Control of Gluconeogenesis in Liver

Glycerol induces G6pc in primary mouse hepatocytes and is the preferred substrate for gluconeogenesis both in vitro and in vivo

Gluconeogenesis (GNG) is de novo production of glucose from endogenous carbon sources. Although it is a commonly studied pathway, particularly in disease, there is a lack of consensus about substrate preference. Moreover, primary hepatocytes are the current gold standard for in vitro liver studies, but no direct comparison of substrate preference at physiological fasting concentrations has been performed. We show that mouse primary hepatocytes prefer glycerol to pyruvate/lactate in glucose production assays and ¹³C isotope tracing studies at the high concentrations commonly used in the literature, as well as at more relevant fasting, physiological concentrations. In addition, when glycerol, pyruvate/lactate, and glutamine are all present, glycerol is responsible for over 75% of all glucose carbons labeled. We also found that glycerol can induce a rate-limiting enzyme of GNG, glucose-6-phosphatase. Lastly, we suggest that glycerol is a better substrate than pyruvate to test in vivo production of glucose in fasting mice. In conclusion, glycerol is the major carbon source for GNG in vitro and in vivo and should be compared with other substrates when studying GNG in the context of metabolic disease states.

Mechanisms of Insulin Action and Insulin Resistance

The 1921 discovery of insulin was a Big Bang from which a vast and expanding universe of research into insulin action and resistance has issued. In the intervening century, some discoveries have matured, coalescing into solid and fertile ground for clinical application; others remain incompletely investigated and scientifically controversial. Here, we attempt to synthesize this work to guide further mechanistic investigation and to inform the development of novel therapies for type 2 diabetes (T2D). The rational development of such therapies necessitates detailed knowledge of one of the key pathophysiological processes involved in T2D: insulin resistance. Understanding insulin resistance, in turn, requires knowledge of normal insulin action. In this review, both the physiology of insulin action and the pathophysiology of insulin resistance are described, focusing on three key insulin target tissues: skeletal muscle, liver, and white adipose tissue. We aim to develop an integrated physiological perspective, placing the intricate signaling effectors that carry out the cell-autonomous response to insulin in the context of the tissue-specific functions that generate the coordinated organismal response. First, in section II, the effectors and effects of direct, cell-autonomous insulin action in muscle, liver, and white adipose tissue are reviewed, beginning at the insulin receptor and working downstream. Section III considers the critical and underappreciated role of tissue crosstalk in whole body insulin action, especially the essential interaction between adipose lipolysis and hepatic gluconeogenesis. The pathophysiology of insulin resistance is then described in section IV. Special attention is given to which signaling pathways and functions become insulin resistant in the setting of chronic overnutrition, and an alternative explanation for the phenomenon of ‟selective hepatic insulin resistanceˮ is presented. Sections V, VI, and VII critically examine the evidence for and against several putative mediators of insulin resistance. Section V reviews work linking the bioactive lipids diacylglycerol, ceramide, and acylcarnitine to insulin resistance; section VI considers the impact of nutrient stresses in the endoplasmic reticulum and mitochondria on insulin resistance; and section VII discusses non-cell autonomous factors proposed to induce insulin resistance, including inflammatory mediators, branched-chain amino acids, adipokines, and hepatokines. Finally, in section VIII, we propose an integrated model of insulin resistance that links these mediators to final common pathways of metabolite-driven gluconeogenesis and ectopic lipid accumulation.

Glucagon: acute actions on hepatic metabolism

Type 2 diabetes mellitus is the result of impaired systemic control of glucose homeostasis, in part through the dysregulation of the hormone glucagon. Glucagon acts on the liver to increase glucose production through alterations in hepatic metabolism, and reducing the elevated glucagon signalling in diabetic patients is an attractive strategy for the treatment of hyperglycaemia. Here we review the actions of the hormone in the liver, focusing on the acute alterations of metabolic pathways. This review summarises a presentation given at the 'Novel data on glucagon' symposium at the 2015 annual meeting of the EASD. It is accompanied by two other reviews on topics from this symposium (by Mona Abraham and Tony Lam, DOI: 10.1007/s00125-016-3950-3 , and by Young Lee and colleagues, DOI: 10.1007/s00125-016-3965-9 ) and an overview by the Session Chair, Isabel Valverde (DOI: 10.1007/s00125-016-3946-z ).

