Regulation of fatty acid and carbohydrate metabolism by insulin, growth hormone and tri-iodothyronine in hepatocyte cultures from normal and hypophysectomized rats

Metabolic Stability of D-Allulose in Biorelevant Media and Hepatocytes: Comparison with Fructose and Erythritol

D-allulose, a C-3 epimer of D-fructose, is a rare monosaccharide used as a food ingredient or a sweetener. In the present study, the in vitro metabolic stability of D-allulose was examined in biorelevant media, that is, simulated gastric fluid (SGF) and fasted state simulated intestinal fluid (FaSSIF) containing digestive enzymes, and in cryopreserved human and rat hepatocytes. The hepatocyte metabolic stabilities of D-allulose were also investigated and compared with those of fructose and erythritol (a sugar-alcohol with no calorific value). D-allulose was highly stable in SGF (97.8% remained after 60 min) and in FaSSIF (101.3% remained after 240 min), indicating it is neither pH-labile nor degraded in the gastrointestinal tract. D-allulose also exhibited high levels of stability in human and rat hepatocytes (94.5–96.8% remained after 240 min), whereas fructose was rapidly metabolized (43.1–52.6% remained), which suggested these two epimers are metabolized in completely different ways in the liver. The effects of D-allulose on glucose and fructose levels were negligible in hepatocytes. Erythritol was stable in human and rat hepatocytes (102.1–102.9% remained after 240 min). Intravenous pharmacokinetic studies in rats showed D-allulose was eliminated with a mean half-life of 72.2 min and a systemic clearance of 15.8 mL/min/kg. Taken together, our results indicate that D-allulose is not metabolized in the liver, and thus, unlikely to contribute to hepatic energy production.

Effects of growth hormone on glucose, lipid, and protein metabolism in human subjects

In evolutionary terms, GH and intracellular STAT 5 signaling is a very old regulatory system. Whereas insulin dominates periprandially, GH may be viewed as the primary anabolic hormone during stress and fasting. GH exerts anabolic effects directly and through stimulation of IGF-I, insulin, and free fatty acids (FFA). When subjects are well nourished, the GH-induced stimulation of IGF-I and insulin is important for anabolic storage and growth of lean body mass (LBM), adipose tissue, and glycogen reserves. During fasting and other catabolic states, GH predominantly stimulates the release and oxidation of FFA, which leads to decreased glucose and protein oxidation and preservation of LBM and glycogen stores. The most prominent metabolic effect of GH is a marked increase in lipolysis and FFA levels. In the basal state, the effects of GH on protein metabolism are modest and include increased protein synthesis and decreased breakdown at the whole body level and in muscle together with decreased amino acid degradation/oxidation and decreased hepatic urea formation. During fasting and stress, the effects of GH on protein metabolism become more pronounced; lack of GH during fasting increases protein loss and urea production rates by approximately 50%, with a similar increase in muscle protein breakdown. GH is a counterregulatory hormone that antagonizes the hepatic and peripheral effects of insulin on glucose metabolism via mechanisms involving the concomitant increase in FFA flux and uptake. This ability of GH to induce insulin resistance is significant for the defense against hypoglycemia, for the development of "stress" diabetes during fasting and inflammatory illness, and perhaps for the "Dawn" phenomenon (the increase in insulin requirements in the early morning hours). Adult patients with GH deficiency are insulin resistant-probably related to increased adiposity, reduced LBM, and impaired physical performance-which temporarily worsens when GH treatment is initiated. Conversely, despite increased LBM and decreased fat mass, patients with acromegaly are consistently insulin resistant and become more sensitive after appropriate treatment.

Triiodo-L-thyronine stimulates glycogen synthesis in rat hepatocyte cultures

The hyperthyroid state is associated with low hepatic glycogen levels, but paradoxically with a high activity of glycogen synthase and low activity of glycogen phosphorylase. We determined the effects of triiodo-L-thyronine (T3) on glycogen synthesis and glycogen synthase activity in rat hepatocytes in vitro. Culture of rat hepatocytes with T3 (100 nM-1 microM) for 16 h-40 h increases glycogen synthesis from glucose and gluconeogenic precursors. The stimulation of glycogen synthesis by T3 was associated with an increase in the activity of glycogen synthase and was additive with the long-term effects of insulin but not with the short-term stimulation of glycogen synthesis by insulin. Culture of hepatocytes with T3 (at concentrations up to 1 microM) did not affect the responsiveness of glycogen synthesis to short-term stimulation by insulin but culture with 10 microM-T3 decreased the responsiveness to insulin without affecting the basal rate. It is suggested that the high activity of glycogen synthase in the hyperthyroid state is due to a direct effect of T3 on the hepatocyte, but the low hepatic glycogen content is probably due to either secondary metabolite and/or endocrine changes or to impaired responsiveness to insulin. T3 may have an anabolic role in the control of hepatic glycogen storage in the euthyroid postprandial state.

