Insulin affects glucose uptake by muscle and mammary tissues of lactating ewes

Effects of insulin on exchanges of glucose across skeletal muscle and mammary tissue were measured in short-term studies in lactating ewes. Insulin secretion was suppressed by a primed/continuous infusion of somatostatin, then insulin was administered by continuous intravenous infusion of doses that were increased, in a step-wise manner, from 0 to 2 U h-1. Plasma glucose was maintained essentially constant by frequent monitoring and intravenous administration of exogenous glucose. Somatostatin suppressed but did not completely inhibit insulin secretion as shown by maintenance of plasma concentration of C-peptide. As plasma insulin was increased, while arterial glucose was maintained stable, uptake of glucose by skeletal muscle increased and glucose uptake by the mammary gland decreased. These observations confirm the role of insulin in regulating glucose uptake by skeletal muscle and raise the possibility that insulin also regulates glucose uptake by the mammary gland.     

Physiological significance of altered insulin metabolism in the conscious rat during lactation.

Uptake of radioactively labelled insulin by the mammary gland of the rat increased 12-fold in lactation compared with non-lactating controls. This uptake was decreased by the presence of unlabelled insulin, indicating that it occurred via insulin receptors. The plasma half-life of insulin is decreased in lactation from 9.4 min to 4.8 min, and the metabolic clearance rate for insulin increased from 7.26 to 13.03 ml/kg body wt. per min. The basal insulin and glucose concentrations in the plasma were decreased in lactation. Infusion of insulin at a dose which led to a small physiological rise in plasma insulin concentration increased lipogenic rates in the mammary gland by 100% without causing marked hypoglycaemia. It is concluded that the lactating mammary gland is a highly insulin-sensitive tissue and that the lower plasma insulin during lactation occurs primarily as a result of this sensitivity increasing extraction of glucose by the gland and thus producing a decrease in the plasma glucose concentration. It is suggested that a secondary result of the fall in plasma insulin concentration is the preferential direction of substrates (glucose and non-esterified fatty acids) towards the lactating mammary gland and away from adipose tissue and the liver.     

Effects of exogenous growth hormone on milk production and nutrient uptake by muscle and mammary tissues of dairy cows in mid-lactation

Responses to exogenous growth hormone were measured in lactating dairy cows surgically prepared to allow measurement of nutrient exchanges across mammary and hind-limb muscle tissues. Cows were injected daily with either saline or growth hormone, at a dose of 0.1 mg/kg liveweight, over periods of 6 days. During administration of growth hormone milk yield, milk fat content and yields of milk fat protein and lactose increased. Arterial plasma concentrations of glucose and non-esterified fatty acids were increased, uptake of glucose by leg muscle tissue decreased, lactate release from leg muscle tended to increase, mammary uptake of non-esterified fatty acids increased, blood flow to leg muscle tended to increase and blood flow to mammary tissue increased during injection of growth hormone. The results show that growth hormone affects supply to and utilization of key nutrients by tissues, resulting in the supply to the mammary gland of additional precursors for milk synthesis.     

Effects of insulin on the metabolism of acetate, beta-hydroxybutyrate and triglycerides by the bovine mammary gland

The effect of insulin on net metabolism of acetate, beta-hydroxybutyrate, glucose and triglycerides by the bovine mammary gland was determined using the glucose clamp technique. Elevation of insulin concentrations in plasma by 50 microU/ml was not associated with significant changes in venoarterial concentration differences and extraction ratios ((A-V)/A). Insulin does not appear to alter the metabolism of acetate, beta-hydroxybutyrate, glucose and triglycerides by direct effects on the mammary gland.     

