1,2-Diacylglycerol (DAG) is an intracellular signal mediator that may initiate
protein synthesis and
cardiac hypertrophy via activation of
protein kinase C. The amounts of 1,2-DAG and its
fatty acid components in the myocardium was assessed and compared with the concentrations of
RNA and
DNA in
cardiac hypertrophy induced by administering
triiodothyronine (T3) with and without
cycloheximide, an inhibitor of
protein synthesis. After the first injection of T3 no
cardiac hypertrophy was observed, and there were no differences in myocardial 1,2-DAG content or in
RNA and
DNA concentrations.
Cardiac hypertrophy was present on days 3 and 7 after a daily injection of T3. Myocardial
RNA concentration was increased by 26% on day 3 and by 34% on day 7, whereas the myocardial 1,2-DAG content was decreased by 8% on day 3 and by 24% on day 7. Pretreatment with
cycloheximide of T3-treated rats prevented the development of
cardiac hypertrophy, but elevated the
RNA concentration and lowered the 1,2-DAG content compared with the findings in T3-treated rats. Analysis of the
fatty acid components of 1,2-DAG revealed that the amounts of 16:0, 18:1 and 18:2 were decreased, accompanied by an elevation of
RNA concentration and a decrease in 1,2-DAG content. It seems that
RNA synthesis preceded the alteration in 1,2-DAG. These findings suggest that 1,2-DAG is not involved in the initiation of
cardiac hypertrophy induced by T3, but is affected by the enhanced concentration of
RNA resulting from the introduction of
thyroid hormones into the myocardium.