Cardiac mRNA levels during the development and growth of the rat

Developmental variations of RNA content and of total mRNA activity were examined in fetal, neonatal and adult rats. The amount of cardiac RNA extractable with the LiCl urea extraction procedure decreased from 3 mg/g tissue in the 18-day-old fetal heart to 1 mg/g in the 30-day-old heart and then to 0.5 mg/g in 100-day-old animals. Translations in the reticulocyte lysate translation system showed a similar pattern for total mRNA activity. It is shown that over this entire period both RNA content and mRNA activity vary as an exponential function of the growth rate of the heart and an explanation for this relationship is proposed. An examination of the translation products of the abundant mRNAs shows that the development of the heart is characterized by a general reduction in all mRNAs.