Visceral fat and metabolic inflammation: the portal theory revisited

Abdominal (central) obesity strongly correlates with (hepatic) insulin resistance and type 2 diabetes. Among several hypotheses that have been formulated, the 'portal theory' proposes that the liver is directly exposed to increasing amounts of free fatty acids and pro-inflammatory factors released from visceral fat into the portal vein of obese patients, promoting the development of hepatic insulin resistance and liver steatosis. Thus, visceral obesity may be particularly hazardous in the pathogenesis of insulin resistance and type 2 diabetes. Herein, we will critically review existing evidence for a potential contribution of portally drained free fatty acids and/or cytokines to the development of hepatic insulin resistance.

Fatty acids-stress attenuates gluconeogenesis induction and glucose production in primary hepatocytes

BACKGROUND

Hepatic gluconeogenesis tightly controls blood glucose levels in healthy individuals, yet disorders of fatty acids (FAs) oxidation are characterized by hypoglycemia. We studied the ability of free-FAs to directly inhibit gluconeogenesis, as a novel mechanism that elucidates the hypoglycemic effect of FAs oxidation defects.

METHODS

Primary rat hepatocytes were pre-treated with FAs prior to gluconeogenic stimuli with glucagon or dexamethasone and cAMP.

RESULTS

Pre-treatment with 1 mM FAs (mixture of 2:1 oleate:palmitate) for 1 hour prior to gluconeogenic induction, significantly decreases the induced expression of the gluconeogenic genes phosphoenolpyruvate carboxykinase (PEPCK) and glucose-6-phosphatase (G6pase) as well as the induced glucose production by the cells. The inhibitory effect of FAs upon gluconeogenesis is abolished when pre-treatment is elongated to 18 hours, allowing clearance of FAs into triglycerides by the cells. Replacement of palmitate with the non-metabolic fatty acid 2-bromopalmitate inhibits esterification of FAs into triglycerides. Accordingly, the increased exposure to unesterified-FAs allows their inhibitory effect to be extended even when pre-treatment is elongated to 18 hours. Similar changes were caused by FAs to the induction of peroxisome-proliferator-activated receptor-γ coactivator 1α (PGC1α) expression, indicating this transcriptional coactivator as the mediating link of the effect. This inhibitory effect of FAs upon gluconeogenic induction is shown to involve reduced activation of cAMP response element-binding (CREB) transcription factor.

CONCLUSIONS

The present results demonstrate that free-FAs directly inhibit the induced gluconeogenic response in hepatocytes. Hence, high levels of free-FAs may attenuate hepatic gluconeogenesis, and liver glucose output.

Making insulin-deficient type 1 diabetic rodents thrive without insulin

Terminally ill insulin-deficient rodents with uncontrolled diabetes due to autoimmune or chemical destruction of beta-cells were made hyperleptinemic by adenoviral transfer of the leptin gene. Within approximately 10 days their severe hyperglycemia and ketosis were corrected. Despite the lack of insulin, moribund animals resumed linear growth and appeared normal. Normoglycemia persisted 10-80 days without other treatment; normal physiological conditions lasted for approximately 175 days despite reappearance of moderate hyperglycemia. Inhibition of gluconeogenesis by suppression of hyperglucagonemia and reduction of hepatic cAMP response element-binding protein, phoshoenolpyruvate carboxykinase, and peroxisome proliferator-activated receptor-gamma-coactivator-1alpha may explain the anticatabolic effect. Up-regulation of insulin-like growth factor 1 (IGF-1) expression and plasma levels and increasing IGF-1 receptor phosphorylation in muscle may explain the increased insulin receptor substrate 1, PI3K, and ERK phosphorylation in skeletal muscle. These findings suggest that leptin reverses the catabolic consequences of total lack of insulin, potentially by suppressing glucagon action on liver and enhancing the insulinomimetic actions of IGF-1 on skeletal muscle, and suggest strategies for making type 1 diabetes insulin-independent.

