The effect of starvation and of protein-deprivation on the extractable amount of cardiac mRNA was investigated in male rats. Cardiac mRNA was determined by either (a) isolation of cardiac mRNA by SDS-Phenol/oligo-dT-cellulose, or by (b) hybridization of cardiac mRNA to 3H-Poly(U). During starvation (1-6 days) the extractable amount of cardiac microsomal RNA decreased from 870 micrograms/g heart (controls) to 606 micrograms/g (3 days) and to 547 micrograms/g (6 days), the extractable amount of mRNA fell from 28.6 micrograms/g heart (controls) to 18.7 micrograms/g (3 days) and to 14.5 micrograms/g (6 days). When a normocaloric but protein-deficient diet was fed, the decreases in cardiac microsomal RNA and mRNA were qualitatively similar, but slightly less severe. An analysis of the intracellular distribution of cardiac microsomal RNA and mRNA in the hearts of normal animals and of animals starved or fed a protein-deficient diet indicates that during starvation cardiac mRNA does not accumulate in the cell sap, but gets rapidly degraded. In the refeeding period, mRNA is transported from the nucleus to the cytoplasm and engages in polyribosome formation. The specific mRNA species coding for the major myofibrillar cardiac proteins are affected to a similar extent by these changes during starvation/protein-deprivation and refeeding.
Using a combination of conventional mRNA-extraction methods and newer hybridization techniques, a method was developed which allows the quantitation of cardiac Poly(A)-containing mRNA in cardiac biopsies. This provides a new tool to directly assess changes in cardiac gene expression in cardiac biopsies. It is proposed that the method described may be valuable in investigations dealing with various human or animal heart diseases where changes in cardiac gene expression are expected and where only biopsy material is available.
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