Adenine nucleotides and inhibition of protein synthesis in isolated hepatocytes incubated under different pO2 levels.

Hepatocytes incubated at a pO2 of 0 mm Hg (N2/CO2, 95%/5%) loose their intracellular ATP content and their ability to synthesize RNA and proteins. Protein synthesis is virtually inhibited from the beginning of the incubation, while ATP content is gradually lost, thus suggesting a primary response of the cell to the absence of O2 rather than to ATP depletion. Such an early decrease of protein synthesis (as estimated as the incorporation of [14C]Leu into cell proteins) is unlikely the result of inhibition of amino acids uptake, enhanced protein degradation, or decreased RNA synthesis. Reoxygenation of such previously hypoxic cells with O2/CO2 at 95%/5% (pO2 of 700 mm Hg), leads to the recovery of both ATP and protein synthesis, even better the hypoxic period is not longer than 30 min. In hepatocytes incubated for 30 min under a pO2 of 700, 80, or 50 mm Hg, cell survival and ADP content are almost identical. Incorporation of radiolabelled leucine is linear in cells incubated under 700 mm Hg O2, but it rather stops at a pO2 of 80 or 50 mm Hg. The time course of both ATP and GTP content behaves in a similar way: it is fairly constant at a pO2 of 700 mm Hg, but a depletion is initiated after 20 min of incubation at a pO2 of 50 or 80 mm Hg. Finally, incubation of hepatocytes either at 700 or 0 mm Hg O2, in the presence of fructose (10 mM), shows that ATP content is maintained at the same level whatever the pO2 level. AMP content is increased only in cells incubated at 0 mm Hg O2 in the absence of fructose. Incorporation of radiolabelled leucine is stopped in such hypoxic cells incubated or not in the presence of fructose. From these results it appears that the presence or the absence of O2 might represent a turn on/off signal to which hepatocytes respond immediately by important metabolic changes like the inhibition of protein synthesis.