Effect of the amino acid alanine on glucagon secretion in non-diabetic and type 1 diabetic subjects during hyperinsulinaemic euglycaemia, hypoglycaemia and post-hypoglycaemic hyperglycaemia

AIMS/HYPOTHESIS

The aim of our study was to establish whether the well-known defective or absent secretion of glucagon in type 1 diabetes in response to hypoglycaemia is selective or includes lack of responses to other stimuli, such as amino acids.

MATERIALS AND METHODS

Responses of glucagon to hypoglycaemia were measured in eight patients with type 1 diabetes and six non-diabetic subjects during hyperinsulinaemic (insulin infusion 0.5 mU kg(-1) min(-1)) and eu-, hypo- and hyperglycaemic clamp studies (sequential steps of plasma glucose 5.0, 2.9, 5.0, 10 mmol/l). Subjects were studied on three randomised occasions with infusion of low- or high-dose alanine, or saline.

RESULTS

With saline, glucagon increased in hypoglycaemia in non-diabetic subjects but not in diabetic subjects. Glucagon increased further with low-dose (181 +/- 16 ng l(-1) min(-1)) and high-dose alanine (238 +/- 20 ng l(-1) min(-1)) in non-diabetic subjects, but only with high-dose alanine in diabetic subjects (area under curve 112 +/- 5 ng l(-1) min(-1)). The alanine-induced glucagon increase in diabetic subjects paralleled the spontaneous glucagon response to hypoglycaemia in non-diabetic subjects not receiving alanine. The greater responses of glucagon to hypoglycaemia with alanine infusion were offset by recovery of eu- or hyperglycaemia.

CONCLUSIONS/INTERPRETATION

In type 1 diabetes, the usually deficient responses of glucagon to hypoglycaemia may improve after increasing the concentration of plasma amino acids. Amino acid-enhanced secretion of glucagon in response to hypoglycaemia remains under physiological control since it is regulated primarily by the ambient plasma glucose concentration. These findings might be relevant to improving counter-regulatory defences against insulin-induced hypoglycaemia in type 1 diabetes.

Changes in hepatic glycogen cycling during a glucose load in healthy humans

AIMS/HYPOTHESIS

Glycogen cycling, i.e. simultaneous glycogen synthesis and glycogenolysis, affects estimates of glucose fluxes using tracer techniques and may contribute to hyperglycaemia in diabetic conditions. This study presents a new method for quantifying hepatic glycogen cycling in the fed state. Glycogen is synthesised from glucose by the direct and indirect (gluconeogenic) pathways. Since glycogen is also synthesised from glycogen, i.e. glycogen-->glucose 1-phosphate-->glycogen, that synthesised through the direct and indirect pathways does not account for 100% of glycogen synthesis. The percentage contribution of glycogen cycling to glycogen synthesis then equals the difference between the sum of the percentage contributions of the direct and indirect pathways and 100.

MATERIALS AND METHODS

The indirect and direct pathways were measured independently in nine healthy volunteers who had fasted overnight. They ingested (2)H(2)O (5 ml/kg body water) and were infused with [5-(3)H]glucose and acetaminophen (paracetamol; 1 g) during hyperglycaemic clamps (7.8 mmol/l) lasting 8 h. The percentage contribution of the indirect pathway was calculated from the ratio of (2)H enrichments at carbon 5 to that at carbon 2, and the contribution of the direct pathway was determined from the (3)H-specific activity, relative to plasma glucose, of the urinary glucuronide excreted between 2 and 4, 4 and 6, and 6 and 8 h.

RESULTS

Glucose infusion rates increased (p<0.01) to approximately 50 mumol kg(-1) min(-1). Plasma insulin and the insulin : glucagon ratio rose approximately 3.6- and approximately 8.3-fold (p<0.001), respectively. From the difference between 100% and the sum of the direct (2-4 h, 54+/-6%; 4-6 h, 59+/-5%; 6-8 h, 63+/-4%) and indirect (32+/-3, 38+/-4, 36+/-3%) pathways, glycogen cycling was seen to be decreased (p<0.05) from 14+/-4% (2-4 h) to 4+/-3% (4-6 h) and 1+/-3% (6-8 h).

