Insulin action on adipocytes. Evidence that the anti-lipolytic and lipogenic effects of insulin are mediated by the same receptor

Acute changes in systemic glycemia gate access and action of GLP-1R agonist on brain structures controlling energy homeostasis

Therapies based on glucagon-like peptide-1 (GLP-1) long-acting analogs and insulin are often used in the treatment of metabolic diseases. Both insulin and GLP-1 receptors are expressed in metabolically relevant brain regions, suggesting a cooperative action. However, the mechanisms underlying the synergistic actions of insulin and GLP-1R agonists remain elusive. In this study, we show that insulin-induced hypoglycemia enhances GLP-1R agonists entry in hypothalamic and area, leading to enhanced whole-body fat oxidation. Mechanistically, this phenomenon relies on the release of tanycyctic vascular endothelial growth factor A, which is selectively impaired after calorie-rich diet exposure. In humans, low blood glucose also correlates with enhanced blood-to-brain passage of insulin, suggesting that blood glucose gates the passage other energy-related signals in the brain. This study implies that the preventing hyperglycemia is important to harnessing the full benefit of GLP-1R agonist entry in the brain and action onto lipid mobilization and body weight loss.

The Effect of High-intensity Interval Training vs Moderate-intensity Continuous Training on Liver Fat: A Systematic Review and Meta-Analysis

CONTEXT

Non-alcoholic fatty liver disease, characterized by excess fat accumulation in the liver, is considered the hepatic manifestation of metabolic syndrome. Recent findings have shown that high-intensity interval training (HIIT) can reduce liver fat but it is unclear whether this form of exercise is superior to traditional moderate-intensity continuous training (MICT).

OBJECTIVE

The aim of this systematic review was to determine the effect of HIIT vs MICT on liver fat in adults. A secondary aim was to investigate the interaction between total weekly exercise volume and exercise-related energy expenditure and change in liver fat.

METHODS

Relevant databases were searched up to December 2020 for randomized trials, comparing HIIT to control, MICT to control, or HIIT to MICT. Studies were excluded if they did not implement 2 or more weeks' intervention or assess liver fat using magnetic resonance-based techniques. Weighted mean differences and 95% CIs were calculated. Regression analyses were undertaken to determine the interaction between weekly exercise volume in minutes and kilocalories (kcal) with change in liver fat content.

RESULTS

Of the 28 268 studies screened, 19 were included involving 745 participants. HIIT and MICT both elicited moderate reductions in liver fat content when compared to control (HIIT: -2.85%, 95% CI, -4.86 to -0.84, P = .005, I2 = 0%, n = 114, low-certainty evidence; MICT: -3.14%, 95% CI, -4.45 to -1.82, P < .001, I2 = 5.2%, n = 533, moderate-certainty evidence). There was no difference between HIIT and MICT (-0.34%, 95% CI, -2.20 to 1.52, P = .721, I2 = 0%, n = 177, moderate-certainty evidence). Neither total exercise volume in minutes (β = .0002, SE = 0.0017, Z = 0.13, P = .89) nor exercise-related energy expenditure in kcal (β = .0003, SE = 0.0002, Z = 1.21, P = .23) were related to changes in liver fat content.

CONCLUSION

HIIT elicits comparable improvements in liver fat to MICT despite often requiring less energy and time commitment. Further studies should be undertaken to assess the relative importance of aerobic exercise prescription variables, such as intensity, on liver fat.

Bidirectional temporal relationship between obesity and hyperinsulinemia: longitudinal observation from a Chinese cohort

INTRODUCTION

Although obesity and hyperinsulinemia are closely intercorrelated, their temporal sequence is still uncertain. This study aims to investigate the temporal relationship patterns between obesity measures and hyperinsulinemia in Chinese adults.

RESEARCH DESIGN AND METHODS

The longitudinal cohort consisted of 2493 participants (860 males and 1633 female, mean age 56.71 years at follow-up) for whom measurements of obesity and hyperinsulinemia measures were collected twice between 2011 and 2014, with an average follow-up time of 3 years. Cross-lagged panel analysis was used to examine the temporal relationship between obesity measures (body mass index (BMI); waist circumference (WC); hip circumference (HC); waist-to-hip ratio (WHR)) and hyperinsulinemia (insulin, homeostasis model assessment of insulin resistance (HOMA-IR), or homeostasis model assessment of beta cell function (HOMA-%β)).

