Glucagon and the permissive action of fatty acids in hepatic gluconeogenesis

Demonstration of a critical role for free fatty acids in mediating counterregulatory stimulation of gluconeogenesis and suppression of glucose utilization in humans

In vitro studies indicate that FFA compete with glucose as an oxidative fuel in muscle and, in addition, stimulate gluconeogenesis in liver. During counterregulation of hypoglycemia, plasma FFA increase and this is associated with an increase in glucose production and a suppression of glucose utilization. To test the hypothesis that FFA mediate changes in glucose metabolism that occur during counterregulation, we examined the effects of acipimox, an inhibitor of lipolysis, on glucose production and utilization ([3-3H]glucose), and incorporation of [U-14C]-alanine into glucose during insulin-induced hypoglycemia. Eight normal volunteers were infused with insulin for 8 h to produce modest hypoglycemia (approximately 3 mM) on two occasions, first without acipimox (control) and then with acipimox administration (250 mg per os at 60 and 240 min). Despite identical plasma insulin concentrations, glucose had to be infused in the acipimox experiments (glucose-clamp technique) to maintain plasma glucose concentrations identical to those in control experiments. Acipimox completely prevented counterregulatory increases in lipolysis so that during the last 4 h plasma FFA were below baseline values and averaged 67 +/- 13 vs. 725 +/- 65 microM in control experiments, P < 0.001. Concomitantly, overall glucose production was reduced by 40% (5.5 +/- 11 vs. 9.3 +/- 0.7 mumol/kg per min, P < 0.001), and gluconeogenesis from alanine was reduced by nearly 70% (0.32 +/- 0.09 vs. 1.00 +/- 0.18 mumol/kg per min, P < 0.001), while glucose utilization increased by 15% (10.8 +/- 1.4 vs. 9.3 +/- 0.7 mumol/kg per min). We conclude that FFA play a critical role in mediating changes in glucose metabolism during counterregulation, and that under these conditions, FFA exert a much more profound effect on hepatic glucose production than on glucose utilization.

The mechanism of the hormonal activation of respiration in isolated hepatocytes and its importance in the regulation of gluconeogenesis

The effects of hormones on the cytochrome spectra of isolated hepatocytes were recorded under conditions of active gluconeogenesis from L-lactate. Glucagon, phenylephrine, vasopressin and valinomycin, at concentrations that caused stimulation of gluconeogenesis, increased the reduction of the components of the cytochrome bc1 complex, just as has been observed in liver mitochondria isolated from glucagon-treated rats [Halestrap (1982) Biochem. J. 204, 37-47]. The effects of glucagon and phenylephrine were additive. The time courses of the increased reduction of cytochrome c/c1 and NAD(P)H/NAD(P)+ caused by hormones, valinomycin, A23187 and ethanol were measured by dual-beam spectrophotometry and fluorescence respectively. Ethanol (14 mM) produced a substantial rise in NAD(P)H fluorescence, beta-hydroxybutyrate/acetoacetate and lactate/pyruvate ratios, no change in cytochrome c/c1 reduction, a 10% decrease in O2 consumption and a 60% decrease in gluconeogenesis. Glucagon, phenylephrine and vasopressin caused a substantial and transient rise in NAD(P)H fluorescence, but a sustained increase in cytochrome c/c1 reduction and the rates of O2 consumption and gluconeogenesis. The transience of the fluorescence response was greater in the absence of Ca2+, when the cytochrome c/c1 response also became transient. The fluorescence response was smaller and less transient, but the cytochrome c/c1 response was greater, in the presence of fatty acids. Both responses were greatly decreased by the presence of 1 mM-pent-4-enoate. Valinomycin (2.5 nM) caused a decrease in NAD(P)H fluorescence coincident with an increase in cytochrome c/c1 reduction and the rate of gluconeogenesis and O2 consumption. A23187 (7.5 mM) caused increases in both NAD(P)H fluorescence and cytochrome c/c1 reduction. The effects of hormones and valinomycin on the time courses of NAD(P)H fluorescence, cytochrome c/c1 reduction and light-scattering by hepatocytes were compared with those of 0.5 microM-Ca2+ or 1 nM-valinomycin on the same parameters of isolated liver mitochondria. It is concluded that hormones increase respiration by hepatocytes in a biphasic manner. An initial Ca2+-dependent activation of mitochondrial dehydrogenases rapidly increases the mitochondrial [NADH], which is followed by a volume-mediated stimulation of fatty acid oxidation and electron flow between NADH and cytochrome c. 10. Amytal (0.5 mM) was able to reverse the effects of hormones on the reduction of cytochromes c/c1 and the rates of gluconeogenesis and O2 consumption without significantly lowering tissue [ATP].(ABSTRACT TRUNCATED AT 400 WORDS)

