Differential Regulation of Slow-skeletal and Cardiac Troponin I mRNA during Development and by Thyroid Hormone in Rat Heart

We have examined mRNA levels for the cardiac troponin I (cTnI) and slow-skeletal (ssTnI) in perinatal rat hearts. Northern blots showed that hypothyroidism was associated with a delay in the expected isoform switching. RNA slot blots showed a six-fold increase in cTnI mRNA from day 3 to day 21 in hearts from postnatal euthyroid rats compared to a three-fold increase in cTnI for the same period in the hypothyroid hearts. On the other hand, ssTnI mRNA levels were higher after 3 days in hearts from the hypothyroid animals and fell to undetectable levels after 21 days. In euthyroid hearts ssTnI was not detectable after 14 days and was not re-expressed in adult hearts. In the ventricles from 28-day-old animals the most significant differences in cTnI mRNA levels were between the euthyroid and T3-treated hypothyroid preparations and in the 87 to 125-day-old group between euthyroid and hypothyroid ventricles. T3 treatment of the 87 to 125-day-old hypothyroid animals did not increase the cTnI mRNA above euthyroid levels despite elevated serum T3. These results show that thyroid hormone influences expression of the cTnI isoform in postnatal and young adult rats, but not to the same extent in animals greater than 28 days of age.