RESULTS

After the adjustment of age, sex, smoking, drinking and follow-up years, in the BMI-insulin model, the path coefficient (β₂=0.229; p<0.001) of baseline BMI to follow-up insulin was significantly greater than the path coefficient (β₁=0.073; p<0.001) of baseline insulin to follow-up BMI (p<0.001 for β₂>β₁). In the WHR-insulin model, the path coefficient (β₁=0.152; p<0.001) of baseline insulin to follow-up WHR was significantly greater than the path coefficient (β₂=0.077; p<0.001) of baseline WHR to follow-up insulin (p=0.007 for β₁>β₂). In the WC/HC-insulin model, the path coefficients of baseline insulin to follow-up WC or HC (β_1s) were also greater than the path coefficients of baseline WC or HC to follow-up insulin (β_2s), but the difference between β_1s and β_2s were not significant. The similar temporal patterns were founded between obesity measures with HOMA-IR or HOMA-%β.

CONCLUSIONS

These findings indicate that there is a bidirectional relationship between obesity and hyperinsulinemia, and abdominal obesity measures are more sensitive to hyperinsulinemia measures than BMI.

Insulin translates unfavourable lifestyle into obesity

Lifestyle factors conferring increased diabetes risk are associated with elevated basal insulin levels (hyperinsulinaemia). The latter predicts later obesity in children and adolescents.A causal role of hyperinsulinaemia for adipose tissue growth is probable because pharmacological reduction of insulin secretion lowers body weight in people who are obese. Genetic inactivation of insulin gene alleles in mice also lowers their systemic insulin levels and prevents or ameliorates high-fat diet-induced obesity. Hyperinsulinaemia causes weight gain because of a physiological property of insulin. Insulin levels that are on the high side of normal, or which are slightly elevated, are sufficient to suppress lipolysis and promote lipogenesis in adipocytes. The effect of insulin on glucose transport or hepatic glucose production requires six or two times higher hormone levels, respectively.It seems justified to suggest a lifestyle that avoids high insulin levels in order to limit anabolic fat tissue activity.

The role of sex steroids in white adipose tissue adipocyte function

With the increasing knowledge that gender influences normal physiology, much biomedical research has begun to focus on the differential effects of sex on tissue function. Sexual dimorphism in mammals is due to the combined effects of both genetic and hormonal factors. Hormonal factors are mutable particularly in females in whom the estrous cycle dominates the hormonal milieu. Given the severity of the obesity epidemic and the fact that there are differences in the obesity rates in men and women, the role of sex in white adipose tissue function is being recognized as increasingly important. Although sex differences in white adipose tissue distribution are well established, the mechanisms affecting differential function of adipocytes within white adipose tissue in males and females remain largely understudied and poorly understood. One of the largest differences in the endocrine environment in males and females is the concentration of circulating androgens and estrogens. This review examines the effects of androgens and estrogens on lipolysis/lipogenesis, adipocyte differentiation, insulin sensitivity and adipokine production in adipocytes from white adipose tissue with a specific emphasis on the sexual dimorphism of adipocyte function in white adipose tissue during both health and disease.

Growth hormone, insulin-like growth factor 1, and insulin signaling-a pharmacological target in body wasting and cachexia

Cachexia is an irreversible process that can develop in the course of chronic disease. It is characterized by the remodeling of the metabolic, inflammatory, and endocrine pathways. Insulin, growth hormone (GH), and insulin-like growth factor 1 (IGF-1) are involved in glucose, protein, and fat metabolism, which regulates body composition. In body wasting and cachexia, their signaling is impaired and causes anabolic/catabolic imbalance. Important mechanisms include inflammatory cytokines and neurohormonal activation. Remodeled post-receptor insulin, GH, and IGF-1 pathways constitute a potential target for pharmacological treatment in the setting of body wasting and cachexia. Peroxisome proliferator-activated receptor gamma agonists, drugs inhibiting angiotensin II action (angiotensin II antagonists and inhibitors of angiotensin-converting enzyme), and testosterone, which interfere with post-receptor pathways of insulin, GH, and IGF-1, were investigated as pharmacological intervention targets and various clinically important implications were reported. There are several other potential targets, but their treatment feasibility and applicability is yet to be established.