CONCLUSIONS/INTERPRETATION

This method allows measurement of hepatic glycogen cycling in the fed state and demonstrates that glycogen cycling occurs most in the early hours after glucose loading subsequent to a fast.

Dysfunctional fat cells, lipotoxicity and type 2 diabetes

Type 2 diabetes is characterized by insulin resistance and impaired insulin secretion. Considerable evidence implicates altered fat topography and defects in adipocyte metabolism in the pathogenesis of type 2 diabetes. In individuals who develop type 2 diabetes, fat cells tend to be enlarged. Enlarged fat cells are resistant to the antilipolytic effects of insulin, leading to day-long elevated plasma free fatty acid (FFA) levels. Chronically increased plasma FFA stimulates gluconeogenesis, induces hepatic and muscle insulin resistance, and impairs insulin secretion in genetically predisposed individuals. These FFA-induced disturbances are referred to as lipotoxicity. Enlarged fat cells also have diminished capacity to store fat. When adipocyte storage capacity is exceeded, lipid 'overflows' into muscle and liver, and possibly the beta-cells of the pancreas, exacerbating insulin resistance and further impairing insulin secretion. In addition, dysfunctional fat cells produce excessive amounts of insulin resistance-inducing, inflammatory and atherosclerosis-provoking cytokines, and fail to secrete normal amounts of insulin-sensitizing cytokines. As more evidence emerges, there is a stronger case for targeting adipose tissue in the treatment of type 2 diabetes. Peroxisome-proliferator activated receptor gamma (PPARgamma) agonists, for example the thiazolidinediones, redistribute fat within the body (decrease visceral and hepatic fat; increase subcutaneous fat) and have been shown to enhance adipocyte insulin sensitivity, inhibit lipolysis, reduce plasma FFA and favourably influence the production of adipocytokines. This article examines in detail the role of adipose tissue in the pathogenesis of type 2 diabetes and highlights the potential of PPAR agonists to improve the management of patients with the condition.

Inhibition of lipolysis causes suppression of endogenous glucose production independent of changes in insulin

We have shown that insulin controls endogenous glucose production (EGP) indirectly, via suppression of adipocyte lipolysis. Free fatty acids (FFA) and EGP are suppressed proportionately, and when the decline in FFA is prevented during insulin infusion, suppression of EGP is also prevented. The present study tested the hypothesis that suppression of lipolysis under conditions of constant insulin would yield a suppression of EGP. N(6)-cyclohexyladenosine (CHA) was used to selectively suppress adipocyte lipolysis during euglycemic clamps in conscious male dogs. FFA suppression by CHA caused suppression of EGP. Liposyn control experiments, which maintained FFA levels above basal during CHA infusion, completely prevented the decline in EGP, whereas glycerol control experiments, which maintained glycerol levels close to basal, did not prevent a decline in EGP. These controls suggest that the EGP suppression was secondary to the suppression of FFA levels specifically. A difference in the sensitivity of FFA and EGP suppression (FFA were suppressed approximately 85% whereas EGP only declined approximately 40%) was possibly caused by confounding effects of CHA, including an increase in catecholamine and glucagons levels during CHA infusion. Thus suppression of lipolysis under constant insulin causes suppression of EGP, despite a significant rise in catecholamines.