Oxidative stress--mediated alterations in glucose dynamics in a genetic animal model of type II diabetes

Insulin resistance, characterized by an inexorable decline in skeletal muscle glucose utilization and/or an excessive hepatic glucose production, constitutes a major pathogenic importance in a cluster of clinical disorders including diabetes mellitus, hypertension, dyslipidemia, central obesity and coronary artery disease. A novel concept suggests that heightened state of oxidative stress during diabetes contributes, at least in part, to the development of insulin resistance. Several key predictions of this premise were subjected to experimental testing using Goto-Kakizaki (GK) rats as a genetic animal model for non-obese type II diabetes. Euglycemic-hyperinsulinemic clamp studies with an insulin infusion index of 5 mU/kg bw/min were used to measure endogenous glucose production (EGP), glucose infusion rate (GIR), glucose disposal rate (GDR) and skeletal muscle glucose utilization index (GUI). Moreover, the status of oxidative stress as reflected by the urinary levels of isoprostane and protein carbonyl formation were also assessed as a function of diabetes. Post-absorptive basal EGP and circulating levels of insulin, glucose and free fatty acid (FFA) were elevated in GK rats, compared to their corresponding control values. In contrast, steady state GIR and GDR of the hyperglycemic/hyperinsulinemic animals were reduced, concomitantly with impaired insulin's ability to suppress EGP. Insulin stimulated [3H]-2-deoxyglucose (2-DG) uptake (a measure of glucose transport activity) by various types of skeletal muscle fibers both in vivo and in vitro (isolated muscle, cultured myoblasts) was diminished in diabetic GK rats. This diabetes-related suppression of skeletal muscle glucose utilization was associated with a decrease in insulin's ability to promote the phosphorylation of tyrosine residues of insulin receptor substrate-1 (IRS-1). Similarly, the translocation of GLUT-4 from intracellular compartment to plasma membrane in response to insulin was also reduced in these animals. Oxidative stress-based markers (e.g. urinary isoprostane, carbonyl-bound proteins) were elevated as a function of diabetes. Nullification of the heightened state of oxidative stress in the GK rats with alpha-lipoic acid resulted in a partial amelioration of the diabetes-related impairment of the in vivo and in vitro insulin actions. Collectively, the above data suggest that 1) insulin resistance in GK rats occurs at the hepatic and skeletal muscle levels, 2) muscle cell glucose transport exhibited a blunted response to insulin and it is associated with a major defect in key molecules of both GLUT-4 trafficking and insulin signaling pathways, 3) skeletal muscle insulin resistance in GK rats appears to be of genetic origin and not merely related to a paracrine or autocrine effect, since this phenomenon is also observed in cultured myoblasts over several passages and finally heightened state of oxidative stress may mediate the development of insulin resistance during diabetes.     

Lactate induces insulin resistance in skeletal muscle by suppressing glycolysis and impairing insulin signaling

Elevation of plasma lactate levels induces peripheral insulin resistance, but the underlying mechanisms are unclear. We examined whether lactate infusion in rats suppresses glycolysis preceding insulin resistance and whether lactate-induced insulin resistance is accompanied by altered insulin signaling and/or insulin-stimulated glucose transport in skeletal muscle. Hyperinsulinemic euglycemic clamps were conducted for 6 h in conscious, overnight-fasted rats with or without lactate infusion (120 micromol x kg(-1) x min(-1)) during the final 3.5 h. Lactate infusion increased plasma lactate levels about fourfold. The elevation of plasma lactate had rapid effects to suppress insulin-stimulated glycolysis, which clearly preceded its effect to decrease insulin-stimulated glucose uptake. Both submaximal and maximal insulin-stimulated glucose transport decreased 25-30% (P < 0.05) in soleus but not in epitrochlearis muscles of lactate-infused rats. Lactate infusion did not alter insulin's ability to phosphorylate the insulin receptor, the insulin receptor substrate (IRS)-1, or IRS-2 but decreased insulin's ability to stimulate IRS-1- and IRS-2-associated phosphatidylinositol 3-kinase activities and Akt/protein kinase B activity by 47, 75, and 55%, respectively (P < 0.05 for all). In conclusion, elevation of plasma lactate suppressed glycolysis before its effect on insulin-stimulated glucose uptake, consistent with the hypothesis that suppression of glucose metabolism could precede and cause insulin resistance. In addition, lactate-induced insulin resistance was associated with impaired insulin signaling and decreased insulin-stimulated glucose transport in skeletal muscle.     

