Comparison of hepatic protein synthesis in vivo versus in vitro in the tumor-bearing rat.

We have used the model of the freshly isolated hepatocyte to study liver metabolism in a rat tumor model and have reported elevations in the rates of protein synthesis, gluconeogenesis, alanine transport, and oxygen utilization by the tumor-influenced hepatocyte. While the metabolic activity of the isolated hepatocyte has been taken to reflect metabolism in the in vivo state, this assumption has not been validated. In the present study, we measure hepatic protein synthesis in vivo using a flooding dose of tracer amino acid, and in vitro in hepatocytes freshly isolated from tumor-bearing (TB) and pair-fed control rats. Increased protein synthesis was observed for the TB rats using both methods of analysis. However, the degree of stimulation seen in the TB animals was much greater in the in vitro assay than in the in vivo approach, suggesting that absolute protein synthetic rates in vitro must be interpreted with caution when extrapolating to the in vivo state.