induced hypertension

Objective: We evaluated the influence of chronic blockade of the renin-angiotensin system on hypertension induced by long-term thyroxine (T-4) administration. To this end, we determined the effects of chronic treatment with captopril on blood pressure, cardiac hypertrophy and other renal and metabolic variables of hypertensive hyperthyroid rats. Methods: T-4 was administered s.c. at 0.38 mu mol/kg per day and captopril was given in the drinking water (1.38 mmol/l). Both treatments were maintained for 6 weeks. Control rats received tap water, After the treatment period, the rats were placed in metabolic cages. Later, blood pressure was measured in conscious rats by intra-arterial determination. Results: T-4-treated rats showed an increased mean arterial pressure (MAP) whereas, in rats treated with T-4 plus captopril, MAP was similar to that of the control group, Captopril did not affect the increased heart rate or ventricular weight/body weight ratio of hyperthyroid rats, but it improved the reduced creatinine clearance of these animals. Conclusions: The elevation in blood pressure produced by long-term T-4 administration was prevented by chronic blockade of the renin-angiotensin system, Captopril improved the renal function of hyperthyroid rats, but did not affect the relative cardiac hypertrophy of these animals.