Alcohol and glucose metabolism in skeletal muscles in the rat

Impact of Alcohol on Glycemic Control and Insulin Action

Alcohol has profound effects on tissue and whole-body fuel metabolism which contribute to the increased morbidity and mortality in individuals with alcohol use disorder. This review focuses on the glucose metabolic effects of alcohol, primarily in the muscle, liver and adipose tissue, under basal postabsorptive conditions and in response to insulin stimulation. While there is a relatively extensive literature in this area, results are often discordant and extrapolating between models and tissues is fraught with uncertainty. Comparisons between data generated in experimental cell and animals systems will be contrasted with that obtained from human subjects as often times results differ. Further, the nutritional status is also an important component of the sometimes divergent findings pertaining to the effects of alcohol on the regulation of insulin and glucose metabolism. This work is relevant as the contribution of alcohol intake to the development or exacerbation of type 2 diabetes remains ill-defined and a multi-systems approach is likely needed as both alcohol and diabetes affect multiple targets within the body.

Chronic ethanol treatment of human hepatocytes inhibits the activation of the insulin signaling pathway by increasing cytosolic free calcium levels

The present study aimed to investigate the effects of ethanol treatment on the induction of intracellular calcium ([Ca(2+)](i)) levels and the inhibition of the activation of the insulin signaling pathway in human hepatocytes. L‑02 cells were treated with various concentrations of ethanol for different periods of time. Cell viability and alanine aminotransferase (ALT)/aspartate aminotransferase (AST) leakage in the culture supernatant were evaluated. Changes in [Ca(2+)](i) levels were detected by flow cytometry and confocal microscopy. Total RNA and protein were extracted to examine the mRNA and protein levels of insulin receptor substrate (IRS)1, IRS2, phosphatidylinositol 3‑kinase (PI3K) and glucose transporter 2 (GLUT2) by reverse transcription-quantitative polymerase chain reaction (RT‑qPCR) and western blot analysis, respectively. Furthermore, insulin was added to the ethanol‑treated L‑02 cells, and the phosphorylation levels of PI3K and protein kinase B (PKB) were determined by western blot analysis before and after Ca(2+) blockage. No significant changes were observed in cell viability, [Ca(2+)](i) levels and in the expression and phosphorylation levels of insulin signal transduction molecules when the L‑02 cells were treated with 0.5 or 1% ethanol. However, treatment with 2 or 4% ethanol resulted in a significant decrease in cell viability and in the mRNA levels of IRS1, IRS2, PI3K (p85α) and GLUT2, as well as in an increase in ALT/AST leakage and in the [Ca(2+)](i) levels (P<0.05). The expression and phosphorylation levels of PI3K (p85α) and PKB were also inhibited by treatment with 2 or 4% ethanol. These cytological effects induced by ethanol treatment were partially reversed by Ca(2+) blockage. These results suggest that ethanol treatment inhibits the activation of the insulin signal transduction pathway in a dose‑, time‑ and Ca(2+)‑dependent manner. The inhibition of IRS1/2, PI3K (p85α), PKB and GLUT2 expression and of PI3K (p85α) and PKB phosphorylation by the high concentrations of ethanol may be the core molecular mechanism of ethanol-induced insulin resistance, and may be related to the induction of [Ca(2+)](i) levels.

Strain-dependent differences for suppression of insulin-stimulated glucose uptake in skeletal and cardiac muscle by ethanol

BACKGROUND

Chronic ethanol (EtOH) consumption impairs the ability of insulin to suppress hepatic glucose production in a strain-dependent manner, with hepatic insulin resistance being greater in Long-Evans (LE) than Sprague-Dawley (SD) rats. We assessed whether strain differences exist for whole-body and tissue glucose uptake under basal and insulin-stimulated conditions and whether they were associated with coordinate strain-dependent elevations in muscle cytokines.

METHODS

Male rats (160 g) were provided the Lieber-DeCarli EtOH-containing (36% total energy) diet or pair-fed a control diet for 8 weeks. Rats were studied in the basal state or during a euglycemic hyperinsulinemic clamp, and whole-body glucose flux assessed using (3) H-glucose and in vivo tissue glucose uptake by (14) C-2-deoxyglucose.

