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This work aimed to determine the role played by the adrenal gland in the fatty acid control of gluconeogenesis in isolated perfused rat livers. The gluconeogenic substrate concentration responses were not altered in adrenalectomized (ADX) rats. This observation indicates that glucocorticoids are not essential to maintain normal basal gluconeogenic rates. In contrast, fatty acid failed to stimulate gluconeogenesis from lactate and elicited attenuated stimulation with pyruvate as substrate in livers from ADX rats. Fatty acid-induced stimulation of respiration and ketone body production were similar in control and ADX rats. Thus the diminished responsiveness of the gluconeogenic pathway to fatty acid cannot be the result of different rates of energy production and/or generation of reducing power. Fatty acids did not inhibit pyruvate decarboxylation in livers from ADX rats. Even though mitochondria isolated from livers of ADX rats showed normal basal rates of pyruvate metabolism, fatty acids failed to inhibit pyruvate decarboxylation and the activity of the pyruvate dehydrogenase complex. This novel observation of the glucocorticoid effect in controlling the pyruvate dehydrogenase complex responsiveness indicates that the mitochondrial partitioning of pyruvate between carboxylation and decarboxylation reactions may be altered in livers from ADX rats. We propose that the diminished effect of fatty acid in stimulating gluconeogenesis in livers from ADX rats is the result of a limited pyruvate availability for the carboxylase reaction due to a lack of inhibition of flux through the pyruvate dehydrogenase complex.

Fatty acids produced a stimulation of gluconeogenesis and either inhibition or no effect on ureagenesis in livers perfused with gluconeogenic substrates and having NH4Cl plus ornithine as the nitrogen source. This finding indicates that stimulation of flux through pyruvate carboxylase is not sufficient to enhance urea production from ammonia. The metabolic action of fatty acids showed the following characteristics: (1) it was concentration-dependent, showing saturation-type kinetics similar to those described for fatty acid oxidation; (2) the stimulatory action on gluconeogenesis was constant and independent of NH4Cl concentration, whereas the inhibition of ureagenesis was variable and dependent on NH4Cl concentration and the degree of reduction of the gluconeogenic substrate; and (3) fatty acids produced apparent reciprocal changes in the state of reduction of the cytosolic and mitochondrial NAD systems. Fatty acid oxidation exerted its effect mainly, if not exclusively, by preventing the gluconeogenic substrate-induced stimulation of ureagenesis. Fatty acids also inhibited ureagenesis without stimulating gluconeogenesis (lactate < 1 mmol/L), ruling out a limiting energy availability as the cause of the inhibition. One or both of the following two mechanisms seem to account for the fatty acid-induced inhibition of ureagenesis from NH4Cl. First, a decreased uptake of ornithine, and second, decreased flux through pyruvate dehydrogenase and probably other NAD(P)-linked mitochondrial dehydrogenases. The correlation found between the ability of fatty acids to inhibit ureagenesis and the state of activation of pyruvate dehydrogenase supports the latter point.

Stimulation of ureogenesis by ornithine and/or NH4Cl inhibited gluconeogenesis from lactate but not from equimolar concentrations of pyruvate in perfused rat liver. Neither a shortage of energy nor a decrease in alpha-ketoglutarate availability seems to be responsible for this inhibition. With lactate as substrate the extracellular concentration of pyruvate attained was approximately equal to 0.15 mM that assuming reflects its cytosolic concentration it would be limiting for its mitochondrial transport. Stimulation of ureogenesis from NH4Cl enhances flux through pyruvate dehydrogenase. Furthermore, activation of pyruvate dehydrogenase by dichloroacetate led to stimulation of ureogenesis and inhibition of glucose production. Conversely, inhibition of pyruvate dehydrogenase flux by fatty acid enhanced glucose production and inhibited ureogenesis. Thus, ornithine and/or NH4Cl seem to inhibit lactate to glucose flux by shifting the mitochondrial partitioning of pyruvate from carboxylation towards decarboxylation with the result of a decreased oxaloacetate formation. Gluconeogenic substrates enhanced the hepatic uptake of ornithine. However, no correlation seems to exist between the uptake of ornithine, ornithine-induced stimulation of ureogenesis and total rates of urea production. Ornithine produced a concentration-dependent acidification of the hepatic outflow perfusate, suggesting that it may be transported in exchange for H+.

Gluconeogenic substrates, lactate or pyruvate, or ornithine produced 100% increase of urea synthesis from NH4Cl. The combined administration of ornithine and lactate (or pyruvate) produced more than additive effects, indicating that they acted at different steps in a potentiating manner. The uptake of ornithine was enhanced by gluconeogenic substrates. This finding may explain, at least in part, the stimulating effect of these substrates on ureagenesis from NH4Cl and ornithine. The gluconeogenic substrate-induced stimulation of ureagenesis from NH4Cl was still observed under conditions of reduced flux through pyruvate carboxylase, ruling out that their action was exclusively mediated by the anaplerotic effect of this enzyme. Pyruvate was a more potent stimulator of ureagenesis than lactate and its effect less sensitive to pyruvate carboxylase inhibition. These observations indicate that a correlation exists between stimulation of ureagenesis by gluconeogenic substrates and flux through pyruvate dehydrogenase. It is concluded that gluconeogenic substrates may stimulate ureagenesis from NH4Cl by 1) increasing intracellular ornithine availability and/or 2) enhancing flux through pyruvate dehydrogenase and consequently the tricarboxylic acid cycle activity.