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The mechanism of isoproterenol and N6,O2-dibutyryl adenosine 3':5'-monophosphate (dibutyryl cAMP) induction of lactate dehydrogenase A subunit mRNA (mRNALDH) was investigated in the rat C6 glioma cell line. During the induction phase the concentration of nuclear mRNALDH sequences increased about 2.5-fold 4 h after the addition of isoproterenol or dibutyryl cAMP. Analysis of nuclear 32P-labeled mRNALDH sequences showed that isoproterenol or dibutyryl cAMP increased the basal rate of in vitro mRNALDH transcription about 3.6-fold within 4 h. The relative rates of in vivo mRNALDH synthesis were additionally measured by pulse-labeling of glioma cells for 15 min with 3Huridine. The induction of mRNALDH in intact glioma cells by isoproterenol and dibutyryl cAMP was quantitatively comparable to that observed in isolated nuclei and the relative rate of 3Huridine incorporation into mRNALDH was maximal 4 to 5 h after the initial induction stimulus. Increased synthesis of mRNALDH in vivo as well as in isolated nuclei occurred only at isoproterenol concentrations that caused elevated levels of glioma cell cAMP. Analysis of the kinetics of decay of 3Huridine-labeled mRNALDH showed a linear rate of decay of non-induced mRNALDH with a t1/2 of 45 min. After isoproterenol stimulation mRNALDH decayed as two populations, one with a t1/2 of 50 min and the other one with a t1/2 of 2.5 h. These results indicate that both isoproterenol and dibutyrl cAMP regulate not only the rate of transcription of mRNALDH but that the stability of mRNALDH is increased during the induction phase.
Jungmann et al. (Fri,) studied this question.