Contribution of glucose and gluconeogenic substrates to insulin-stimulated glycogen synthesis in cultured fetal hepatocytes

The influence of medium composition on basal and insulin-stimulated glycogenesis was studied in cultured 17-day-old rat fetal hepatocytes, which contain no glycogen at the time of transplantation. Continuous-labeling ¹⁴C-glucose experiments were used to determine both glycogen content and glycogen labeling. The specific activity of glucose units in the newly formed glycogen (a) was compared to that of the medium glucose (b): the ratio a/b expresses the contribution of medium glucose to glycogen formation. In standard medium (5.5 mM glucose), this ratio averaged 0.60. Variations of glucose concentration in the medium from 1 to 40 mM were accompanied by a progressive increase in both glycogen content and the ratio a/b (up to 0.80). Supplementation of standard medium with fructose, galactose, glycerol, or lactate-pyruvate decreased the hepatocyte glucose uptake from the medium. Galactose (1 to 5 mM) or lactate-pyruvate (5 mM) enhanced the glycogen content whereas glycerol or fructose (1 to 5 mM) had no effect. The ratio a/b, not modified by glycerol or lactate-pyruvate, was decreased to 0.45 by fructose (5 mM). Galactose at concentrations as low as 1 to 2 mM brought the ratio down to 0.30, indicating that it is a superior precursor of glycogen as compared to glucose. When the hepatocytes were grown in the presence of 10 nM insulin, the glycogen content was constantly higher than in the absence of the hormone (2-fold stimulation). Also the amplitude of the glycogenic effect of insulin was similar whatever the modifications of the medium, whereas ratio a/b and glucose uptake were hardly increased by insulin. Thus several substrates can contribute to glycogen formation (especially galactose) in cultured fetal hepatocytes and the essential effect of insulin is a stimulation of the final step of the glycogenosynthetic pathway.     

Focal adhesion kinase (FAK) regulates insulin-stimulated glycogen synthesis in hepatocytes.

Experimental data support a role for FAK, an important component of the integrin signaling pathway, in insulin action. To test the hypothesis that FAK plays a regulatory role in hepatic insulin action, we overexpressed wild type (WT), a kinase inactive (KR), or a COOH-terminal focal adhesion targeting (FAT) sequence-truncated mutant of FAK in HepG2 hepatoma cells. In control untransfected (NON) and vector (CMV2)- and WT-transfected cells, insulin stimulated an expected 54 +/- 13, 37 +/- 4, and 47 +/- 12 increase in [U-(14)C]glucose incorporation into glycogen, respectively. This was entirely abolished in the presence of either KR (-1 +/- 7%) or FAT mutants (0 +/- 8%, n = 5, p < 0.05 for KR or FAT versus other groups), and this was associated with a significant attenuation of incremental insulin-stimulated glycogen synthase (GS) activity. Insulin-stimulated serine phosphorylation of Akt/protein kinase B was significantly impaired in mutant-transfected cells. Moreover, the ability of insulin to inactivate GS kinase-3beta (GSK-3beta), the regulatory enzyme immediately upstream of GS, by serine phosphorylation (308 +/- 16, 321 +/- 41, and 458 +/- 34 optical densitometric units (odu) in NON, CMV2, and WT, respectively, p < 0.02 for WT versus CMV2) was attenuated in the presence of either FAT (205 +/- 14, p < 0.01) or KR (189 +/- 4, p < 0.005) mutants. FAK co-immunoprecipitated with GSK-3beta, but only in cells overexpressing the KR (374 +/- 254 odu) and FAT (555 +/- 308) mutants was this association stimulated by insulin compared with NON (-209 +/- 92), CMV2 (-47 +/- 70), and WT (-39 +/- 31 odu). This suggests that FAK and GSK-3beta form both a constitutive association and a transient complex upon insulin stimulation, the dissociation of which requires normal function and localization of FAK. We conclude that FAK regulates the activity of Akt/protein kinase B and GSK-3beta and the association of GSK-3beta with FAK to influence insulin-stimulated glycogen synthesis in hepatocytes. Insulin action may be subject to regulation by the integrin signaling pathway, ensuring that these growth and differentiation-promoting pathways act in a coordinated and/or complementary manner.     