Leucine deprivation stimulates fat loss via increasing CRH expression in the hypothalamus and activating the sympathetic nervous system

We previously showed that leucine deprivation decreases abdominal fat mass largely by increasing energy expenditure, as demonstrated by increased lipolysis in white adipose tissue (WAT) and uncoupling protein 1 (UCP1) expression in brown adipose tissue (BAT). The goal of the present study was to investigate the possible involvement of central nervous system (CNS) in this regulation and elucidate underlying molecular mechanisms. For this purpose, levels of genes and proteins related to lipolysis in WAT and UCP1 expression in BAT were analyzed in wild-type mice after intracerebroventricular administration of leucine or corticotrophin-releasing hormone antibodies, or in mice deleted for three β-adrenergic receptors, after being maintained on a leucine-deficient diet for 7 d. Here, we show that intracerebroventricular administration of leucine significantly attenuates abdominal fat loss and blocks activation of hormone sensitive lipase in WAT and induction of UCP1 in BAT in leucine-deprived mice. Furthermore, we provide evidence that leucine deprivation stimulates fat loss by increasing expression of corticotrophin-releasing hormone in the hypothalamus via activation of stimulatory G protein/cAMP/protein kinase A/cAMP response element-binding protein pathway. Finally, we show that the effect of leucine deprivation on fat loss is mediated by activation of the sympathetic nervous system. These results suggest that CNS plays an important role in regulating fat loss under leucine deprivation and thereby provide novel and important insights concerning the importance of CNS leucine in the regulation of energy homeostasis.

Structural basis of the aberrant receptor binding properties of hagfish and lamprey insulins

The insulin from the Atlantic hagfish (Myxine glutinosa) has been one of the most studied insulins from both a structural and a biological viewpoint; however, some aspects of its biology remain controversial, and there has been no satisfying structural explanation for its low biological potency. We have re-examined the receptor binding kinetics, as well as the metabolic and mitogenic properties, of this phylogenetically ancient insulin, as well as that from another extant representative of the ancient chordates, the river lamprey (Lampetra fluviatilis). Both insulins share unusual binding kinetics and biological properties with insulin analogues that have single mutations at residues that contribute to the hexamerization surface. We propose and demonstrate by reciprocal amino acid substitutions between hagfish and human insulins that the reduced biological activity of hagfish insulin results from unfavorable substitutions, namely, A10 (Ile to Arg), B4 (Glu to Gly), B13 (Glu to Asn), and B21 (Glu to Val). We likewise suggest that the altered biological activity of lamprey insulin may reflect substitutions at A10 (Ile to Lys), B4 (Glu to Thr), and B17 (Leu to Val). The substitution of Asp at residue B10 in hagfish insulin and of His at residue A8 in both hagfish and lamprey insulins may help compensate for unfavorable changes in other regions of the molecules. The data support the concept that the set of unusual properties of insulins bearing certain mutations in the hexamerization surface may reflect those of the insulins evolutionarily closer to the ancestral insulin gene product.

AAS, growth hormone, and insulin abuse: psychological and neuroendocrine effects

The nontherapeutic use of prescription medicines by individuals involved in sport is increasing. Anabolic-androgenic steroids (AAS) are the most widely abused drug. Much of our knowledge of the psychological and physiological effects of human growth hormone (hGH) and insulin has been learned from deficiency states. As a consequence of the Internet revolution, previously unobtainable and expensive designer drugs, particularly recombinant human growth hormone (rhGH) and insulin, have become freely available at ridiculously discounted prices from countries such as China and are being abused. These drugs have various physiological and psychological effects and medical personnel must become aware that such prescription medicine abuse appears to be used not only for performance and cosmetic reasons, but as a consequence of psychological pre-morbidity.

Pregnancy Effects on Rat Adipose Tissue Lipolytic Capacity are Dependent on Anatomical Location

