Oxidative stress impairs insulin internalization in endothelial cells in vitro

Aims/hypothesis. Because oxidative stress has been suggested to be a significant contributing factor in the development of endothelial dysfunction and insulin resistance, we investigated whether reactive oxygen species contribute to insulin resistance by impairing insulin uptake through an effect on endothelial insulin receptor function. Methods. Following a 2-h pro-oxidant challenge with xanthine oxidase, we examined the temporal pattern of insulin processing in the human umbilical endothelial cell line Ea.Hy926 and bovine aortic endothelial cells equilibrated with [¹²⁵I]-insulin. Insulin receptor mRNA concentrations were analysed by RT-PCR and insulin receptor tyrosine phosphorylation and protein concentrations were estimated by western blotting. Results. Xanthine oxidase exposure resulted in a major reduction in total insulin receptor-mediated [¹²⁵I]-insulin internalization over a 1-h period in both Ea.Hy926 and bovine aortic endothelial cells. After 15 min, untreated bovine aortic endothelial cells internalized fivefold more cell-bound [¹²⁵I]-insulin than pro-oxidant treated cells. The [¹²⁵I]-insulin disappeared from the cell surface at a similar rate in both pro-oxidant and untreated cells, with relatively more [¹²⁵I]-insulin being released into the medium in pro-oxidant treated cells. Although xanthine oxidase reduced insulin receptor mRNA and protein concentrations, cell surface insulin binding capacity was not affected. Following 5 min insulin exposure, insulin receptor auto-phosphorylation was considerably reduced in cells challenged with xanthine oxidase for 2 h, which could be important for insulin receptor activation and internalization. Conclusion/interpretation. Oxidative stress impairs insulin endocytosis in both arterial and venous endothelial cell lines. This was not a consequence of modified insulin binding capacity but could involve insufficient insulin receptor activation. [Diabetologia (2001) 44: 605–613] Received: 2 October 2000 and in revised from: 18 December 2000     

Angiotensin II enhances insulin sensitivity in vitro and in vivo

The renin-angiotensin system plays a critical role in the pathogenesis of obesity, obesity-associated hypertension, and insulin resistance. However, the biological actions of angiotensin II (AII) on insulin sensitivity remain controversial. Because angiotensinogen and AII receptors are expressed on adipose tissue, we investigated the effect of AII on the insulin sensitivity of isolated rat adipocytes. The results of a receptor binding assay showed the maximal AII binding capacity of adipocytes to be 8.3 +/- 0.9 fmol/7 x 10(6) cells and the dissociation constant to be 2.72 +/- 0.11 nM. Substantial expression of both type 1 and 2 AII (AT1 and AT2) receptors was detected by RT-PCR. AII had no effect on basal glucose uptake, but significantly potentiated insulin-stimulated glucose uptake; this effect was abolished by the AT1 antagonist, losartan. In addition, AII did not alter the insulin binding capacity of adipocytes, but increased insulin-stimulated tyrosine phosphorylation of the insulin receptor beta-subunit, Akt phosphorylation, and translocation of glucose transporter 4 to the plasma membrane. AII potentiated insulin-stimulated glucose uptake through the AT1 receptor and by alteration of the intracellular signaling of insulin. Intraperitoneal injection of Sprague Dawley rats with AII increased insulin sensitivity in vivo. In conclusion, we have shown that AII enhances insulin sensitivity both in vitro and in vivo, suggesting that dysregulation of the insulin-sensitizing effect of AII may be involved in the development of insulin resistance.     