Receptor-mediated insulin degradation and insulin-stimulated glycogenesis in cultured foetal hepatocytes.

Insulin-stimulated glycogenesis and insulin degradation were studied simultaneously at 37 degrees C in cultured foetal hepatocytes grown for 2-3 days in the presence of cortisol. Degradation of cell-associated insulin, as measured by trichloroacetic acid precipitation, was significant after 4 min in the presence of 1-3 nM-125I-labelled insulin. This process became maximal (30% of insulin degraded) after 20 min, a time when binding-state conditions were achieved. No insulin-degradative activity was detected in a medium that had been exposed to cells. At steady-state, the appearance of insulin degradation products in the medium was linearly dependent on time (1.5 fmol/min per 10(6) cells at 1nM-125I-labelled insulin). Chloroquine (3-50 microM), bacitracin (0.1-10 mM) and NH4Cl (1-10 mM) inhibited insulin degradation as soon as this became detectable and caused an increase in the association of insulin to hepatocytes after 20 min. Lidocaine and dansylcadaverine had similar effects, whereas N-ethylmaleimide, aprotinin, phenylmethanesulphonyl fluoride and leupeptin were found to be ineffective. Chloroquine, and also bacitracin, at concentrations that inhibited insulin degradation, decreased the insulin-stimulated incorporation of [14C]glucose into glycogen over 2 h. This effect of chloroquine was specific, since it did not modify the basal glycogenesis, or the glycogenic effect of a glucose load in the absence of insulin. It therefore appears that the receptor-mediated insulin degradation (or some associated pathway) is functionally related to the glycogenic effect of insulin in foetal hepatocytes.     

Physiological hyperinsulinemia impairs insulin-stimulated glycogen synthase activity and glycogen synthesis

Although chronic hyperinsulinemia has been shown to induce insulin resistance, the basic cellular mechanisms responsible for this phenomenon are unknown. The present study was performed 1) to determine the time-related effect of physiological hyperinsulinemia on glycogen synthase (GS) activity, hexokinase II (HKII) activity and mRNA content, and GLUT-4 protein in muscle from healthy subjects, and 2) to relate hyperinsulinemia-induced alterations in these parameters to changes in glucose metabolism in vivo. Twenty healthy subjects had a 240-min euglycemic insulin clamp study with muscle biopsies and then received a low-dose insulin infusion for 24 (n = 6) or 72 h (n = 14) (plasma insulin concentration = 121 +/- 9 or 143 +/- 25 pmol/l, respectively). During the baseline insulin clamp, GS fractional velocity (0.075 +/- 0.008 to 0.229 +/- 0.02, P < 0.01), HKII mRNA content (0.179 +/- 0.034 to 0.354 +/- 0.087, P < 0.05), and HKII activity (2.41 +/- 0.63 to 3.35 +/- 0.54 pmol x min(-1) x ng(-1), P < 0.05), as well as whole body glucose disposal and nonoxidative glucose disposal, increased. During the insulin clamp performed after 24 and 72 h of sustained physiological hyperinsulinemia, the ability of insulin to increase muscle GS fractional velocity, total body glucose disposal, and nonoxidative glucose disposal was impaired (all P < 0.01), whereas the effect of insulin on muscle HKII mRNA, HKII activity, GLUT-4 protein content, and whole body rates of glucose oxidation and glycolysis remained unchanged. Muscle glycogen concentration did not change [116 +/- 28 vs. 126 +/- 29 micromol/kg muscle, P = nonsignificant (NS)] and was not correlated with the change in nonoxidative glucose disposal (r = 0.074, P = NS). In summary, modest chronic hyperinsulinemia may contribute directly (independent of change in muscle glycogen concentration) to the development of insulin resistance by its impact on the GS pathway.     