Pregnancy is characterized by changes in maternal adiposity. The aim of this study was to carry out a detailed analysis of the different steps of the adrenergic pathway, lipoprotein lipase (LPL) levels and adipocyte size, in order to evaluate the response of white adipose tissue (WAT) to the metabolic changes during pregnancy depending on the anatomical location. In general, the levels of the proteins of the lipolytic pathway decreased with pregnancy. In retroperitoneal WAT adenylate cyclase (AC) levels decreased from 100% in controls to 44% by day 13 and 11% by day 20. In mesenteric WAT the alpha (2A)/beta (3)-adrenergic receptor balance seemed to be one of the main regulatory points of the lipolytic pathway and the reduction in the postreceptor element levels was clearly lower than for the other two depots (PKA levels reduced from 100% in controls to 72% by day 20, while in the other two depots it decreased to 30%, and AC and HSL levels did not show statistically significant changes in this depot). In contrast, the LPL-to-HSL ratio may be a major regulatory point in gonadal WAT. In summary, we describe regional differences in the regulation of WAT metabolism throughout pregnancy, which may be of great importance to determine the role of the different fat depots during late pregnancy. Thus, gonadal and mesenteric WAT changed to a lipolytic state to sustain the rapid foetal growth, although with differences between them in the main regulatory points, while retroperitoneal WAT could have a role later on, during lactation.

Fat accumulation in the rat during early pregnancy is modulated by enhanced insulin responsiveness

Insulin sensitivity has been implicated in the variation of fat accumulation in early gestation by as-yet-unknown mechanisms. In the present study, we analyzed the insulin sensitivity of lipolysis and lipogenesis in lumbar adipocytes from rats at 0, 7, 14, and 20 days of gestation. In adipocytes of 7-day pregnant rats, we found a twofold decrease in both beta-agonist (isoproterenol and BRL-37344)-stimulated lipolysis and beta3-adrenoceptor protein but not in lipolysis initiated by forskolin or isobutylmethylxanthine, suggesting a modification of the lipolytic pathway at the receptor level. Whereas adipocytes from 7-day pregnant rats showed a twofold increase in fatty acid synthesis from glucose, those from 20-day pregnant animals displayed a decreased lipogenic activity. Insulin responsiveness of the lipolytic and lipogenic pathways was analyzed by dose-response experiments, giving evidence for the involvement of improved insulin responsiveness in the enhanced lipogenic and reduced lipolytic activities of adipocytes in early pregnancy. In contrast, insulin resistance is responsible for lower antilipolytic and lipogenic actions of insulin in late pregnant animals. In conclusion, the present study shows that enhanced adipose tissue insulin responsiveness during early pregnancy contributes to maternal fat accumulation, whereas decreased insulin responsiveness during late gestation modulates fat breakdown.

Influence of mental stress and circadian cycle on postprandial lipemia

BACKGROUND

Mental stress produces alterations in serum lipids and lipoproteins.

OBJECTIVE

The objective was to assess the effect of mental stress during the day and night on postprandial lipoproteins.

DESIGN

Fourteen healthy subjects aged 26.6 +/- 5.0 y were given randomly the same meal either at night (0100) or during the day (1300), with or without (control session) a mental stress challenge. The meal contained 40% of estimated daily energy needs. The mental task was performed on a computer and consisted of a task of choice reaction. Blood samples were drawn at baseline and hourly for 7 h after the meal.

RESULTS

Urinary epinephrine concentrations were higher (P < 0.012) during the mental task than during the control sessions. Repeated-measures analysis of variance showed that mean postprandial triacylglycerol concentrations were significantly higher (P < 0.02) and total cholesterol (P < 0.0001) and HDL-cholesterol concentrations were significantly lower (P < 0.0001) at night than during the day. The mean postprandial VLDL-triacylglycerol concentration was significantly higher (P < 0.04) during the mental task than during the control sessions. Similarly, the VLDL-cholesterol response, calculated as the area under the postprandial curve, was significantly greater (P < 0.02) during the mental task than during the control sessions. There was no interaction between mental stress and nyctohemeral cycle on postprandial lipoprotein responses, suggesting that both indexes act independently on postprandial lipid metabolism.

CONCLUSIONS

Mental stress is associated with increased concentrations of postprandial triacylglycerol-rich lipoprotein fractions. Therefore, postprandial hyperlipidemia is one possible mechanism contributing to the higher risk of ischemic heart disease in stressed people.

Effects of fasting on tissue glucose utilization in conscious resting rats. Major glucose-sparing effect in working muscles

The effects of fasting on glucose metabolism in the conscious resting rat were studied. Fasting decreased whole-body glucose utilization by 40%. The fast induced a decrease in glucose utilization in muscles which are constantly working even in the resting state, i.e. heart, diaphragm and postural muscles. No modification was observed in other tissues.