Neuroendocrine perturbations as a cause of insulin resistance

Insulin resistance is followed by several prevalent diseases. The most common condition with insulin resistance is obesity, particularly when localized to abdominal, visceral regions. A summary of recent reviews on the pathogenesis of systemic insulin resistance indicates that major factors are decreased insulin effects on muscular glycogen synthase or preceding steps in the insulin signalling cascade, on endogenous glucose production and on circulating free fatty acids (FFA) from adipose tissue lipolysis. Contributions of morphologic changes in muscle and other factors are considered more uncertain. Newly developed methodology has made it possible to determine more precisely the neuroendocrine abnormalities in abdominal obesity including increased cortisol and adrenal androgen secretions. This is probably due to a hyperactivity of the hypothalamic-pituitary-adrenal (HPA) axis, amplified by inefficient feedback inhibition by central glucocorticoid receptors, associated with molecular genetic defects. Secondly, secretion of gender-specific sex steroid hormones becomes inhibited and the sympathetic nervous system activated. At this stage the HPA axis shows signs of a 'burned-out' condition, and cortisol secretion is no longer elevated. Cortisol counteracts the insulin activation of glycogen synthase in muscle, the insulin inhibition of hepatic glucose production and the insulin inhibition of lipolysis in adipose tissue, leading to the well-established systemic insulin resistance caused by excess cortisol. This is exaggerated by increased free fatty acid mobilization, particularly with a concomitant elevation of the activity of the sympathetic nervous system. Furthermore, capillarization and fiber composition in muscle are changed. These are the identical perturbations responsible for insulin resistance in recent reviews. The diminished sex steroid secretion in abdominal obesity has the same consequences. It is thus clear that insulin resistance may be induced by neuroendocrine abnormalities, such as those seen in abdominal obesity. These endocrine perturbations also direct excess fat to visceral fat depots via mechanisms that are largely known, indicating why abdominal obesity is commonly associated with insulin resistance. This possible background to the most prevalent condition of insulin resistance has been revealed by development of methodology that allows sufficiently sensitive measurements of HPA axis activity. These findings demonstrate the power of neuroendocrine regulations for somatic health.

A high-fat diet worsens metabolic control in streptozotocin-treated rats by increasing hepatic glucose production

The aim of this study was to determine the mechanism by which a high-fat diet exacerbates the diabetes produced by a low dose of streptozotocin (STZ). The glucose clamp technique was used to determine hepatic glucose production (HGP) and the disappearance rate (Rd) of glucose, basally and during insulin infusions of 1.0 and 3.0 mU/kg/min in control of STZ-treated rats fed either a low-fat or high-fat diet. Fasting plasma glucose in the high fat-STZ (HFS) group was significantly higher than in any of the other groups: low fat-STZ (LFS), high-fat controls (HFC), or low-fat controls (LFC) (18.1 +/- 1.6 v 8.1 +/- 0.8 mmol/L, P less than .001; 6.0 +/- 0.2 mmol/L, P less than .001; 5.4 +/- 0.1 mmol/L, P less than .001, respectively). Basal HGP was markedly higher in the HFS group compared with each of the other three groups (98.8 +/- 5.9 v 61.4 +/- 3.7, P less than .001; 42.9 +/- 1.6, P less than .001; 39.6 +/- 1.3 mumol/kg/min, P less than .001; HFS v LFS, HFC, and LFC, respectively). Following insulin infusion, no differences were observed in HGP between the LFC and LFS groups at either insulin dose. However, HGP was not suppressed to control levels in either of the high-fat diet groups, and this defect was more marked in the HFS group. It is concluded that a high-fat diet exacerbates mild STZ diabetes primarily by increasing HGP.

Modulation of hepatic glucose production by non-esterified fatty acids in type 2 (non-insulin-dependent) diabetes mellitus