Pathways of glycogen synthesis from glucose during the glycogenic response to insulin in cultured foetal hepatocytes.

The pathways of glycogen synthesis from glucose were studied using double-isotope procedures in 18-day cultured foetal-rat hepatocytes in which glycogenesis is strongly stimulated by insulin. When the medium containing 4 mM-glucose was supplemented with [2-3H,U-14C]glucose or [3-3H,U-14C]glucose, the ratios of 3H/14C in glycogen relative to that in glucose were 0.23 +/- 0.04 (n = 6) and 0.63 +/- 0.09 (n = 8) respectively after 2 h. This indicates that more than 75% of glucose was first metabolized to fructose 6-phosphate, whereas 40% reached the step of the triose phosphates prior to incorporation into glycogen. The stimulatory effect of 10 nM-insulin on glycogenesis (4-fold) was accompanied by a significant increase in the (3H/14C in glycogen)/(3H/14C in glucose) ratio with 3H in the C-2 position (0.29 +/- 0.05, n = 6, P less than 0.001) or in the C-3 position (0.68 +/- 0.09, n = 8, P less than 0.01) of glucose, whereas the effect of a 12 mM-glucose load (3.5-fold) did not alter these ratios. Fructose (4 mM) displaced [U-14C]glucose during labelling of glycogen in the presence and absence of insulin by 50 and 20% respectively, and produced under both conditions a similar increase (45%) in the (3H/14C in glycogen)/(3H/14C in glucose) ratio when 3H was in the C-2 position. 3-Mercaptopicolinate (1 mM), an inhibitor of gluconeogenesis from lactate/pyruvate, further decreased the already poor labelling of glycogen from [U-14C]alanine, whereas it increased both glycogen content and incorporation of label from [U-14C]serine and [U-14C]glucose with no effect on the relative 3H/14C ratios in glycogen and glucose with 3H in the C-3 position of glucose. These results indicate that an alternative pathway in addition to direct glucose incorporation is involved in glycogen synthesis in cultured foetal hepatocytes, but that insulin preferentially favours the classical direct route. The alternative foetal pathway does not require gluconeogenesis from pyruvate-derived metabolites, contrary to the situation in the adult liver.     

Regulation of protein synthesis by estradiol-17beta in cultured hepatocytes

Estradiol-17beta added to cultured chick embryo hepatocytes induced the appearance in the medium of a phosphoprotein, identified as phosvitin on the basis of: (i) its behaviour on ionic exchange columns; (ii) its SDS-acrylamide gel electrophoretic mobility; (iii) its amino acid composition. The hormone treatment was also followed by a decreased synthesis of other proteins secreted by the hepatocytes.     

Effect of down-regulation and return of insulin receptors on glycogen synthesis in cultured rat hepatocytes

The dependence of the regulation of insulin receptors by insulin on the time hepatocytes were maintained in culture and the relationship between the return of down-regulated receptors and glycogen synthesis from labelled glucose were investigated in primary cultures of adult rat hepatocytes. Insulin receptor numbers, but not ligand affinity, decreased significantly within the first 24 h of culture, even in the absence of insulin, and then returned to the immediate 'post-attachment' level during 24-48 h. Therefore, down-regulation of insulin receptors by 10 nmol/l insulin was only minor during the 1st day in culture, but amounted to 50% of control levels after the 2nd day, whereas the rate of insulin degradation remained unaltered throughout the entire period of culture. When down-regulated monolayers were switched to insulin-free medium, receptors returned to control levels within 5-10 h. The reduced basal rate of glycogenesis as well as insulin-sensitivity and insulin responsiveness of this metabolic pathway also gradually increased to control levels. However, the time-dependent receptor return was dissociated from the increase in insulin-sensitivity, emphasising the importance of postbinding events. Since the changes both in basal rates and in insulin responsiveness of glycogenesis during the period of receptor return were inversely related to differences in the actual glycogen content between control and down-regulated cells, cellular glycogen content might participate in the regulation of glycogenesis as a 'feedback inhibitor'.     

