Levels of ornithine aminotransferase messenger RNA under conditions of cyclic AMP induction in cultured hepatocytes.

In primary cultures of adult rat hepatocytes, the relative rate of ornithine aminotransferase synthesis is increased approximately 7- to 8-fold in response to dibutyryl cyclic AMP. The corresponding increase in ornithine aminotransferase messenger RNA under these conditions was observed to be only 2- to 3-fold. The levels of ornithine aminotransferase mRNA were measured by hybridization of cellular RNA to a cloned cDNA probe. The ribosomal transit time (20-25 min) along ornithine aminotransferase mRNA was the same in both control and induced cultures. The level of ornithine aminotransferase nascent chains in cAMP-induced cultures was increased 7-fold over the level in control cultures. In the face of a 2- to 3-fold increase in the amount of ornithine aminotransferase mRNA, this suggests an approximately 3-fold increase in the rate of initiation per copy of ornithine aminotransferase messenger RNA. These results indicate that cAMP may regulate ornithine aminotransferase synthesis at both transcriptional and translational levels in cultured hepatocytes.