RESULTS

EtOH impaired whole-body insulin-mediated glucose uptake (IMGU) more in SD than LE rats. This difference was due to impaired IMGU by gastrocnemius and heart in EtOH-fed SD versus LE rats. However, decreased IMGU in adipose tissue (epididymal and perirenal) produced by EtOH was comparable between strains. EtOH-induced insulin resistance in muscle from SD rats was associated with reduced AKT and AS160 phosphorylation and plasma membrane-localized GLUT4 protein as well as enhanced phosphorylation of c-Jun N-terminal kinase (JNK) and IRS-1 (S307), changes which were absent in muscle from LE rats. EtOH increased tumor necrosis factor alpha (TNFα) mRNA in gastrocnemius and fat under basal conditions in both SD and LE rats; however, hyperinsulinemia decreased TNFα in skeletal muscle from LE, but not SD rats. Interleukin (IL)-6 mRNA in gastrocnemius was increased under basal conditions and increased further in response to insulin in SD rats, but no EtOH- or insulin-induced change was detected in muscle IL-6 of LE rats.

CONCLUSIONS

These data indicate strain-dependent differences in EtOH-induced IMGU in skeletal and cardiac muscle, but not fat, associated with sustained increases in TNFα and IL-6 mRNA and JNK activation and decreased plasma membrane GLUT4 in response to insulin.

Chronic ethanol consumption resulting in the downregulation of insulin receptor-beta subunit, insulin receptor substrate-1, and glucose transporter 4 expression in rat cardiac muscles

The objective of this study was to investigate the effect of chronic ethanol intake on the expression of insulin receptor beta subunit (IR-beta), insulin receptor substrate-1 (IRS-1), and glucose transporter 4 (Glut4) in rat cardiac muscle. Forty-eight male Wistar rats were randomly classified into four groups and to each group, ethanol was administered daily at the respective doses of 0 (control, C), 0.5 g kg(-1) (low ethanol, L), 2.5 g kg(-1) (middle ethanol, M), and 5 g kg(-1) (high ethanol, H). Twenty-two weeks later, the rats were anesthetized, and the left ventricle muscles were excised. The IR-beta, IRS-1, and Glut4 mRNA levels in the cardiac muscle tissues were detected by reverse-transcription polymerase chain reaction (RT-PCR); the IR-beta, tyrosine phosphorylation of IR-beta (PY-IR-beta), IRS-1, tyrosine phosphorylation of IRS-1 (PY-IRS-1), and Glut4 protein levels were measured by Western blotting. Compared to the control group, the IR-beta, IRS-1, and Glut4 mRNA levels in groups H and M decreased remarkably. In addition, the protein levels of IR-beta, IRS-1, and Glut4 showed a corresponding decline in ethanol-treated groups, especially in group H. Moreover, the PY-IR-beta and PY-IRS-1 protein levels decreased in the hearts of ethanol-treated rats with corresponding changes in the IR-beta and IRS-1 protein levels. The present study showed that chronic ethanol intake could downregulate the expression levels of IR-beta, IRS-1, and Glut4 in rat cardiac muscles.

Simultaneous determination of blood glucose and isoleucine levels in rats after chronic alcohol exposure by microwave-assisted derivatization and isotope dilution gas chromatography/mass spectrometry

Blood glucose and isoleucine are two biomarkers of chronic alcohol exposure. Simultaneous determination of blood glucose and isoleucine levels helps to illuminate the influence of alcohol on the metabolism of glucose and amino acids. The most accurate method for the detection of serum glucose is isotope dilution gas chromatography/mass spectrometry (ID GC/MS). In this study, a rapid, simple and sensitive technique was developed for the quantitative analysis of glucose and isoleucine in rats after chronic alcohol exposure by microwave-assisted derivatization (MAD) and ID GC/MS. Serum glucose and isoleucine were rapidly derivatized by N-methyl-N-trimethylsilyltrifluoroacetamide (MSTFA) with microwave irradiation, and the trimethylsilyl derivatives were analyzed by GC/MS. This technique was used to demonstrate that pyrroloquinoline quinone (PQQ), a non-covalently bound prosthetic group in some quinoproteins involved in the metabolism of some sugar or alcohol, could reverse alcohol exposure induced glucose elevation. On the other hand, it did not affect the metabolism of isoleucine whose level was elevated along with serum glucose. The combination of MAD and ID GC/MS has been shown to be an accurate, rapid, simple and sensitive method for the quantification of glucose and isoleucine in serum samples.

Chronic alcohol consumption yields sex differences in whole-body glucose production in rats

AIMS

The effects of chronic alcohol consumption (8 weeks) on glucose kinetics, in the absence (water, 4 g/kg) and presence of an acute ethanol dose (4 g/kg), were examined in 48 h fasted male and female Wistar rats.