Regulation of glycogen synthesis by amino acids in cultured human muscle cells

Insulin and a number of metabolic factors stimulate glycogen synthesis and the enzyme glycogen synthase. Using human muscle cells we find that glycogen synthesis is stimulated by treatment of the cells with lithium ions, which inhibit glycogen synthase kinase 3. Insulin further stimulates glycogen synthesis in the presence of lithium ions, an effect abolished by wortmannin and rapamycin. We report also that amino acids stimulate glycogen synthesis and glycogen synthase, these effects also being blocked by rapamycin and wortmannin. Amino acids stimulate p70(s6k) and transiently inhibit glycogen synthase kinase 3 without effects on the activity of protein kinase B or the mitogen-activated protein kinase pathway. Thus, the work reported here demonstrates that amino acid availability can regulate glycogen synthesis. Furthermore, it demonstrates that glycogen synthase kinase 3 can be inactivated within cells independent of activation of protein kinase B and p90(rsk).     

Regulation of glycogen synthesis from glucose and gluconeogenic precursors by insulin in periportal and perivenous rat hepatocytes.

Glycogen synthesis in hepatocyte cultures is dependent on: (1) the nutritional state of the donor rat, (2) the acinar origin of the hepatocytes, (3) the concentrations of glucose and gluconeogenic precursors, and (4) insulin. High concentrations of glucose (15-25 mM) and gluconeogenic precursors (10 mM-lactate and 1 mM-pyruvate) had a synergistic effect on glycogen deposition in both periportal and perivenous hepatocytes. When hepatocytes were challenged with glucose, lactate and pyruvate in the absence of insulin, glycogen was deposited at a linear rate for 2 h and then reached a plateau. However, in the presence of insulin, the initial rate of glycogen deposition was increased (20-40%) and glycogen deposition continued for more than 4 h. Consequently, insulin had a more marked effect on the glycogen accumulated in the cell after 4 h (100-200% increase) than on the initial rate of glycogen deposition. Glycogen accumulation in hepatocyte cultures prepared from rats that were fasted for 24 h and then re-fed for 3 h before liver perfusion was 2-fold higher than in hepatocytes from rats fed ad libitum and 4-fold higher than in hepatocytes from fasted rats. The incorporation of [14C]lactate into glycogen was 2-4-fold higher in periportal than in perivenous hepatocytes in both the absence and the presence of insulin, whereas the incorporation of [14C]glucose into glycogen was similar in periportal and perivenous hepatocytes in the absence of insulin, but higher in perivenous hepatocytes in the presence of insulin. Rates of glycogen deposition in the combined presence of glucose and gluconeogenic precursors were similar in periportal and perivenous hepatocytes, whereas in the presence of glucose alone, rates of glycogen deposition paralleled the incorporation of [14C]glucose into glycogen and were higher in perivenous hepatocytes in the presence of insulin. It is concluded that periportal and perivenous hepatocytes utilize different substrates for glycogen synthesis, but differences between the two cell populations in the relative utilization of glucose and gluconeogenic precursors are dependent on the presence of insulin and on the nutritional state of the rat.     

Regulation of glycogen synthase activation in isolated hepatocytes

Molecular and Cellular Biochemistry - Glycogen synthase, the regulatory enzyme of glycogen synthesis undergoes multisite phosphorylation leading to its inactivation. The kinases responsible for...     