Effect of insulin and contraction up on glucose transport in skeletal muscle

The major glucose transporter protein expressed in skeletal muscle is GLUT4. Both muscle contraction and insulin induce translocation of GLUT4 from the intracellular pool to the plasma membrane. The intracellular pathways that lead to contraction- and insulin-stimulated GLUT4 translocation seem to be different, allowing the attainment of a maximal effect when acting together. Insulin utilizes a phosphatidylinositol 3-kinase-dependent mechanism, whereas the exercise signal may be initiated by calcium release from the sarcoplasmic reticulum or from autocrine- or paracrine-mediated activation of glucose transport. During exercise skeletal muscle utilizes more glucose than when at rest. However, endurance training leads to decreased glucose utilization during sub-maximal exercise, in spite of a large increase in the total GLUT4 content associated with training. The mechanisms involved in this reduction have not been totally elucidated, but appear to cause the decrease of the amount of GLUT4 translocated to the plasma membrane by altering the exercise-induced enhancement of glucose transport capacity. On the other hand, the effect of resistance training is controversial. Recent studies, however, demonstrated the improvement in insulin sensitivity correlated with increasing muscle mass. New studies should be designed to define the molecular basis for these important adaptations to skeletal muscle. Since during exercise the muscle may utilize insulin-independent mechanisms to increase glucose uptake, the mechanisms involved should provide important knowledge to the understanding and managing peripheral insulin resistance.     

Insulin action in skeletal muscle from patients with NIDDM

Insulin resistance in peripheral tissues is a common feature of non insulin-dependent diabetes mellitus (NIDDM). The decrease in insulin-mediated peripheral glucose uptake in NIDDM patients can be localized to defects in insulin action on glucose transport in skeletal muscle. Following short term in vitro exposure to both submaximal and maximal concentrations of insulin, 3-O-methylglucose transport rates are 40-50% lower in isolated skeletal muscle strips from NIDDM patients when compared to muscle strips from nondiabetic subjects. In addition, we have shown that physiological levels of insulin induce a 1.6-2.0 fold increase in GLUT4 content in skeletal muscle plasma membranes from control subjects, whereas no significant increase was noted in NIDDM skeletal muscle. Impaired insulin-stimulated GLUT4 translocation and glucose transport in NIDDM skeletal muscle is associated with reduced insulin-stimulated IRS-1 tyrosine phosphorylation and PI3-kinase activity. The reduced IRS-1 phosphorylation cannot be attributed to decreased protein expression, since the IRS-1 protein content is similar between NIDDM subjects and controls. Altered glycemia may contribute to decreased insulin-mediated glucose transport in skeletal muscle from NIDDM patients. We have shown that insulin-stimulated glucose transport is normalized in vitro in the presence of euglycemia, but not in the presence of hyperglycemia. Thus, the circulating level of glucose may independently regulate insulin stimulated glucose transport in skeletal muscle from NIDDM patients via a down regulation of the insulin signaling cascade.     

Inhibition of lipolysis does not affect insulin sensitivity to glucose uptake in the mourning dove

Birds have much higher plasma glucose and fatty acid levels compared to mammals. In addition, they are resistant to insulin-induced decreases in blood glucose. Recent studies have demonstrated that decreasing fatty acid utilization alleviates insulin resistance in mammals, thereby decreasing plasma glucose levels. This has yet to be examined in birds. In the present study, the levels of glucose and beta-hydroxybutyrate (BOHB), a major ketone body and indicator of fatty acid utilization, were measured after the administration of chicken insulin, acipimox (an anti-lipolytic agent), or insulin and acipimox in mourning doves (Zenaidura macroura). Insulin significantly decreased whole blood glucose levels (19%), but had no effect on BOHB concentrations. In contrast, acipimox decreased blood BOHB levels by 41%, but had no effect on whole blood glucose. In addition to changes in blood composition, levels of glucose uptake by various tissues were measured after the individual and combined administration of insulin and acipimox. Under basal conditions, the uptake of glucose appeared to be greatest in the kidney followed by the brain and skeletal muscle with negligible uptake by heart, liver and adipose tissues. Acipimox significantly decreased glucose uptake by brain (58% in cortex and 55% in cerebellum). No significant effect of acipimox was observed in other tissues. In summary, the acute inhibition of lipolysis had no effect on glucose uptake in the presence or absence of insulin. This suggests that free fatty acids alone may not be contributing to insulin resistance in birds.     

