The present study aimed at determining the effect of cell volume changes on protein synthesis, measured as the incorporation of [(3)H]leucine into acid-precipitable protein, in isolated hepatocytes of air-breathing walking catfish (Clarias batrachus). The rate of protein synthesis, which was recorded to be 10.02 +/- 0.10 (n = 25) nmoles mg(-1) cell protein h(-1) in isotonic incubation conditions, increased/decreased significantly by 18 and 48%, respectively, following hypo- (-80 mOsmol l(-1))/hypertonic (+80 mOsmol l(-1)) incubation conditions (adjusted with NaCl), with an accompanying increase/decrease of hepatic cell volume by 12 and 20%, respectively. Similar cell volume-sensitive changes of protein synthesis were also observed when the anisotonicity of incubation medium was adjusted with mannitol. Increase of hepatic cell volume by 9%, due to addition of glutamine plus glycine (5 mM each) to the isotonic control incubation medium, led to a significant increase of protein synthesis by 14%. Decrease of hepatic cell volume by 15 and 18%, due to addition of dibutyl-cAMP and adenosine in isotonic control incubation medium, led to a significant decrease of protein synthesis by 30 and 34%, respectively. Thus, it appears that the increase/decrease of hepatic cell volume, caused either by changing the extracellular osmolarity or by the presence of amino acids or certain other metabolites, leads to increase/decrease of protein synthesis, respectively, and shows a direct correction (r = 0.99) between the hepatic cell volume and protein synthesis in walking catfish. These cell volume-sensitive changes of protein synthesis probably help this walking catfish in fine tuning the different metabolic pathways for better adaptation during cell volume changes and also to avoid the adverse affects of osmotic stress. This is the first report of cell volume-sensitive changes of protein synthesis in hepatic cells of any teleosts.