Regulation of apo-A-I processing in cultured hepatocytes

Apo-A-I, the major protein component of high density lipoproteins, appears intracellularly as an intermediate precursor (pro-apo-A-I) with a hexapeptide extension (RHFWQQ) at its amino terminus. Proteolytic processing of pro-apo-A-I to apo-A-I has been shown to occur extracellularly in cell and organ cultures from rat and human tissues. Recently, however, intracellular conversion has been detected in chickens. To determine what distinguishes and regulates these two processing methods, the proteolytic processing and secretion of apo-A-I was studied by metabolic labeling in chick hepatocytes and in Hep-G2 cells (derived from a human hepatocellular carcinoma). The proportions of intracellular and secreted pro-apo-A-I and apo-A-I were measured by sequencing NH2-terminal portions of the proteins and determining the location of radio-labeled amino acids. Chick hepatocytes cultured in the absence of hormones or fetal bovine serum secreted primarily processed apo-A-I (83%). In the presence of serum these cells secreted only pro-apo-A-I, whereas incubation with a combination of hormones (insulin, triiodothyronine, dexamethasone) resulted in secretion of a nearly equal mixture of the pro- and processed forms of the protein. In contrast, Hep-G2 cells, maintained in the absence of serum, secreted only pro-apo-A-I; when grown in the presence of serum these cells secreted a mixture of pro- and processed apo-A-I. Under conditions in which chick hepatocytes and Hep-G2 cells secreted both forms of the protein, a mixture of pro- and processed apo-A-I was also found intracellularly; when only the pro-form was secreted, the cells likewise contained only pro-apo-A-I. Under all the above conditions, the secreted apo-A-I exhibited similar isoform patterns in two-dimensional gel electrophoresis. These data show that both chick hepatocytes and human hepatoma cells are capable of intracellularly processing pro-apo-A-I to apo-A-I, and that the extent of intracellular processing is controlled by the cell's hormonal environment.     

Regulation of bacterial glycogen synthesis

The formation of the alpha 1,4 glucosidic linkages of bacterial glycogen occurs first by synthesis of ADPglucose from ATP and alpha glucose 1-P and then transfer of the glucose moiety from the formed sugar nucleotide to a pre-existing glucan primer. Unlike mammalian glycogen synthesis, regulation occurs at the synthesis of the sugar nucleotide. Generally glycolytic intermediates activate ADPglucose synthesis while AMP, ADP and/or Pi inhibit ADPglucose synthesis. A variation of activator specificity is is seen when the enzyme is isolated from different bacteria and is thought to be related to the predominant type of carbon assimilation or dissimilation pathways present in the particular organism. Evidence indicating that the allosteric activation effects observed in vitro are physiologically pertinent for the regulation of glycogen synthesis is reviewed. The recent experiments in identifying the allosteric activator site of the Escherichia coli ADPglucose pyrophosphorylase as well as other chemical modification studies identifying amino acid residues essential for allosteric activation and for catalytic activity are discussed. Evidence is also presented for the covalent modification of the Rhodopseudomonas sphaeroides ADPglucose pyrophosphorylase by bromopyruvate at its allosteric activator site. Regulation of the biosynthesis of glycogen also occurs at the genetic level and the current evidence for the existence of a glycogen operon is presented. In addition the current studies concerning the cloning of the DNA region containing the Escherichia coli structural genes coding for the glycogen biosynthetic enzymes as well as the nucleotide sequence of the E. coli ADPglucose pyrophosphorylase are presented.     

Muscle glycogen content affects insulin-stimulated glucose transport and protein kinase B activity

We investigated the possible regulatory role of glycogen in insulin-stimulated glucose transport and insulin signaling in skeletal muscle. Rats were preconditioned to obtain low (LG), normal, or high (HG) muscle glycogen content, and perfused isolated hindlimbs were exposed to 0, 100, or 10,000 microU/ml insulin. In the fast-twitch white gastrocnemius, insulin-stimulated glucose transport was significantly higher in LG compared with HG. This difference was less pronounced in the mixed-fiber red gastrocnemius and was absent in the slow-twitch soleus. In the white gastrocnemius, insulin activation of insulin receptor tyrosine kinase and phosphoinositide 3-kinase was unaffected by glycogen levels, whereas protein kinase B activity was significantly higher in LG compared with HG. In additional incubation experiments on fast-twitch epitrochlearis muscles, insulin-stimulated cell surface GLUT-4 content was significantly higher in LG compared with HG. The data indicate that, in fast-twitch muscle, the effect of insulin on glucose transport and cell surface GLUT-4 content is modulated by glycogen content, which does not involve initial but possibly more downstream signaling events.     