Insulin regulation of amino-acid metabolism in the mammary gland of sheep in early lactation and fed fresh forage

Insulin plays an important role in regulating the partitioning of nutrients to the mammary gland, particularly in lactating ruminants fed concentrate-based diets. There is evidence that the nutritional status of the animals might also affect their response to insulin. This is largely untested in early lactating ruminants fed fresh forage. To investigate nutritional effects on insulin response, 12 lactating sheep, housed indoors, were allocated to one of two treatment groups (hyperinsulinaemic euglycaemic clamp (HEC) or control) in a randomised block design and fed perennial ryegrass (Lolium perenne)/white clover (Trifolium repens) pasture. Mammary amino acid (AA) net uptake from plasma and utilisation for milk protein synthesis was measured during the 4th day of the HEC using arterio-venous concentration differences, and 1-13C-leucine was used to estimate whole body and mammary gland leucine kinetics. There was no change in feed intake, milk protein output and mammary blood flow during the HEC (P > 0.1). The HEC decreased (P < 0.1) the arterial concentrations of all essential AA (EAA) except histidine. The mammary net uptake of some EAA (isoleucine, leucine, methionine and phenylalanine) was reduced by the HEC (P < 0.1). Leucine oxidation in the mammary gland was not altered during the HEC (P > 0.1) but mammary protein synthesis was reduced by the HEC (P < 0.05). These results show that sheep mammary gland can adapt to changing AA precursor supply to maintain milk protein production during early lactation, when fed fresh forage. How this occurs remains unclear, and this area deserves further study.     

Substrate regulation of the glucose transport system in rat skeletal muscle. Characterization and kinetic analysis in isolated soleus muscle and skeletal muscle cells in culture

A self-regulatory mechanism of the glucose transport in rat skeletal muscle cells is described. In isolated rat soleus muscles and rat skeletal myocytes and myotubes in culture, pre-exposure to varying glucose concentrations modulated the rate of 2-deoxyglucose uptake. Maximal uptake was observed at glucose concentrations below 3 mM. Between 2.5 and 4.0 mM glucose it was reduced by 25-35%; further elevation of the glucose concentration resulted in a gradual decrease of the transport rate by approximately 2% for each millimolar glucose. The effect of glucose was time-dependent and fully reversible. Insulin rapidly increased the 2-deoxyglucose uptake in the soleus muscle; however, the insulin effect depended on the glucose concentration of the preincubation. Insulin was totally ineffective in muscles pre-exposed to 1.0-3.0 mM glucose, whereas its stimulatory action increased with increasing glucose concentrations above 4 mM. The effect of low glucose and insulin were not additive, and the maximal 2-deoxyglucose uptake rates induced by both conditions were of identical magnitude. It is postulated that glucose may "up- and down-regulate" its transport by affecting the number of active glucose transporters in the plasma membrane, and that insulin exerts its stimulatory effect only when the extracellular glucose reaches a threshold concentration.     

Metabolic Adaptations to Dexamethasone‐Induced Insulin Resistance in Healthy Volunteers