Substrate cycling between triglyceride and fatty acid in human adipocytes

Substrate cycles in metabolism require energy and generate heat, and they may be involved in thermogenesis. We have studied one such cycle between triglyceride and fatty acid in isolated human adipocytes using a nonisotopic technique. In the absence of added hormone, and with 5 mmol/L (90 mg/dL) glucose in the incubation medium, lipolysis and fatty acid reesterification coexisted such that 40 +/- 4% (mean +/- SEM) of the fatty acid produced was cycled back into triglyceride. In 51 individual subjects the range was from 0% to 100%. Both lipolysis and the quantity of fatty acid recycled correlated positively with cell volume (P less than .001 and P less than .005, respectively). Norepinephrine (10(-6) mol/L) alone (33 experiments) increased lipolysis 3.1-fold, and reduced the percentage of fatty acid reesterified. Cycling was similar to that in the basal state. Lipolysis was inhibited 46% by postabsorptive levels of insulin alone (18 experiments), but the proportion of fatty acid reesterified increased such that the quantity cycled back into triglyceride was similar to that observed in the basal state. In the presence of both norepinephrine and insulin (18 experiments), lipolysis was increased by 58% while 31 +/- 4% of the fatty acid released was reesterified. In consequence, the quantity of fatty acid cycled back into triglyceride increased 2.1-fold. Increasing the insulin level fivefold or the medium glucose concentration to 20 mmol/L produced no further increase in the quantity of fatty acid reesterified. A substrate cycle exists, therefore, between triglyceride and fatty acid in human adipose tissue, and its activity is modified by norepinephrine and insulin.(ABSTRACT TRUNCATED AT 250 WORDS)

Insulin binding and its action in adipocytes of hyperthyroid and hypothyroid rats

The studies of the effects of thyroid hormone on insulin secretion and glucose metabolism have been made, but the results have been controversial. In order to evaluate the effects of thyroid hormone at the cellular level, insulin binding and insulin-induced lipogenesis in isolated rat epididymal fat cells were studied in control, hyperthyroid and hypothyroid rats. Hyperthyroidism was induced by daily intraperitoneal injection of sodium L-thyroxine and hypothyroidism by a single injection of ¹³¹I. The adipocytes were isolated by treatment of collagenase as originally described by Gliemann (1967). The results were as follows: 1) The fasting serum insulin levels of hyperthyroid (21.6 ± 3.7 uU/ml) and hypothyroid groups (20.5 ± 7.0 uU/ml) were not different from the value of control (23.1 ± 11 uU/ml). The fasting blood glucose level of hypothyroid group (164.9 ± 12.0 mg/dl) was higher than the values of the controls (148.2 ± 13.2 mg/dl) or the hyperthyroid group (147.0 ± 12.5 mg/dl), (p<.005). 2) The specific ¹²⁵I-insulin binding of the hyperthyroid group was not different from the value of the controls, but the value of the hypothyroid group was higher than the value of the controls (p<.005). 3) The insulin receptor concentration of the hypothyroid group (1.09 ± 0.02 ng/0.5×10⁵ cells) was higher than the value of the controls (0.72 ± 0.01 ng/0.5×10⁵ cells) or the hyperthyroid group (0.79 ± 0.02 ng/0.5×10⁵ cells), (p<.05). 4) The average affinities of the receptors in all groups showed an inverse correlation with the insulin concentration. The average affinity of the hypothyroid group was higher than the value of the control or the hyperthyroid group. 5) Insulin-induced lipogenesis was reduced proportionately in all insulin concentrations in both the hyperthyroid and hypothyroid groups compared with the dose-response curve of the control group. The maximal amount of lipogenesis of the hyperthyroid and hypothyroid groups were 63.4% (p<.05) and 32.3% (p<.005) of the controls, respectively. These studies suggest that thyroid hormone may regulate the concentration of insulin receptors, and altered thyroid states reduce insulin-induced lipogenesis in the adipocyte at postreceptor levels.

