Effects of 10- to 12-day treatment with L-thyroxine in rats with myocardial infarction.

The effects of treatment with L-thyroxine (T4) for 10-12 days on left ventricular (LV) function and myosin isoenzyme content in rats with LV dysfunction after myocardial infarction were studied. Electrocardiogram (ECG) criteria were used to select animals with large myocardial infarctions. These animals were divided into six groups: control and five treatment groups that received daily subcutaneous injections of T4 (1.5, 3, 6, 10, or 15 micrograms/100 g body wt) for 10-12 days. Intravascular pressures were then measured with micromanometer-tipped catheters, and the hearts were analyzed for myosin isoenzyme distribution. The lowest dose of T4 (1.5 micrograms) produced no changes in heart rate, LV pressure or aortic pressure (AoP), myosin isoenzyme composition, or serum T4 levels. Larger doses of T4 (3, 6, 10, and 15 micrograms) produced increases in LV rate of pressure development (dP/dt), heart rate, and T4 levels but did not change LV pressure or AoP. Myosin isoenzyme composition in the control group showed increases in the proportion of the V3 form and decreases in the V1 form compared with the normal pattern. T4 administration produced dose-dependent increases in the V1 myosin isoform and corresponding decreases in the amount of the V2 and V3 forms. Heart rate increases observed with larger doses of T4 could be prevented by addition of propranolol (500 mg/l) to the drinking water. However, in rats treated with T4 and propranolol, LV pressure, AoP, LV dP/dt, and myosin isoenzyme composition were unchanged compared with T4-treated rats.(ABSTRACT TRUNCATED AT 250 WORDS)