Objective : Insulin resistance is observed in individuals with normal glucose tolerance. This indicates that increased insulin secretion can compensate for insulin resistance and that additional defects are involved in impaired glucose tolerance or type 2 diabetes. The objective of this study was to evaluate a procedure aimed at assessing the compensatory mechanisms to insulin resistance. Research Methods and Procedures : Eight healthy nonobese female patients were studied on two occasions, before and after administration of 2 mg/d dexamethasone for 2 days during a two‐step hyperglycemic clamp. Insulin secretion was assessed from plasma insulin concentrations. Insulin sensitivity was assessed from the ratio of whole‐body glucose use (6, 6 ²H₂ glucose) to plasma insulin concentrations. This procedure is known to induce a reversible impairment of glucose tolerance and insulin resistance. Results : In all subjects, dexamethasone induced a decrease in insulin sensitivity and a proportionate increase in first‐phase insulin secretion and in insulin concentrations at both steps of glycemia. The resulting hyperinsulinemia allowed the restoration of normal whole‐body glucose uptake and the suppression of plasma free fatty acids and triglycerides. In contrast, the suppression of endogenous glucose production was impaired after dexamethasone (p < 0.01). Discussion : Increased insulin secretion fully compensates dexamethasone‐induced insulin resistance in skeletal muscle and adipose tissue but not in the liver. This suggests that failure to overcome hepatic insulin resistance can impair glucose tolerance. The compensatory insulin secretion in response to insulin resistance can be assessed by means of a hyperglycemic clamp after a dexamethasone challenge.     

Influence of TNF-alpha and IL-6 infusions on insulin sensitivity and expression of IL-18 in humans

Inflammation is associated with insulin resistance, and both tumor necrosis factor (TNF)-alpha and interleukin (IL)-6 may affect glucose uptake. TNF induces insulin resistance, whereas the role of IL-6 is controversial. High plasma levels of IL-18 are associated with insulin resistance in epidemiological studies. We investigated the effects of TNF and IL-6 on IL-18 gene expression in skeletal muscle and adipose tissue. Nine human volunteers underwent three consecutive interventions, receiving an infusion of recombinant human (rh)IL-6, rhTNF, and saline. Insulin sensitivity was assessed by measurement of whole body glucose uptake with the stable isotope tracer method during a euglycemic hyperinsulinemic clamp (20 mU.min(-1).kg(-1)), which was initiated 1 h after the IL-6-TNF-saline infusion. Cytokine responses were measured in plasma, muscle, and fat biopsies. Plasma concentrations of TNF and IL-6 increased 10- and 38-fold, respectively, during the cytokine infusions. Whole body insulin-mediated glucose uptake was significantly reduced during TNF infusion but remained unchanged during IL-6 infusion. TNF induced IL-18 gene expression in muscle tissue, but not in adipose tissue, whereas IL-6 infusion had no effect on IL-18 gene expression in either tissue. We conclude that TNF-induced insulin resistance of whole body glucose uptake is associated with increased IL-18 gene expression in muscle tissue, indicating that TNF and IL-18 interact, and both may have important regulatory roles in the pathogenesis of insulin resistance.     

Changes in bovine mammary insulin binding during pregnancy and lactation

1. Binding of insulin to microsomes from mammary glands of pregnant and lactating dairy cows was characterized. 2. Binding affinities of the insulin receptor did not change from pregnancy to lactation. 3. Maximal specific binding occurred in microsomes from cows in mid-pregnancy and declined in microsomes from cows in late pregnancy. 4. Insulin binding continued to decrease from early to mid-lactation and increased during late lactation. 5. Results indicate that decreased sensitivity in mammary tissue from lactating dairy cows is at least in part a result of a reduction in insulin receptor number. 6. Results demonstrate further physiological differences between the ruminant and non-ruminant mammary gland.     

Lactogenic and somatogenic binding sites in intact and detergent-solubilized membrane preparations of female rat liver.

1. Lactation results in decreased glucose and acetate utilization and increased lactate output by sheep adipose tissue. 2. The ability of insulin to stimulate acetate uptake was lost in adipose tissue from lactating sheep, whereas both the response and the sensitivity (ED50) for insulin for stimulation of glucose conversion into products other than lactate were decreased. These impairments were partly restored by prolonged incubation of adipose tissue for 48 h. 3. The ability of insulin to stimulate lactate output was not altered by lactation. 4. Dexamethasone inhibited glucose uptake, lactate output and glycerol output in adipose tissue from both non-lactating and lactating sheep, with an ED50 of about 1 nM. Dexamethasone inhibited acetate uptake by adipose tissue from non-lactating sheep, but this effect was not observed with adipose tissue from lactating sheep. 5. Dexamethasone inhibited the stimulation of glucose uptake at all concentrations of insulin used; the effect varied with insulin concentration and resulted in an accentuation of the insulin dose-response curve. The insulin dose-response curve in the presence of dexamethasone was muted during lactation. 6. The overall effect of these adaptations is to ensure that glucose and acetate utilization by adipose tissue after an insulin surge is diminished during lactation.     

