Aortic stenosis produces hypertrophy of the rat heart without causing an accumulation of N1-acetylspermidine.

Isoprenaline treatment causes cardiac hypertrophy and an accumulation of N1-acetylspermidine in the rat heart. To determine whether the cardiac hypertrophy is the cause of the increase in N1-acetylspermidine, we produced cardiac hypertrophy by constriction of the aorta and analyzed polyamines in the hearts of these rats 1, 3, and 10 days after the aortic constriction. Our results show that compared to sham-operated animals, this treatment caused a 60% increase in putrescine and a 30% increase in spermidine by day 10, but not the expected increase in N1-acetylspermidine. We conclude that N1-acetylspermidine is not induced by a cardiac overload.

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.

Heating RNA before cell-free translation gives no selective increases among the translation products of RNA from rat heart and mammary gland, rabbit reticulocyte membranes, or trout liver.

A recent study has reported that the heating of a population of guanidinium-extracted mRNAs prior to translation causes a selective increase in the translation of certain mRNAs. To determine if this phenomenon is a general property of mRNAs, we carried out a comparison of the translation products obtained when phenol-extracted rat heart and mammary gland RNAs, rabbit reticulocyte membrane RNAs, and trout liver RNAs were translated in the reticulocyte translation system, with and without a prior heat treatment. Our results show that no selective increase in the translation of mRNAs was observed for any of these samples. Among the 14 RNA preparations examined, one total mammary RNA preparation did display a twofold increase in the translation of all mRNAs after heat treatment. It is shown that the heat enhancement of translational activity observed for this sample was due to the reversible formation of intermolecular aggregates with a contaminant that can be removed by chromatography on oligo(dT)-cellulose. Since heat treatment did not selectively enhance the activity of any mRNA in these samples, our results show that the current practice of translating phenol-extracted RNAs without a prior heat treatment should be satisfactory for the translation of most mRNA populations.

N1-monoacetylation abolishes the inhibitory effect of spermine and spermidine in the reticulocyte lysate translation system.

At optimum magnesium, the translation of rat heart mRNA in the nuclease treated rabbit reticulocyte lysate system was inhibited by low concentrations of spermidine or spermine but not of putrescine. Spermidine and spermine cause a general reduction in the translation of all the heart mRNAs since no differential effects were observed when the translation products were examined by gel electrophoresis. Spermine was a five times more potent inhibitor than spermidine but no inhibition was obtained with N1-acetylspermidine or N1-acetylspermine. Since analyses of endogenous polyamines demonstrate that the inhibitory concentrations of spermine could be obtained by converting a small fraction of the endogenous spermidine to spermine, these results indicate that interconversions of the polyamines might be a sensitive regulatory mechanism for protein synthesis.