The effect of triiodothyronine on the cardiac mRNA

Abstract

Cardiac hypertrophy was induced in rats by daily injections of L-triiodothyronine (0.2 mg/kg body wt, s.c., 1 to 3 days). Regression of hypertrophy was studied after 3 days of injection for a period of 1 to 16 days. Cardiac RNA was isolated by phenolextraction, mRNA by affinity chromatography of cardiac total RNA on oligo-dT-cellulose. In addition, cardiac mRNA was also quantitated by hybridization to 3H-Poly(U). After 3 days of T 3-treatment, the extractable amount of cardiac total RNA increased from 759 ± 181 μg/g heart (controls) to 980 ± 119 μg/g heart (T 3-rats), and that of mRNA from 24.7 ± 2.0 (controls) to 39.4 ± 7.0 (T 3-rats), increases of 29.1% (total RNA) and 59.5% (mRNA). 16 days after cessation of T 3-treatment, total RNA and mRNA levels had returned to normal. The increase in cardiac mRNA in T 3-rats was investigated in further detail by in vitro translation of the isolated mRNA in the mRNA-dependent reticulocyte lysate system. Incorporation of 35S-methionine into total protein was identical for cardiac mRNA from control and T 3-rats. Separation of the in vitro synthesized proteins on SDS-Polyacrylamide-Slabgels revealed the complete synthesis of actin, troponin-T, tropomyosin, myoglobin and myosin-light-chains 1 and 2. The mRNAs coding for these proteins represented identical fractions of total mRNA in control and T 3-rats. This leads to the conclusion that T 3 had stimulated the synthesis of all of these specific mRNAs in an unselective manner. From our data and from the results of other authors we conclude that changes in cardiac mRNA levels are the major regulatory factor in causing changes in cardiac protein synthesis rates leading to cardiac hypertrophy in hyperthyroidism.