Differential signaling of insulin and IGF-1 receptors to glycogen synthesis in murine hepatocytes.

We have used SV40-transformed hepatocytes from insulin receptor-deficient mice (-/-) and normal mice (WT) to investigate the different abilities of insulin and IGF-1 receptors to stimulate glycogen synthesis. We report that insulin receptors are more potent than IGF-1 receptors in stimulating glycogen synthesis. Both receptors stimulate glycogen synthesis in a PI 3-kinase-dependent manner, but only the effect of insulin receptors is partially rapamycin-dependent. Insulin and IGF-1 receptors activate Akt to a similar extent, whereas GSK-3 inactivation in response to IGF-1 is considerably lower in both -/- and WT cells, compared to the effect of insulin in WT cells. The findings indicate that (i) the potency of insulin and IGF-1 receptors in stimulating glycogen synthesis correlates with their ability to inactivate GSK-3, (ii) the extent of GSK-3 inactivation does not correlate with the extent of Akt activation mediated by insulin or IGF-1 receptors, indicating that the effect of insulin on GSK-3 requires additional kinases, and (iii) the pathways required for insulin stimulation of glycogen synthesis in mouse hepatocytes are PI 3-kinase-dependent and rapamycin-sensitive.     

Swelling of rat hepatocytes stimulates glycogen synthesis

In hepatocytes from fasted rats, several amino acids are known to stimulate glycogen synthesis via activation of glycogen synthase. The hypothesis that an increase in cell volume resulting from amino acid uptake may be involved in the stimulation of glycogen synthesis is supported by the following observations. 1) The extent of stimulation of glycogen synthesis by both metabolizable and nonmetabolizable amino acids was directly proportional to their ability to increase cell volume, except for proline, which stimulated glycogen synthesis more than could be accounted for by the increase in cell volume. 2) Both cell swelling and stimulation of glycogen synthesis by amino acids were prevented when hepatocytes were incubated in hyperosmotic media containing sucrose or raffinose. 3) Increasing the cell volume by incubating hepatocytes in Na(+)-depleted media in the absence of amino acids also stimulated glycogen synthesis. 4) Stimulation of glycogen synthesis by Na+ depletion was prevented by restoring the normal osmolarity with sucrose, but not with choline chloride which, by itself, stimulated glycogen synthesis and increased the cell volume. It is concluded that stimulation of glycogen synthesis by amino acids is due, at least in part, to an increase in hepatocyte volume resulting from amino acid uptake, and that hepatocyte swelling per se stimulates glycogen synthesis.     

Stimulation of glycogen synthesis and lipogenesis by glutamine in isolated rat hepatocytes.

Glutamine stimulated glycogen synthesis and lactate production in hepatocytes from overnight-fasted normal and diabetic rats. The effect, which was half-maximal with about 3 mM-glutamine, depended on glucose concentration and was maximal below 10 mM-glucose. beta-2-Aminobicyclo[2.2.1.]heptane-2-carboxylic acid, an analogue of leucine, stimulated glutaminase flux, but inhibited the stimulation of glycogen synthesis by glutamine. Various purine analogues and inhibitors of purine synthesis were found to inhibit glycogen synthesis from glucose, but they did not abolish the stimulatory effect of glutamine on glycogen synthesis. The correlation between the rate of glycogen synthesis and synthase activity suggested that the stimulation of glycogen synthesis by glutamine depended solely on the activation of glycogen synthase. This activation of synthase was not due to a change in total synthase, nor was it caused by a faster inactivation of glycogen phosphorylase, as was the case after glucose. It could, however, result from a stimulation of synthase phosphatase, since, after the addition of 1 nM-glucagon or 10 nM-vasopressin, glutamine did not interfere with the inactivation of synthase, but did promote its subsequent re-activation. Glutamine was also found to inhibit ketone-body production and to stimulate lipogenesis.     