Biological effects of sulphated insulin in adipocytes and hepatocytes

The binding affinity of sulphated insulin compared with unmodified, neutral insulin has been reported to be approximately four times lower in human and rat adipocytes but over twenty times lower in rat hepatocytes. In the present study the biological action of sulphated insulin was assessed in rat hepatocytes and human and rat adipocytes. To achieve half-maximal stimulation of fatty acid synthesis in rat hepatocytes about twenty one times higher concentrations of sulphated than neutral insulin were required (15.07 +/- 5.50 vs 0.71 +/- 0.34 nmol/l), this ratio being similar to the ratio of binding affinity in rat hepatocytes. In human adipocytes, half-maximal stimulation of initial rates of glucose uptake was observed at 11.6 +/- 5.1 vs 2.9 +/- 1.3 pmol/l for sulphated and neutral insulin respectively, and half-maximal inhibition of lipolysis at 31.0 +/- 13.5 vs 7.3 +/- 2.5 pmol/l respectively. These data are consistent with the four-fold lower binding affinity of sulphated insulin to human adipocytes. However, in rat adipocytes the biological potency of sulphated insulin was found to be much lower than anticipated from the binding data, half-maximal stimulation of initial rates of glucose uptake being observed at 757 +/- 299 vs 35 +/- 13 pmol/l respectively and half-maximal inhibition of lipolysis at 35.9 +/- 12.1 vs 1.5 +/- 0.5 pmol/l respectively. Thus, in rat adipocytes, approximately 22 times the concentration of sulphated insulin was required to achieve equivalent biological effect. A discrepancy between binding affinity and biological action with respect to sulphated insulin was identified in rat adipocytes but not human adipocytes nor rat hepatocytes suggesting differences in the binding-action linkage in these cells.

Triacylglycerol synthesis in the premolt field crab Paratelphusa hydrodromus (Milne-Edwards) (Crustacea)

Triacylglycerol synthesis was investigated in the hepatopancreas of the premolt field crab Paratelphusa hydrodromus using uniformally labelled [14C] acetate. Maximum incorporation occurred after 180 min into the triacylglycerol fraction of the hepatopancreas in both the sexes. Sexual dimorphism was indicated by a greater accumulation of labelled acetate in the triacylglycerol fraction of the hepatopancreas of the female than that of the male crab. The conversion of labelled acetate to triacylglycerol was greater in fed- than starved crabs. Haemolymph seemed to contain factor(s) which enhanced the lipogenic capacity of the hepatopancreas.

Covalently-linked insulin dimers: their metabolism and biological effects in vivo as partial competitive antagonists of insulin clearance

The biological properties of three covalently-linked insulin dimers were studied in greyhounds. Constant infusions showed that the plasma distribution kinetics were slower for the dimers than for insulin. The metabolic clearance rates of the three dimers (10.3 +/- 0.4, 8.8 +/- 0.5, 8.2 +/- 0.5 ml X min-1 X kg-1; mean +/- SEM) were significantly lower than that of insulin (19 +/- 0.8 ml X min-1 X kg-1), and their hypoglycaemic effects (11.2%, 3% and 0.3%) were markedly reduced compared with their lipogenic potencies in vitro (80%, 30% and 13%, respectively). A low dose infusion of insulin or an equipotent dose of one of the dimers significantly prolonged the effects of an insulin bolus on plasma glucose but not on non-esterified fatty acids. The apparent distribution space (106.4 +/- 11.9 ml/kg) and clearance rate (14.7 +/- 0.5 ml X min-1 X kg-1) of an insulin bolus were significantly reduced by one dimer (44.5 +/- 8.4 ml/kg and 10.7 +/- 2.8 ml X min-1 X kg-1) but not by the equipotent insulin infusion (102.7 +/- 8.2 ml/kg and 16.4 +/- 0.07 ml X min-1 X kg-1). The apparent partial competitive antagonism of insulin by the dimers that has been reported in vitro can be observed in vivo, in that antagonism of insulin metabolism was directly demonstrated with one of the dimers.

Effects of streptozotocin diabetes and insulin treatment on adipose tissue glucose utilization in the rat

[U-14C] glucose utilization by pieces of epididymal fat-pad was studied in streptozotocin-diabetic rats, food-restricted animals, and normal controls. Both CO2 and fatty acids formation were greatly reduced in tissues from the diabetic animals treated or not with insulin and this change was milder in tissues from food-restricted rats. A positive correlation was found between [14C] glucose utilization in vitro and plasma RIA-insulin levels when values from all animals were pooled. The percentual response to high doses of insulin was small in adipose tissue from streptozotocin-diabetic animals for CO2 and total lipids formation, but it was high for fatty acids synthesis; the response in food-restricted animals was high and was greatly augmented in tissues from the streptozotocin-diabetic rats treated with insulin. Results indicate that in adipose tissue from streptozotocin-diabetic animals the alteration of insulin responsiveness occurs distal to the receptor events and that impaired basal glucose metabolism is a direct consequence of their hypoinsulinemia.