To study the effect of changes in plasma non-esterified fatty acid concentration on suppression of hepatic glucose production by insulin eight Type 2 (non-insulin-dependent) diabetic patients participated in three euglycaemic, hyperinsulinaemic (108pmol.m2-1.min-1) clamp studies combined with indirect calorimetry and infusion of [3-3H]-glucose and [1-14C]palmitate; (1) a control experiment with infusion of NaCl 154 mmol/l, (2) heparin was infused together with insulin, and (3) an antilipolytic agent, Acipimox, was administered at the beginning of the experiment. Six healthy volunteers participated in the control experiment. Plasma non-esterified fatty acid concentrations during the insulin clamp were in diabetic patients: (1) 151 +/- 36 mumol/l, (2) 949 +/- 178 mumol/l, and (3) 65 +/- 9 mumol/l; in healthy control subjects 93 +/- 13 mumol/l. Non-esterified fatty acid transport rate, oxidation and non-oxidative metabolism were significantly higher during the heparin than during the Acipimox experiment (p less than 0.001). Suppression of hepatic glucose production by insulin was impaired in the diabetic compared to control subjects (255 +/- 42 vs 51 +/- 29 mumol/min, p less than 0.01). Infusion of heparin did not affect the suppression of hepatic glucose production by insulin (231 +/- 49 mumol/min), whereas Acipimox significantly enhanced the suppression (21 +/- 53 mumol/min, p less than 0.001 vs 154 mmol/l NaCl experiment). We conclude that insulin-mediated suppression of hepatic glucose production is not affected by increased non-esterified fatty acid availability. In contrast, decreased non-esterified fatty acid availability enhances the suppression of hepatic glucose production by insulin.

Gluconeogenesis in isolated chicken hepatocytes: effect of fatty acids, beta-hydroxybutrate, ethanol, and various pyruvate/lactate ratios

The effect of fatty acids beta-hydroxybutyrate, ethanol, and different pyruvate/lactate ratios on gluconeogenesis in isolated chicken hepatocytes was investigated. Glucogenesis was significantly affected by a change in the oxidation-reduction (pyruvate/lactate) ratio, and this effect was greater than could be accounted for by the additive effects of these substrates. Substituting lactate with nongluconeogenic substrates, such as beta-hydroxybutyrate or ethanol, increased the formation of glucose by 80 and 200%, respectively, demonstrating the beneficial effect of the increased reducing equivalents in the hepatocytes. Oleic acid per se had no effect but when added complexed with albumin, it had a negative effect on gluconeogenesis.

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.

Regulation of hepatic free fatty acid metabolism by glucagon and insulin

The roles of glucagon and insulin in the direct short-term regulation of hepatic free fatty acid (FFA) metabolism were studied in hepatocytes isolated from fed, fasted, and streptozotocin-induced diabetic rats. In fed animals, the principal metabolic product of palmitate metabolism was triglyceride, whereas ketones were the major product in fasted and diabetic animals. Glucagon at physiological concentrations increased ketogenesis and decreased triglyceride synthesis from palmitate in hepatocytes from fed rats at FFA concentrations 1.0 mM or less. Insulin had no effect on FFA metabolism when present as the sole hormone, but could antagonize the actions of submaximal concentrations of glucagon. The metabolism of palmitate in fasted or diabetic hepatocytes was unaffected by either hormone. Ketogenesis from octanoate was also unaffected by hormone addition in all cell types. These data are consistent with a locus of hormonal regulation at a step prior to beta-oxidation of fatty acid. Glucagon and insulin may modulate FFA metabolism by both intrahepatic and extrahepatic mechanisms.

The metabolism of oleic acid by the perfused rat liver in experimental diabetes induced by antiinsulin serum

The metabolism of varying quantities of oleic acid was examined in isolated perfused livers from normal fed rats and from animals made diabetic by pretreatment with guinea pig antiinsulin serum (AIS). The data presented reemphasize the fact that the quantity of free fatty acid (FFA) coming to the liver is a necessary, but not the most important, factor affecting the subsequent metabolism of the FFA. Rates of ketogenesis and output of triglyceride and the terminal concentration of hepatic triglyceride were proportional to uptake of FFA in certain concentration ranges. For equal rates of uptake of FFA, ketogenesis was greater, and the quantity of triglyceride secreted or accumulated within the liver was less, with livers from diabetic animals than with livers from normal animals. In confirmation of previous data, the liver was observed to have a maximal capacity to secrete triglyceride. Triglyceride accumulated in livers from normal-fed and diabetic animals only when uptake of FFA was more than sufficient to saturate the secretory process. Since proportionately more FFA was catabolized by livers from AIS treated animals, greater uptake of FFA was required to produce maximal rates of output of triglyceride and accumulation in livers from diabetic than from normal animals. Rates of ketogenesis by livers from normal fed animals increased minimally with increasing uptake of FFA (up to 1.0 mM free fatty acid). Even when uptake increased considerably with FFA concentrations of approximately 2.5 mM, rates of ketogenesis by livers from normal animals were less than half those of livers from diabetic rats, and maximal rates were not achieved by the normal controls. It is evident that changes in hepatic metabolism of FFA in the intact diabetic animal result from simultaneous alterations of supply of FFA and hormonally induced metabolic changes in the liver. Moreover, although hepatic secretion and accumulation of triglyceride is greater in isolated perfused livers from normal rats than from diabetic animals when the livers are exposed to equal quantities of FFA, the diabetic livers can accumulate more triglyceride, secrete more triglyceride, and oxidize more FFA to ketone bodies than can the normal under conditions in which considerably more substrate is available to the diabetic rather than to the normal livers. These differences might also be expected to occur in the acutely insulin deficient intact animal, in which changes in hormonal status and substrate (FFA) availability occur simultaneously, and might, in part, explain the ketonemia, hypertriglyceridemia, and hepatic steatosis often observed in vivo.