Insulin resistance in soleus muscle from obese Zucker rats. Involvement of several defective sites

1. The effect of insulin upon glucose transport and metabolism in soleus muscles of genetically obese (fa/fa) and heterozygote lean Zucker rats was investigated at 5–6 weeks and 10–11 weeks of age. Weight-standardized strips of soleus muscles were used rather than the intact muscle in order to circumvent problems of diffusion of substrates. 2. In younger obese rats (5–6 weeks), plasma concentrations of immunoreactive insulin were twice those of controls, whereas their circulating triacylglycerol concentrations were normal. Insulin effects upon 2-deoxyglucose uptake and glucose metabolism by soleus muscles of these rats were characterized by both a decreased sensitivity and a decrease in the maximal response of this tissue to the hormone. 3. In older obese rats (10–11 weeks), circulating concentrations of insulin and triacylglycerols were both abnormally elevated. A decrease of 25–35% in insulin-binding capacity to muscles of obese rats was observed. The soleus muscles from the older obese animals also displayed decreased sensitivity and maximal response to insulin. However, at a low insulin concentration (0.1m-i.u./ml), 2-deoxyglucose uptake by muscles of older obese rats was stimulated, but such a concentration was ineffective in stimulating glucose incorporation into glycogen, and glucose metabolism by glycolysis. 4. Endogenous lipid utilization by muscle was calculated from the measurements of O₂ consumption, and glucose oxidation to CO₂. The rate of utilization of fatty acids was normal in muscles of younger obese animals, but increased in those of the older obese rats. Increased basal concentrations of citrate, glucose 6-phosphate and glycogen were found in muscles of older obese rats and may reflect intracellular inhibition of glucose metabolism as a result of increased lipid utilization. 5. Thus several abnormalities are responsible for insulin resistance of muscles from obese Zucker rats among which we have observed decreased insulin binding, decreased glucose transport and increased utilization of endogenous fatty acid which could inhibit glucose utilization.     

Rapid effects of insulin and glucose on the hepatic incorporation of gluconeogenic substrates into glyceride glycerol and glycogen

1. The hepatic utilization of gluconeogenic substrates was investigated shortly after portal infusion of either insulin or glucose in fasted rats. 2. After 20 min of insulin infusion blood glucose concentration decreased. However, neither glucose generation from precursors such as alanine or pyruvate nor their incorporation into fatty acids was modified. Under these conditions, insulin rapidly increased the incorporation of gluconeogenic substrates into the hepatic glyceride glycerol fraction. Insulin treatment led to a decrease in substrate incorporation into liver glycogen. 3. After 20 min of portal glucose infusion both plasma insulin and glucose concentrations increased and the incorporation of pyruvate into hepatic glyceride glycerol and into glycogen was also stimulated. 4. A close relationship was observed between blood glucose concentrations and the level of incorporation of gluconeogenic substrates into liver glycogen. 5. In conclusion, during fasting insulin stimulates the incorporation of gluconeogenic substrates into the glycerol moiety of hepatic glycerides, which may be the preferential mechanism through which fatty acid esterification is accomplished during refeeding. This effect of insulin is rapid and detected even before other classical modifications induced by the hormone such as gluconeogenesis inhibition or lipogenesis activation. Furthermore, the effect is not related to insulin-induced hypoglycemia since glucose infusion mimics insulin action on glyceride glycerol synthesis.     

