Effects of ammonia and norvaline on lactate metabolism by hepatocytes from starved rats. The use of 14C-labelled lactate in studies of hepatic gluconeogenesis

Abstract

1. Hepatocytes from starved rats were incubated with l-lactate and NH₄Cl or norvaline, and the rates of the tricarboxylic acid cycle and of gluconeogenesis were calculated from changes in metabolite concentrations or from radioisotopic data from incubations with labelled lactate or propionate. 2. Gluconeogenesis was stimulated by the addition of 10mm-NH₄Cl, 5mm-norvaline or 1mm-oleate by 27, 45 and 59% respectively. NH₄Cl or norvaline also increased lactate uptake. Norvaline inhibited urea synthesis from NH₄Cl by 85%. 3. The effects of NH₄Cl and norvaline were not additive. However, NH₄Cl inhibited and norvaline was without effect on gluconeogenesis from pyruvate, indicating that the two compounds act by different mechanisms. 4. The tricarboxylic acid-cycle flux was increased 80% by lactate, and NH₄Cl caused a further 25% stimulation. Norvaline had no effect on the tricarboxylic acid-cycle flux. NH₄Cl and norvaline tripled and doubled, respectively, flux through pyruvate dehydrogenase. 5. Total ATP formation was calculated to range from 470 to 830μmol/h per 100mg of protein, of which the basic metabolic activity accounted for 400–450μmol/h per 100mg of protein. ATP formation does not seem to be rate-limiting for gluconeogenesis. 6. Pyruvate recycling was estimated from the ¹⁴C yield from [1-¹⁴C]propionate in lactate and glucose to be 10–30% of the flux of phosphoenolpyruvate to glucose. The further addition of NH₄Cl more than doubled the recycling of pyruvate. 7. [1,4-¹⁴C]Succinate was rapidly metabolized by hepatocytes. About 20% of the radioactivity was recovered in glucose, indicating that succinate is also metabolized by intact (non-damaged) hepatocytes. 8. It is concluded that the metabolism of lactate by the liver is too complex to allow simple rate measurements with labelled compounds.