Stimulation of gluconeogenesis by palmitic acid in rat hepatocytes: evidence that this effect can be dissociated from the provision of reducing equivalents

When hepatocytes isolated from fasted rats were incubated in medium containing 5 mmol/L pyruvate, addition of albumin-bound palmitate (0.5 mmol/L) increased fatty acid oxidation and the conversion of pyruvate to glucose. Similar stimulation of gluconeogenesis occurred when palmitate was added to hepatocytes in medium containing 5 mmol/L alanine. Addition of 0.5 mmol/L (+)-octanoylcarnitine, an inhibitor of fatty acid oxidation, prevented the increment in beta-oxidation, but not the increase in glucose formation from pyruvate or alanine, induced by palmitate. These studies and other data to be considered subsequently indicate that palmitate can stimulate hepatic gluconeogenesis from three-carbon precursors under conditions that preclude an increase in the formation of reducing equivalents by beta-oxidation.

Distinctive roles of oleate and glucagen in gluconeogenesis

1. The effect of oleate on the subcellular distribution of ATP, 2-oxoglutarate, glutamate, citrate, malate and phosphoenolpyruvate was studied in hepatocytes from rats starved for 48 h by applying a modified digitonin method. The results markedly differ from those observed after glucagon [Siess, E. A., Brocks, D. G., Lattke, H. K., and Wieland, O. H. (1977) Biochem J. 166, 225-235]. Total cellular amounts and the distribution of ATP and 2-oxoglutarate remained unchanged. In the mitochondrial matrix glutamate was increased, while mitochondrial phospho-enolpyruvate was decreased. Citrate and malate were increased both in the mitochondrial and cytosolic space. 2. In contrast to the effect of glucagon, gluconeogenesis from dihydroxyacetone, fructose or glutamine was not stimulated by oleate. Gluconeogenesis from propionate was even inhibited by the fatty acid. 3. The stimulation by glucagon of glucose production from dihydroxyacetone or fructose was undiminished in biotin-deficient hepatocytes. Glucose formation from lactate, however, was stimulated only in biotin-substituted hepatocytes. 4. The results indicate that oleate stimulates gluconeogenesis by increasing pyruvate carboxylase activity (EC 6.4.1.1), whereas glucagon displays a more complex mode of action.

Distinctive effects of glucagon on gluconeogenesis and ketogenesis in hepatocytes isolated from normal and biotin-deficient rats

In hepatocytes from 48 h-starved rats identical glucagon dose-response curves were obtained for the stimulation of gluconeogenesis from lactate, for ketogenesis and for the decreasing of the C5-dicarboxylate pool. Glucagon (20 nM) caused a 5-fold increase in 3-hydroxybutyrate formation, but decreased acetoacetate production 50% of that of the control. In hepatocytes from biotin-deficient rats glucagon no longer stimulated gluconeogenesis from lactate, but still produced its effects on the mitochondrial redox state and the C5-dicarboxylate pool. The results suggest that the primary site of the hormone action on gluconeogenesis is located within the mitochondria rather than in the cytosol.

Glucagon and insulin control of gluconeogenesis in the perfused isolated rat liver. Effects on cellular metabolite distribution

The metabolic effects of glucagon and glucagon plus insulin on the isolated rat livers perfused with 10 mM sodium L-lactate as substrate were studied. Glucagon stimulated gluconeogenesis, ketogenesis and ureogenesis at the concentration used of 2.1 nM. The addition of insulin to give a glucagon-to-insulin ratio of 0.2 reversed all the glucagon effects. The glucagon enhancement of gluconeogenesis was accompanied by a rise in cytosolic and mitochondrial state of reduction of the NAD system and a fall in the [ATP]/[ADP] ratio. The analysis of the intermediary metabolite concentrations suggested, as possible sites of glucagon action, the steps between pyruvate and phosphoenolpyruvate as well as the reactions catalyzed by phosphofructokinase and/or fructose bisphosphatase. All the changes in metabolite contents were abolished when insulin was present. Glucagon increased the intramitochondrial concentration of all the metabolites, whose intracellular distribution was calculated. The finding of a significant rise in the calculated intramitochondrial concentration of oxaloacetate points to pyruvate carboxylation as an important site of glucagon interaction with the gluconeogenic pathway. A primary event in the glucagon action redistributing intracellular metabolites seems to be the mitochondrial entry of malate. The possibility is discussed that the changes in metabolite cellular distribution were brought about by the increased cellular state of reduction caused by the hormone.