Regulation of glycogen synthase in rat hepatocytes. Evidence for multiple signaling pathways.

We examined the signaling pathways regulating glycogen synthase (GS) in primary cultures of rat hepatocytes. The activation of GS by insulin and glucose was completely reversed by the phosphatidylinositol 3-kinase inhibitor wortmannin. Wortmannin also inhibited insulin-induced phosphorylation and activation of protein kinase B/Akt (PKB/Akt) as well as insulin-induced inactivation of GS kinase-3 (GSK-3), consistent with a role for the phosphatidylinositol 3-kinase/PKB-Akt/GSK-3 axis in insulin-induced GS activation. Although wortmannin completely inhibited the significantly greater level of GS activation produced by the insulin-mimetic bisperoxovanadium 1,10-phenanthroline (bpV(phen)), there was only minimal accompanying inhibition of bpV(phen)-induced phosphorylation and activation of PKB/Akt, and inactivation of GSK-3. Thus, PKB/Akt activation and GSK-3 inactivation may be necessary but are not sufficient to induce GS activation in rat hepatocytes. Rapamycin partially inhibited the GS activation induced by bpV(phen) but not that effected by insulin. Both insulin- and bpV(phen)-induced activation of the atypical protein kinase C (zeta/lambda) (PKC (zeta/lambda)) was reversed by wortmannin. Inhibition of PKC (zeta/lambda) with a pseudosubstrate peptide had no effect on GS activation by insulin, but substantially reversed GS activation by bpV(phen). The combination of this inhibitor with rapamycin produced an additive inhibitory effect on bpV(phen)-mediated GS activation. Taken together, our results indicate that the signaling components mammalian target of rapamycin and PKC (zeta/lambda) as well as other yet to be defined effector(s) contribute to the modulation of GS in rat hepatocytes.     

Brief food restriction increases FA oxidation and glycogen synthesis under insulin-stimulated conditions

To determine the effects of brief food restriction on fatty acid (FA) metabolism, hindlimbs of F344/BN rats fed either ad libitum (AL) or food restricted (FR) to 60% of baseline food intake for 28 days were perfused under hyperglycemic-hyperinsulinemic conditions (20 mM glucose, 1 mM palmitate, 1,000 microU/ml insulin, [3-(3)H]glucose, and [1-(14)C]palmitate). Basal glucose and insulin levels were significantly lower (P < 0.05) in FR vs. AL rats. Palmitate uptake (34.3 +/- 2.7 vs. 24.5 +/- 3.1 nmol/g/min) and oxidation (3.8 +/- 0.2 vs. 2.7 +/- 0.3 nmol.g(-1).min(-1)) were significantly higher (P < 0.05) in FR vs. AL rats, respectively. Glucose uptake was increased in FR rats and was accompanied by significant increases in red and white gastrocnemius glycogen synthesis, indicating an improvement in insulin sensitivity. Although muscle triglyceride (TG) levels were not significantly different between groups, glucose uptake and total preperfusion TG concentration were negatively correlated (r(2) = 0.27, P < 0.05). In conclusion, our results show that under hyperglycemic-hyperinsulinemic conditions, brief FR resulted in an increase in FA oxidative disposal that may contribute to the improvement in insulin sensitivity.     

Autoregulatory control of actin synthesis in cultured rat hepatocytes.

ADP-ribosylation of actin by Clostridium botulinum C2 toxin resulted in a depolymerization of filamentous F-actin and an increase of monomeric G-actin in cultured hepatocytes. Simultaneously the de novo synthesis of actin was largely reduced, while the synthesis of albumin and of other proteins was not significantly impaired. The specific decrease of actin mRNA to 30% of the control indicates a down-regulation of actin synthesis at a pretranslational level. On the other hand, treatment with the mycotoxin phalloidin resulted in an increase of F-actin and a decrease of monomeric G-actin. Under this condition the de novo synthesis of actin was specifically enhanced and the level of actin mRNA was increased to 600% of the control. The data suggest an autoregulatory control of the actin synthesis.