Effects of oleic acid infusion on plasma free fatty acids and blood ketone bodies in the fasting rat

Oleic acid emulsions stabilized with albumin were infused into fasted rats. Blood samples taken before and during infusion were analyzed for free fatty acids (FFA), ketone bodies, glucose, and insulin. Turnover rates of FFA and ketone bodies were also determined using constant infusion of radioactive tracers. During oleic acid infusions at a rate of 2 mumoles/min/100 g body weight, FFA concentrations increased for a short time and then decreased to preinfusion levels. The decreases in concentrations were due to decreases in the endogenous rates of appearance of FFA into the blood. When oleic acid was infused at a rate of 3.5 mumoles/min/100 g body weight, FFA concentrations increased and remained elevated throughout the infusion. Ketone body concentrations more than doubled during infusions at 2 and 3.5 mumoles/min/100 g body weight and showed no signs of decreasing even when FFA concentrations decreased. Insulin concentrations doubled during infusion and glucose concentrations decreased. Insulin injected during infusion had little effect on concentrations of FFA or ketone bodies. It was concluded that infusions of oleic acid inhibit adipose tissue lipolysis and increase blood ketone concentrations in intact fasted rats. The injection of insulin does not inhibit ketogenesis when blood FFA levels are maintained by infusion.

Coronary vasodilation by fatty acids

Fatty acids increase the coronary flow rate of rat hearts, perfused according to the Langendorff technique. Long-chain and medium-chain fatty acids are more effective vasodilators than short-chain fatty acids. The vasodilatation by fatty acids does not proceed through the intermediate formation of the vasodilator adenosine, nor by stimulation of adenylcyclase activity. Since at low Ca2+ concentrations fatty acids not only stimulate the coronary flow rate but also cardiac contractility, it is suggested that especially the lipophilic fatty acids have calcium ionophoric properties leading to increased Ca2+ removal from smooth muscle cytosol and hence to vasodilatation. Preliminary experiments, moreover, indicate that both medium- and long-chain fatty acids, like prostaglandin E1 and Ca2+, inhibit membrane ATPase(s) of aorta smooth muscle cells, suggesting increased Ca2+ binding to vascular smooth muscle cell membranes.

Octanoate metabolism in the isolated perfused rat liver. II. Comparison with a long chain fatty acid

Liver of fed rats was perfused in vitro with 10 mmoles/litre lactic acid. Supplementary addition of octanoic acid (1.2 mmoles/litre) increased production of ketone bodies. By doubling the quantity of medium chain fatty acid, production of ketone bodies was doubled. Octanoic acid is as ketogenic aspalmitoleic acid if the two fatty acids are compared molecule by molecule. However, C8 : O is more ketogenic that C16: 1 when the comparison is effected on the basis of an identical number of acetyl groups. Octanoic acid increases glucose production by the liver. It was not possible to show the same phenomenon in the presence of palmitoleic acid. During oxidation of fatty acids the beta-hydroxybutyrate/acetoacetate and lactate/pyruvate ratios of the perfusion medium were increased.