Synthesis and Degradation of Liver Ribosomal RNA in Fed and Fasted Rats

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Abstract
The turnover of liver ribosomal RNA in fed and fasted rats was examined by observing the rate of loss of radioactivity over a period of several days after injection of 3H-orotic acid. In fed rats, the fractional catabolic and synthetic rates were 14 and 16% per day, respectively. During starvation, degradation of RNA was at first accelerated to about 50% per day during which time synthetic rate was also increased, but after 48 hours of fasting the fractional rate of degradation fell to 10% per day. This drop in degradation was accompanied by a greater fall in synthesis so that fasted animals showed a slow but continuous loss of liver RNA. Coincident with the phase of extensive loss of RNA in fasted rats was the virtually complete disaggregation of liver polysomes as well as a marked increase in membrane-free ribosomes. The results indicate that a) synthesis of liver RNA is not depressed by fasting until significant losses of RNA and polysomes have occurred, b) the effect of fasting on degradative rate varies with the duration of starvation, and c) ribosomal RNA turnover in rat liver is in part regulated through changes in the population of membrane-free ribosomes and subunits, the abundance of which is affected by food deprivation.