Treatment of 18 h-starved rats with dexamethasone and subsequent isolation and incubation of the hepatocytes in the presence of the steroid increased gluconeogenic flux with both 1.0 mM pyruvate and 1.0 mM lactate plus 0.2 mM pyruvate as the substrate. The magnitude of stimulation was comparable with both substrates. The increase in glucose output was accompanied by an increased flux through pyruvate carboxylase, although the absolute flux and magnitude were considerably less in the presence of the more reduced substrate. The effect of the steroid on the flux through pyruvate dehydrogenase was substrate-dependent, an inhibition occurring with the more oxidized substrate. There was no effect of steroid treatment on [1-14C]lactate or pyruvate oxidation or on tricarboxylic-acid-cycle flux as measured by [3-14C]pyruvate oxidation. Dexamethasone treatment resulted in a parallel increase in both pyruvate kinase flux and glucose synthesis with both substrates employed, indicating that the steroid had no effect on the partitioning of phosphoenolpyruvate between pyruvate and lactate formation and gluconeogenesis. Similarly there was no effect of the steroid on either the activity ratio or the total pyruvate kinase activity in the cells. It is suggested that the acute effect of the dexamethasone to increase gluconeogenesis resides at the level of phosphoenolpyruvate formation, i.e. pyruvate carboxylase and possibly phosphoenolpyruvate carboxykinase.

The effects of [leucine]enkephalin and angiotensin on hepatic carbohydrate and cyclic nucleotide metabolism are compared. Both peptides stimulated glycogenolysis as a result of an increase in phosphorylase a activity and enhanced glucose synthesis from [2-14C]pyruvate, although neither had any significant effect on pyruvate kinase activity. Although the magnitudes of the effects of both peptides on glycogenolysis were comparable and unaffected by the presence of insulin. [Leu]enkephalin proved to be more efficacious in enhancing gluconeogenesis, the response being comparable with that to glucagon. Both effectors decreased the intracellular concentration of cyclic AMP in hepatocytes when incubated under control conditions and after addition of sub-optimal concentrations of glucagon. This was correlated with the ability of the two peptides to inhibit both basal and hormone-stimulated adenylate cyclase activity in purified liver plasma membranes.

The possibility that the effects of [Leu]enkephalin in vitro on hepatic carbohydrate metabolism are mediated by interaction with angiotensin II receptors has been examined. Preincubation of hepatocytes with either the angiotensin II receptor antagonist [Sar1,Ile8]angiotensin II or 10 mM-dithiothreitol abolished the ability of both angiotensin II and [Leu]enkephalin to increase phosphorylase a in hepatocytes prepared from fed rats. Dithiothreitol had no effect on the stimulation of phosphorylase in the presence of glucagon or phenylephrine, although it also inhibited the response to vasopressin. [Leu]enkephalin displaced specifically bound 125I-labelled angiotensin II from hepatic plasma membranes over a concentration range of 10(-7)-10(-5) M. This correlated with the dose-response required to stimulate phosphorylase activity in intact hepatocytes and suggests that the effects of the opioid peptides on carbohydrate metabolism in liver are the result of cross-reactivity of the peptides with angiotensin II receptors. Addition of 10(-5) M-[Leu]enkephalin to isolated kidney tubule fragments stimulated gluconeogenesis from 5 mM-pyruvate, the magnitude of stimulation being comparable to that by either angiotensin II or adrenaline. This effect of the opioid peptide was also abolished by pretreatment of the tubules with [Sar1,Ile8]angiotensin II, suggesting that the ability of [Leu]enkephalin to interact with angiotensin II receptors is not restricted to the liver, but may occur in other tissues where both receptors occur together.

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