Insulin sensitivity of rates of glycolysis and glycogen synthesis in soleus, stripped soleus, epitrochlearis, and hemi-diaphragm muscles isolated from sedentary rats

The effect of insulin concentrations on the rates of glycolysis and glycogen synthesis in four different in vitro rat muscle preparations (intact soleus, stripped soleus, epitrochlearis, and hemi-diaphragm) were investigated: the concentrations of insulin that produced half-maximal stimulation of the rates of these two processes in the four muscle preparations were similar - about 100 muunits/ml. This is at least 10-fold greater than the concentration that produced half-maximal inhibition of lipolysis in isolated adipocytes. Since 100 muunits/ml insulin is outside the normal physiological range in the rat, it is suggested that, in vivo, insulin influences glucose utilization in muscle mainly indirectly, via changes in the plasma fatty acid levels and the 'glucose/fatty acid cycle'. Consequently the view that insulin stimulates glucose utilization in muscle mainly by a direct effect on membrane transport must be treated with caution.

Glucose and ketone body kinetics in diabetic ketoacidosis

The hyperglycaemia and hyperketonaemia of diabetic ketoacidosis are initiated primarily by overproduction of these substrates; subsequent maintenance of hyperglycaemia occurs, in large part, due to impaired utilization of glucose, whereas overproduction of ketone bodies continues to be the major mechanism for maintenance of hyperketonaemia. Insulin deficiency results in increased rates of lipolysis and provides increased substrate (free fatty acids) for ketogenesis. Hyperglucagonaemia can augment ketogenesis further in the setting of insulin deficiency. It is likely that other counter-insulin hormones (growth hormone, catecholamines) also contribute to the pathogenesis of DKA, though their role is less well defined. Insulin corrects DKA largely via suppression of lipolysis (and thus ketone body production); insulin suppresses glucose production at lower levels than it does ketone body production.

Glucose stimulation of the antilipolytic effect of insulin in humans

Dose-response studies of the inhibition of lipolysis by insulin in isolated human adipocytes were conducted with the use of a sensitive bioluminescent assay of glycerol release. The addition of glucose to the incubation medium was associated with an increase in insulin sensitivity and an increase in the maximum insulin effect. The results suggest that glucose plays an important role in regulating the antilipolytic action of insulin in humans.

Morphological studies on long-term culture of marrow cells: characterization of the adherent stromal cells and their interactions in maintaining the proliferation of hemopoietic stem cells

In long-term cultures of bone marrow, the adherent stromal cells provide support for the proliferation and maintenance of hemopoietic stem cells. These stromal cells and their interactions were characterized by means of scanning (SEM) and transmission (TEM) electron microscopy in correlation with functional studies. Cultures were initiated by establishing the adherent stromal layer as a "soil" which was then "seeded" after 3 weeks by the addition of another marrow-cell suspension. Clonal assay of the supernatant demonstrated the continuous proliferation of the hemopoietic stem cell. The stroma essentially consisted of two cell types, macrophages and epithelioid cells. Macrophages were smaller, 10-15 microns, phagocytosed latex and carbon particles, and contained lysosomes. Their surface did not stain with polycationic ferritin (PCF). Epithelioid cells were much larger, more than 100 microns; contained numerous thin, elongated mitochondria; did not phagocytose latex particles; but did display strong surface staining with PCF. The appearance of epithelioid cells in TEM depended on their state of development and whether the section was parallel or perpendicular to the substratum. Epithelioid cells displayed a maturational spectrum, at two ends of which were synthetic and storage phases. In the synthetic phase, the cell contained numerous profiles of rough endoplasmic reticulum, and in the storage phase, numerous storage granules. These two phases were best appreciated in sections perpendicular to the substratum, demonstrating synthetic cells on top settling over the substratum upon maturation into the storage cells. Both macrophages and epithelioid cells contained fat globules which increased in number and size with the addition of hydrocortisone to the culture medium. A distinct fat-cell type, as has been claimed, was not found in this study. Granulopoiesis was observed in the culture system in the absence of colony-stimulating activity in the supernatant, suggesting direct cellular interaction or short-range factors in the induction of granulopoiesis. Widespread cellular interactions were noted between macrophages and epithelioid cells, the latter often completely embracing the former and both extending cytoplasmic processes toward each other. This is reminiscent of the cooperative interaction of endoderm and mesoderm in chick embryo hemopoiesis and may be necessary for the maintenance of stem cells in these cultures.