Evidence for dissociation of gluconeogenesis stimulated by non-esterified fatty acids and changes in fructose 2,6-bisphosphate in cultured rat hepatocytes

In order to examine the role of fructose 2,6-bisphosphate (Fru-2,6-P2) in non-esterified-fatty-acid-stimulated gluconeogenesis, Fru-2,6-P2 levels were measured in cultured rat hepatocytes under conditions mimicking the fasted state. After addition of either 1.5 mM-palmitate or 10 nM-glucagon, [U-14C]lactate incorporation into glucose increased 2-fold, but only glucagon suppressed Fru-2,6-P2. Prevention of palmitate oxidation with a carnitine palmitoyltransferase-I inhibitor (2-bromopalmitate) diminished glucose production and Fru-2,6-P2 levels. Addition of exogenous glucose to the media increased Fru-2,6-P2 in a dose-related manner, which was further augmented by addition of palmitate. When Fru-2,6-P2 levels were examined in cells cultured under conditions mimicking the fed state (significantly higher basal Fru-2,6-P2 levels and lower glucose production), palmitate oxidation was associated with a significant fall in Fru-2,6-P2. In conclusion, the present studies have demonstrated a dissociation between fatty-acid-stimulated gluconeogenesis and changes in Fru-2,6-P2 in cultured rat hepatocytes. Further experiments suggest that the accumulation of intracellular hexose 6-phosphate as a result of fatty-acid-stimulated gluconeogenesis masks a putative inhibitory effect of fatty acids on Fru-2,6-P2 concentrations.

Adrenergic mechanisms contribute to the late phase of hypoglycemic glucose counterregulation in humans by stimulating lipolysis

Three studies were performed on nine normal volunteers to assess whether catecholamine-mediated lipolysis contributes to counterregulation to hypoglycemia. In these three studies, insulin was intravenously infused for 8 h (0.30 mU.kg-1.min-1 from 0 to 180 min, and 0.40 mU.kg-1.min-1 until 480 min). In study I (control study), only insulin was infused; in study II (direct + indirect effects of catecholamines), propranolol and phentolamine were superimposed to insulin and exogenous glucose was infused to reproduce the same plasma glucose (PG) concentration of study I. Study III (indirect effect of catecholamines) was the same as study II, except heparin (0.2 U.kg-1.min-1 after 80 min), 10% Intralipid (1 ml.min-1 after 160 min) and variable glucose to match PG of study II, were also infused. Glucose production (HGO), glucose utilization (Rd) [3-3H]glucose, and glucose oxidation and lipid oxidation (LO) (indirect calorimetry) were determined. In all three studies, PG decreased from approximately 4.8 to approximately 2.9 mmol/liter (P = NS between studies), and plasma glycerol and FFA decreased to a nadir at 120 min. Afterwards, in study I plasma glycerol and FFA increased by approximately 75% at 480 min, but in study II they remained approximately 40% lower than in study I, whereas in study III they rebounded as in study I (P = NS). In study II, LO was lower than in study I (1.69 +/- 0.13 vs. 3.53 +/- 0.19 mumol.kg-1.min-1, P less than 0.05); HGO was also lower between 60 and 480 min (7.48 +/- 0.57 vs. 11.6 +/- 0.35 mumol.kg-1.min-1, P less than 0.05), whereas Rd was greater between 210 and 480 min (19 +/- 0.38 vs. 11.4 +/- 0.34 mumol.kg-1.min-1, respectively, P less than 0.05). In study III, LO increased to the values of study I; between 4 and 8 h, HGO increased by approximately 2.5 mumol.kg-1.min-1, and Rd decreased by approximately 7 mumol.kg-1.min-1 vs. study II. We conclude that, in a late phase of hypoglycemia, the indirect effects of catecholamines (lipolysis mediated) account for at least approximately 50% of the adrenergic contribution to increased HGO, and approximately 85% of suppressed Rd.

Regulation of fatty acid metabolism and gluconeogenesis by growth hormone and insulin in sheep hepatocyte cultures. Effects of lactation and pregnancy

Primary monolayer hepatocyte cultures derived from non-mated, pregnant and lactating sheep were used to investigate the interactions between the effects of growth hormone and insulin on (i) the partitioning of fatty acid metabolism between oxidation and esterification, and (ii) the rate of gluconeogenesis. In hepatocytes from lactating sheep the rates of gluconeogenesis, ketogenesis and very-low-density lipoprotein secretion were approx. 2-fold higher than in cells from non-mated or pregnant animals. There was no apparent difference in the rates of fatty acid uptake between the three groups of sheep cells. Growth hormone stimulated gluconeogenesis only in hepatocytes from non-mated sheep. It has no effect on the flux of fatty acid towards ketone body formation. Growth hormone inhibited intracellular accumulation of acylglycerol from exogenous fatty acid. Insulin alone had no such effect, but it blunted the effect of growth hormone when the two hormones were present together. The data suggest that major differences may exist between ruminants and non-ruminants in the response of liver metabolism both to lactation per se and to the effects of growth hormone and insulin.