METHODS

Primed continuous infusions of [6-3H]- and [U-14C]glucose were employed to assess rates of glucose appearance (Ra), glucose disappearance (Rd), and apparent glucose carbon recycling.

RESULTS

After injecting the male and female controls with water, there were no significant alterations in glucose kinetics. Compared to controls, chronic alcohol-fed female animals (injected with water) demonstrated significantly lower: glucose Ra, blood glucose concentration, and apparent glucose carbon recycling for a majority of the experimental period. In separate groups injected with ethanol, the glucose Ra fell by 31% for male rats fed the control diet (MC), 43% for male rats fed the ethanol diet (ME), 29% for female rats fed the control diet (FC), and 42% for female rats fed the ethanol diet (FE). Further, compared to controls (MC and FC), the blood glucose concentration was significantly lower prior to and following the ethanol injection for FE. In addition, FE animals had significantly lower rates of glucose Ra and glucose carbon recycling compared to controls prior to and after the ethanol injection. ME animals demonstrated similar declines in glucose Ra (compared to FE), but only after the ethanol injection. Conversely, ME were able to match the decrease in glucose Ra with comparable declines in glucose Rd resulting in blood glucose concentrations that did not differ from controls.

CONCLUSIONS

Chronic alcohol consumption results in sex differences in whole-body glucose production and glucose regulation.

Effect of alcohol intake on muscle glycogen storage after prolonged exercise

We studied the effects of alcohol intake on postexercise muscle glycogen restoration with samples from vastus lateralis being collected immediately after glycogen-depleting cycling and after a set recovery period. Six well-trained cyclists undertook a study of 8-h recovery (2 meals), and another nine cyclists undertook a separate 24-h protocol (4 meals). In each study, subjects completed three trials in crossover order: control (C) diet [meals providing carbohydrate (CHO) of 1.75 g/kg]; alcohol-displacement (A) diet (1.5 g/kg alcohol displacing CHO energy from C) and alcohol + CHO (AC) diet (C + 1.5 g/kg alcohol). Alcohol intake reduced postmeal glycemia especially in A trial and 24-h study, although insulin responses were maintained. Alcohol intake increased serum triglycerides, particularly in the 24-h study and AC trial. Glycogen storage was decreased in A diets compared with C at 8 h (24.4 +/- 7 vs. 44.6 +/- 6 mmol/kg wet wt, means +/- SE, P < 0.05) and 24 h (68 +/- 5 vs. 82 +/- 5 mmol/kg wet wt, P < 0.05). There was a trend to reduced glycogen storage with AC in 8 h (36.2 +/- 8 mmol/kg wet wt, P = 0.1) but no difference in 24 h (85 +/- 9 mmol/kg wet wt). We conclude that 1). the direct effect of alcohol on postexercise glycogen synthesis is unclear, and 2). the main effect of alcohol intake is indirect, by displacing CHO intake from optimal recovery nutrition practices.

Metabolic effects of alcohol on skeletal muscle

The purpose of this review is to summarize our current understanding of the acute and chronic interactions between alcohol and nutrient metabolism in skeletal muscle. Insulin is well known to play an important regulatory role in nutrient, especially glucose, uptake and utilization in skeletal muscle. Several studies have shown that alcohol can acutely reduce the normal metabolic responses of skeletal muscle to the action of insulin. The most obvious of these is an acute impairment in glucose metabolism associated with alcohol consumption. While the exact mechanism(s) underlying this acute insulin resistance is presently unclear, several possible factors are discussed in this review. In contrast to these short-term effects, the effects of alcohol on skeletal muscle insulin sensitivity in chronic alcohol abusers are not as well established. Chronic alcohol abuse is known to be associated with skeletal myopathies, believed to result from alcohol induced abnormalities in muscle protein synthesis. Finally, the alcohol-mediated impairments of many aspects of skeletal muscle metabolism are discussed in relation to the insulin resistance associated broad spectrum of common lifestyle-related disorders, including non-insulin dependent diabetes mellitus and obesity, the consequences of which may be important to the pathogenesis of alcohol-related diseases.