Oxidative stress impairs insulin signal in skeletal muscle and causes insulin resistance in postinfarct heart failure

Insulin resistance has been shown to occur as a consequence of heart failure. However, its exact mechanisms in this setting remain unknown. We have previously reported that oxidative stress is enhanced in the skeletal muscle from mice with heart failure after myocardial infarction (MI) (30). This study is aimed to investigate whether insulin resistance in postinfarct heart failure is due to the impairment of insulin signaling in the skeletal muscle caused by oxidative stress. Mice were divided into four groups: sham operated (sham); sham treated with apocynin, an inhibitor of NAD(P)H oxidase activation (10 mmol/l in drinking water); MI; and MI treated with apocynin. After 4 wk, intraperitoneal insulin tolerance tests were performed, and skeletal muscle samples were obtained for insulin signaling measurements. MI mice showed left ventricular dilation and dysfunction by echocardiography and increased left ventricular end-diastolic pressure and lung weight. The decrease in glucose level after insulin load significantly attenuated in MI compared with sham. Insulin-stimulated serine phosphorylation of Akt and glucose transporter-4 translocation were decreased in MI mice by 61 and 23%, respectively. Apocynin ameliorated the increase in oxidative stress and NAD(P)H oxidase activities measured by the lucigenin assay in the skeletal muscle after MI. It also improved insulin resistance and inhibited the decrease of Akt phosphorylation and glucose transporter-4 translocation. Insulin resistance was induced by the direct impairment of insulin signaling in the skeletal muscle from postinfarct heart failure, which was associated with the enhanced oxidative stress via NAD(P)H oxidase.     

Physiological significance of the concentration of human milk glucose

Summary The concentration of glucose in human milk, [Glucose]_milk, was directly related to the volume of milk secreted not only during lactogenesis and weaning but also during full lactation. To investigate the mechanism for this observation we first established that glucose equilibrates across the apical membrane of the mammary alveolar cell, using infusion of stable isotopically labelled glucose into lactating women. Our results indicate that [Glucose]_milk can be used to measure the glucose concentration in the mammary alveolar cell, [Glucose]_cell. We then investigated the regulation of glucose transport into the mammary alveolar cell using glucose clamp methodology in fully lactating and weaning women. Maintenance of high plasma insulin concentrations for four hours under euglycemic conditions had no effect on [Glucose]_milk, demonstrating that insulin does not regulate glucose transport into the mammary gland. On the other hand, maintaining the [Glucose]_plasma at twice the fasting level resulted in a 3-fold increase in the steady state [Glucose]_milk in fully lactating women and a 5-fold increase in [Glucose]_milk in weaning women. Kinetic analysis of the data showed that the V_max for glucose transport into the mammary alveolar cell across the basolateral membrane is regulated by the level of synthetic activity in the mammary alveolar cell.Dedicated to Professor Stuart Patton on the occasion of his 70th birthday.     

Late effects of postnatal administration of monosodium glutamate on insulin action in adult rats

Early postnatal administration of monosodium glutamate (MSG) to rats induces obesity, hyperinsulinemia and hyperglycemia in adulthood, thus suggesting the presence of insulin resistance. We therefore investigated the effects of insulin on glucose transport and lipogenesis in adipocytes as well as insulin binding to specific receptors in the liver, skeletal muscle and fat tissues. An increase of plasma insulin, glucose and leptin levels was found in 3-month-old rats treated with MSG during the postnatal period. The attenuation of insulin stimulatory effect on glucose transport was observed in MSG-treated rats. Despite the lower basal and insulin-stimulated glucose uptake, the incorporation of glucose into lipids was significantly higher in MSG-treated rats, suggesting a shift in glucose metabolism towards lipid synthesis in fat tissue. Insulin binding to plasma membranes from the liver, skeletal muscle and adipocytes was decreased in MSG-treated rats. This is in agreement with the lower insulin effect on glucose transport in these animals. Furthermore, a decreased amount of GLUT4 protein was found in adipocytes from MSG-treated obese rats. The results demonstrated an attenuation of insulin effect on glucose transport due to a lower insulin binding and lower content of GLUT4 protein in MSG-treated rats. However, the effect of insulin on lipogenesis was not changed. Our results indicated that early postnatal administration of MSG exerts an important effect on glucose metabolism and insulin action in adipocytes of adult animals.