CARDIOVASCULAR-RESPONSE TO TRIIODOTHYRONINE IN SPRAGUE-DAWLEY AND SPONTANEOUSLY HYPERTENSIVE RATS

In this study we have compared the influence of triiodothyronine (T3) on cardiovascular function and heart weight in Sprague-Dawley and spon taneously hypertensive rats. We also investigated whether the effects induced by T3 are reversible. T3 was administered daily (0.2 mg/kg sub cutaneously) for 14 days in female Sprague-Dawley and spontaneously hy pertensive rats. Treatment was then stopped for 14 days. At the end of the treatment and of the period without T3, hemodynamic measurements were made in the intact, anesthetized animals by catherization of the left ventricle with a Millar ultraminiature catheter pressure transduc er. Cardiac output was measured by thermodilution. The development and regression of myocardial hypertrophy was measured by the ratio of hea rt to body weight. The systolic pressure in the left ventricle was hig her in spontaneously hypertensive rats than in Sprague-Dawley rats mat ched for body weight. At the end of treatment with T3, the heart rate was increased to exactly the same extent in both groups. Left ventricu lar systolic pressure was increased by 15 % in Sprague-Dawley rats, bu t was not altered significantly in spontaneously hypertensive rats. T3 induced an increase in left ventricular dp/dt(max) by 106 % in Spragu e-Dawley rats, but by only 32 % in spontaneously hypertensive rats, Ca rdiac output increased by 63 % in Sprague-Dawley rats, but by only 31 % in spontaneously hypertensive rats (statistically not significant). When T3 treatment was stopped for 14 days, all the functional changes returned to control values. T3 induced an increase in the ratio of hea rt to body weight by 57 % in Sprague-Dawley rats, but by only 38 % in spontaneously hypertensive rats, which bad already developed a signifi cant degree of cardiac hypertrophy at the beginning of T3 treatment. T hus, except for the heart rate, the functional and growth response of the heart to T3 was attenuated in spontaneously hypertensive rats.