Acute ethanol-mediated insulin resistance in the rat: the role of ethanol oxidation

Ethanol causes an acute and profound insulin resistance in humans and in the rat. Recent studies indicate that defects in skeletal muscle glucose uptake and utilization make a major contribution to this insulin resistance. In this study, we used the euglycaemic hyperinsulinaemic clamp to examine the role that hepatic ethanol oxidation via alcohol dehydrogenase (ADH) plays in the acute insulin resistance caused by ethanol in the rat. Treatment with the ADH inhibitor 4-methylpyrazole (4-MP) failed to abolish the insulin resistance as expressed as a decrease in the rate of glucose infusion required to maintain euglycaemia (GIR). A decrease in GIR was also observed in response to tert-butanol, an alcohol that is not a substrate for hepatic ADH. These results indicate that oxidation via ADH is not a prerequisite for the inhibition by ethanol of whole-body glucose utilization. In a separate study, we examined the relationship between blood ethanol concentration and GIR in order to determine the potency of ethanol in causing insulin resistance. These experiments showed that even at low blood concentrations (<2 mM), ethanol caused a profound decrease in GIR, similar in magnitude to that observed at higher blood concentrations (approximately 40 mM)

Ethanol acutely impairs glycogen repletion in skeletal muscle following high intensity short duration exercise in the rat

Ethanol is recognized to affect adversely carbohydrate metabolism in skeletal muscle. This paper seeks to establish whether ethanol acutely impairs glycogen repletion during recovery from high intensity short duration exercise in the rat. High intensity exercise caused the massive mobilization of glycogen stores in muscles rich in type IIa and IIb fibres and marked increases in plasma and muscle lactate levels. During the 30-minute recovery period, there was substantial glycogen repletion in these muscles in both the ethanol-treated and control rats. Ethanol, however, was associated with reduced glycogen resynthesis in both the tibialis anterior (by 22%) and red gastrocnemius (by 31%) muscles but not in the white gastrocnemius muscle. This reduction in post-exercise glycogen deposition was accompanied by decreased lactate disposal and elevated plasma glucose levels. These effects of ethanol on glycogen repletion may involve interactions with hepatic gluconeogenesis, glucose uptake and utilization in muscle, muscle glycogen synthesis and lactate glyconeogenesis. The ethanol-mediated impairment in post-exercise glycogen repletion may have important implications for the pathogenesis of chronic alcoholic skeletal myopathy.

No change in apoptosis in skeletal muscle exposed acutely or chronically to alcohol

The pathogenic mechanisms responsible for the deleterious changes in ethanol-exposed skeletal muscle are unknown, although apoptosis may be a causal process. We therefore investigated the responses of skeletal muscle to acute or chronic ethanol exposure in male Wistar rats. In acute studies, rats were dosed with ethanol (75 mmol (3.46 g)/kg BW) and killed after either 2.5 or 6 hours. In chronic studies, rats were fed ethanol as 35% of total dietary energy for 6 weeks. Apoptosis was determined by either DNA fragmentation or TUNEL (terminal deoxynucleotidyl transferase mediated dUTP nick end labelling) assays. The results showed that apoptosis was not increased in the ethanol-exposed muscle in both acute and chronic studies compared to appropriate controls.

Alcoholic myopathy: biochemical mechanisms

Between one- and two-thirds of all alcohol abusers have impairment of muscle function that may be accompanied by biochemical lesions and/or the presence of a defined myopathy characterised by selective atrophy of Type II fibres. Perturbations in protein metabolism are central to the effects on muscle and account for the reductions in muscle mass and fibre diameter. Ethanol abuse is also associated with abnormalities in carbohydrate (as well as lipid) metabolism in skeletal muscle. Ethanol-mediated insulin resistance is allied with the inhibitory effects of ethanol on insulin-stimulated carbohydrate metabolism. It acutely impairs insulin-stimulated glucose and lipid metabolism, although it is not known whether it has an analogous effect on insulin-stimulated protein synthesis. In alcoholic cirrhosis, insulin resistance occurs with respect to carbohydrate metabolism, although the actions of insulin to suppress protein degradation and stimulate amino acid uptake are unimpaired. In acute alcohol-dosing studies defective rates of protein synthesis occur, particularly in Type II fibre-predominant muscles. The relative amounts of mRNA-encoding contractile proteins do not appear to be adversely affected by chronic alcohol feeding, although subtle changes in muscle protein isoforms may occur. There are also rapid and sustained reductions in total (largely ribosomal) RNA in chronic studies. Loss of RNA appears to be related to increases in the activities of specific muscle RNases in these long-term studies. However, in acute dosing studies (less than 1 day), the reductions in muscle protein synthesis are not due to overt loss of total RNA. These data implicate a role for translational modifications in the initial stages of the myopathy, although changes in transcription and/or protein degradation may also be superimposed. These events have important implications for whole-body metabolism.