Insulin responsiveness of superficial forearm tissues in type 2 (non-insulin dependent) diabetes

Forearm perfusion studies were carried out to determine the responsiveness to insulin of the superficial forearm tissues in non-obese Type 2 (non-insulin-dependent) diabetics, and the interrelationships among plasma concentrations of glucose, insulin and non-esterified fatty acids (NEFA), tissue uptake of glucose and insulin and tissue release of NEFA. It was found that: (1) in normal subjects, uptake of glucose was dependent on glucose concentration. It was also dependent on insulin concentration in the range of 0-30 mU/l, but not over a wider range of insulin concentration (less than 66 mU/l), indicating that the insulin effect was maximal at approximately 30 mU/l. In contrast, glucose uptake in diabetics was independent of glucose concentration but dependent on insulin uptake over an insulin concentration range up to 140 mU/l; glucose uptake reached the same levels as in control subjects but only at higher concentration and higher uptake of insulin. (2) Insulin uptake was directly dependent on insulin concentration and the regression coefficients were very similar in the two groups. (3) NEFA concentration fell to comparable levels in the two groups of subjects in response to insulin. It is concluded that in Type 2 diabetes: (1) the superficial forearm tissues show decreased responsiveness to the stimulatory effect of both hyperglycaemia and hyperinsulinaemia on glucose utilization but the NEFA-lowering effect of insulin is undiminished, and (2) tissue uptake of insulin is normal, despite the decrease in receptor capacity that has been demonstrated by others.

Effects of insulin on lipolysis and lipogenesis in hamster white adipocytes with high sensitivity to hormones

A modified procedure for preparation of hamster adipocytes by collagenase digestion under carefully controlled conditions has been developed. The adipocytes were 4- to 8-fold more sensitive to catecholamine stimulation of lipolysis than cells prepared by a commonly used method (Hittelman, K.J., Wu, C.F. and Butcher, R.W. (1973) Biochim. Biophys. Acta 304, 188-196) and also more sensitive to the anti-lipolytic action of insulin. The effects of insulin on lipogenesis, measured as [3H]glucose conversion to cell lipids, and on catecholamine-stimulated lipolysis were compared under identical conditions with the same cell batch. Isoprenaline-stimulated lipolysis was found to be half-maximally inhibited by an insulin concentration 8-fold lower than that stimulating lipogenesis to a corresponding extent (half-maximal effects at insulin concentrations of 40 vs. 300 pM). A similar difference was found when cells had been stimulated with adrenaline instead of isoprenaline.

Insulin resistance in adipocytes from fed and fasted obese rats: dissociation of two insulin actions

Adipocytes from 26-week-old Zucker rats (greater than 600 g) and controls of the same strain and age (greater than 400 g) served as models of insulin resistance and obesity. Adipocytes from fatty rats demonstrated insulin resistance of glucose metabolism and resistance to the antilipolytic effect of insulin which was not improved by fasting. Adipocytes from fatty rats were also resistant to the lipolytic action of norepinephrine. Control rats displayed insulin resistance of glucose metabolism and resistance to the antilipolytic effect of insulin. However, fasting for 6.5 days restored the antilipolytic action of insulin without affecting the insulin sensitivity of glucose metabolism. This dissociation of the responsiveness of insulin actions by fasting indicates that there may be at least two mechanisms of insulin resistance in the same cell.

Conversion of [U-14C] glucose to lipid in the American cockroach, Periplaneta americana L

The incorporation of D-[U-14C] glucose into fat body acylglycerols was studied in fed and starved adult cockroaches, Periplaneta americana. About 7-10% of injected glucose is incorporated into the triacylglycerol component of fat body within three hours of injection. The capacity for conversion of glucose to triacylglycerol is greater in fed than starved Insects and the differences in lipogenic capacity of fat body from fed and starved cockroaches are retained in vitro. Incubation of fat body in the presence of haemolymph enhances the lipogenic capacity of fat body.