Selective stimulation by tri-iodothyronine of myocardial deoxyribonucleic acid polymerase-alpha in neonatal rats

Three forms of DNA polymerase (alpha, beta and gamma) were separated from isolated rat myocardial cells on the basis of template, pH and ionic requirements, sensitivity to N-ethylmaleimide and position on sucrose gradients. Tri-iodothyronine administration (20mug/100g intraperitoneally) to 3-week-old rats resulted in selective stimulation of DNA polymerase-alpha (198+/-7.1 versus 102+/-5.8pmol of [(3)H]dTMP/30min per mg of protein in untreated controls, P<0.01), with no change in polymerases-beta and -gamma. [(3)H]Thymidine incorporation into myocardial DNA was also enhanced in tri-iodothyronine-treated neonatal rats (132+/-11.2 versus 53+/-4.1c.p.m./mug of DNA in controls, P<0.001). Increased incorporation was associated with an expansion of deoxyribonucleoside 5'-triphosphate pools, especially that of dTTP (24+/-1.6 versus 10+/-1.1pmol/mg of DNA, P<0.01). Neither DNA polymerase activities nor [(3)H]thymidine incorporation were changed in 6-month-old rats in response to tri-iodothyronine. Unstimulated adult myocardial cells had DNA polymerase activities comparable with those in 3-week-old animals, but significantly lower [(3)H]-thymidine incorporation and deoxyribonucleoside triphosphate concentrations. Enhancement of both DNA polymerase-alpha activity and [(3)H]thymidine incorporation in tri-iodothyronine-treated young rats was prevented by concomitant administration of either vinblastine (1mug/g) or daunomycin (2mug/g); actinomycin D (0.1mug/g) or cycloheximide (8mug/g), on the other hand, prevented the increase in [(3)H]thymidine incorporation, but not DNA polymerase-alpha activation. These results demonstrate an age-dependent stimulation of myocardial DNA replication by tri-iodothyronine and suggest an inter-relationship between DNA synthesis and subsequent entry into mitosis.