Ethanol stimulates glucose uptake and translocation of GLUT-4 in H9c2 myotubes via a Ca(2+)-dependent mechanism

Short-term exposure to ethanol impairs glucose homeostasis, but the effects of ethanol on individual components of the glucose disposal pathway are not known. To understand the mechanisms by which ethanol disrupts glucose homeostasis, we have investigated the direct effects of ethanol on glucose uptake and translocation of GLUT-4 in H9c2 myotubes. Short-term treatment with 12.5-50 mM ethanol increased uptake of 2-deoxyglucose by 1.8-fold in differentiated myotubes. Pretreatment of H9c2 myotubes with 100 nM wortmannin, an inhibitor of phosphatidylinositol 3-kinase, had no effect on ethanol-induced increases in 2-deoxyglucose uptake. In contrast, preincubation with 25 microM dantrolene, an inhibitor of Ca(2+) release from the sarcoplasmic reticulum, blocked the stimulation of 2-deoxyglucose uptake by ethanol. Increased 2-deoxyglucose uptake after ethanol treatment was associated with a decrease in small intracellular GLUT-4 vesicles and an increase in GLUT-4 localized at the cell surface. In contrast, ethanol had no effect on the quantity of GLUT-1 and GLUT-3 at the plasma membrane. These data demonstrate that physiologically relevant concentrations of ethanol disrupt the trafficking of GLUT-4 in H9c2 myotubes resulting in translocation of GLUT-4 to the plasma membrane and increased glucose uptake.

Metabolic consequences of alcohol ingestion

Many of the pathophysiological effects of alcohol ingestion relate to the pathways of ethanol metabolism. However, some of the acute and chronic effects of ethanol use are also attributable to the direct effects of ethanol, e.g. on membrane fluidity. Oxidation of ethanol to acetaldehyde is catalysed by alcohol dehydrogenase (ADH). There are at least six classes of ADH, some of which show inter-individual variation, i.e. genetic polymorphism, that influences the rate of ethanol oxidation. A consequence of ethanol oxidation is an increase in the NADH/NAD redox potential within the cytosol and mitochondria with subsequent alteration in several tissue metabolites. The popular hypothesis that most, if not all, of the consequences of chronic alcohol ingestion can be explained by these redox changes is still unproven. This should be considered in the context that most metabolic pathways of the liver are affected by alcohol, as are several endocrine axes in the whole body. In fact most, if not all, tissues and organs are deleteriously affected by chronic ingestion. Acetaldehyde, the product of ethanol oxidation, is chemically highly reactive, toxic and immunogenic. However, the concentrations achieved in vivo usually fall short of those used to produce these toxic effects in experimental situations. Oxidation of acetaldehyde is also coupled to redox changes, although primarily affecting the intra-mitochondrial redox. In addition, further oxidative pathways of ethanol metabolism can lead to the formation of fatty acid ethyl esters, hydroxyethyl free radicals and reactive oxygen species via the ethanol-specific cytochrome P450-2E1 system. There is no conclusive evidence that nutrient supplementation has beneficial effects on overall ethanol-mediated tissue damage.

Alcohol-related diols cause acute insulin resistance in vivo

Epidemiological studies suggest that alcohol consumption is an independent risk factor for the development of non-insulin-dependent diabetes mellitus (NIDDM). Alcoholism is known to be associated with increased plasma levels of two novel diols, 2,3-butanediol and 1,2-propanediol, metabolites known to impair insulin action in isolated adipocytes. This study examines whether 2,3-butanediol and 1,2-propanediol have the capacity to impair insulin action acutely in vivo in the rat. Using the euglycemic-hyperinsulinemic clamp, it is shown that the two diols reduce whole-body glucose utilization (by approximately 30%), with the onset of insulin resistance in vivo occurring at plasma concentrations of 2,3-butanediol (33 micromol/L) at least one order of magnitude (P < .001) lower than 1,2-propanediol (432 micromol/L). Tracer methodologies using [U-14C]glucose and 2-deoxy[1-(3)H]glucose indicate that the reduction in whole-body glucose utilization is accompanied by a reduction in glucose uptake and glycogen synthesis in the skeletal muscle and heart. The association between elevated plasma diol levels and insulin resistance demonstrated in this report raises the question of whether there is a link between the high plasma diol levels in alcohol abusers and their increased susceptibility to NIDDM.