Effects of cortisone on orotic acid transport and RNA synthesis in rat liver

Abstract

The present studies evaluated hormonal effects upon cellular transport processes and upon pyrimidine nucleotide and RNA metabolism. Adrenalectomized rats which had received a single dose of 5 mg cortisone acetate per 100 g body weight intraperitoneally were sacrificed at various times thereafter; in each instance, radioactive orotic acid was administered 20 min prior to sacrifice and the total radioactivities as well as the specific activities of the total acid-soluble UMP fraction, RNA, and RNA-uracil were determined. It was found that, in addition to glucocorticoidal acceleration of nucleotide incorporation into RNA (representing RNA synthesis), over half of the hormonally enhanced incorporation into RNA reflect the increased specific activities of the uracil precursors of the acid-soluble pool. Furthermore, indirect evidence indicates that the hormonally induced increase in the specific activities of the acid-soluble uridine nucleotides was due neither to a change in the size of the pool nor to an increased rate of conversion of 3H-orotate to uracil nucleotides, but rather was due to an hormonally enhanced transport of the 3H-orotate precursor into the liver. Studies on the rate of disappearance of various labeled RNA precursors from the acid-soluble fraction indicate that the radioactivity of orotic acid had a half-life of about 8 hr for cortisone-treated animals, and 12 hr for controls. On the other hand, the half-life for the disappearance of radioactivity of uridine and guanine were only 30 and 70 min respectively, and were independent of hormone treatment. Orotic acid is, therefore, most useful in long-term incorporation studies; whereas, uridine and guanine, essentially pulse-labeling in nature, are best utilized for RNA turnover studies. With identical 20-min pulse-labeling incorporation periods, the incorporation efficiency into RNA for orotic acid was 15-fold that of uridine and 5-fold that of guanine.