α-cell loss from islet impairs its insulin secretion in vitro and in vivo

Pancreatic islet transplantation is an effective treatment for diabetes mellitus. But it is not clear whether α-cell loss during islets isolation could impair the insulin release from β-cell. To unravel this issue, human islets with α-cell deficiency were prepared by prolonged enzyme digestion, as confirmed by immunocytochemistry, immunofluorescence staining and islet insulin/glucagon content analysis. The functions of islets with α-cell deficiency were evaluated in vitro and in vivo. In vitro, human islets with α-cell deficiency exhibited low glucose-induced insulin release compared with intact islets. In islets deficient in α-cells, exogenous glucagon did not alone stimulate insulin release in the absence of glucose, but increased the glucose-induced insulin release in a dose-dependent manner. In intact islets, glucagon did not significantly change the glucose-stimulated insulin secretion. In vivo, transplantation of human islets with α-cell deficiency did not effectively correct hyperglycemia in diabetic C57BL/6 mice. In diabetic nude mice transplanted with islets deficient in α-cells, administration of exogenous glucagon significantly decreased glycemia, while withdrawing glucagon increased glycemic levels as compared with relevant controls. In addition, the survival of diabetic nude mice grafted with islets deficient in α-cells was significantly shorter than the survival of nude mice grafted with intact islets. These results indicated that glucagon-secreting α-cellls have an important role in maintaining glucose-stimulated insulin release from β-cells, and that α-cell loss from islets during isolation has a deleterious effect on insulin secretion.     

Methylglyoxal impairs insulin signalling and insulin action on glucose-induced insulin secretion in the pancreatic beta cell line INS-1E

Aims/hypothesis  

Chronic hyperglycaemia aggravates insulin resistance, at least in part, by increasing the formation of advanced glycation end-products (AGEs). Methylglyoxal (MGO) is the most reactive AGE precursor and its abnormal accumulation participates in damage in various tissues and organs. Here we investigated the ability of MGO to interfere with insulin signalling and to affect beta cell functions in the INS-1E beta cell line.     

Osteoprotegerin increases leukocyte adhesion to endothelial cells both in vitro and in vivo

Recombinant osteoprotegerin (OPG) promoted the adhesion of both primary polymorphonuclear neutrophils (PMNs) and leukemic HL60 cells to endothelial cells. Leukocyte/endothelial cell adhesion was promoted by short (peak at 1 hour) preincubation of either endothelial cells or PMNs with OPG, and the peak of proadhesive activity was observed in the same range of OPG concentrations detected in the sera of patients affected by cardiovascular diseases. Although the cognate high-affinity ligands for OPG, membrane receptor activator of nuclear factor-kappaB ligand (RANKL) and tumor necrosis factor-related apoptosis-inducing ligand (TRAIL), were detected at significant levels on both PMNs and HL60 cells, they were not expressed on the surface of endothelial cells. However, preincubation of OPG with heparin abrogated its proadhesive activity, whereas pretreatment of endothelial cells with chondroitinase plus heparinases significantly decreased the proadhesive activity of OPG. Taken together, these findings suggest the involvement of both the ligand binding and the N-terminal heparin-binding domains of OPG in mediating its pro-adhesive activity. The relevance of these in vitro findings was underscored by in vivo experiments, in which the topical administration of recombinant OPG increased leukocyte rolling and adhesion to rat mesenteric postcapillary venules. Our data suggest that a pathological increase of OPG serum levels might play an important role in promoting leukocyte/endothelial cell adhesion.     

Acute mental stress impairs insulin sensitivity in IDDM patients

Summary The effect of acute mental stress on insulin sensitivity was evaluated in ten IDDM patients, studied on two occasions (test day and control day) in random order and separated by a period of 1–3 weeks. Mental stress was evoked by a modified filmed version of Stroop's CWT for 20 min. On the control day, the patients were resting quietly during the corresponding period. Insulin sensitivity was estimated by an insulin (0.4 mU · kg^–1 · min^–1)-glucose (4.5 mg · kg^–1 · min^–1)-infusion test (IGIT) for 6.5 h. Mental stress evoked significant responses for adrenaline, cortisol and GH, their respective peak values being 0.27 ± 0.05 nmol/l, 426 ± 27 nmol/l and 7.6 ± 1.8 g/l, as well as increases in systolic and diastolic blood pressure and pulse rate The steady-state blood glucose levels, i.e. the mean blood glucose levels 3–6.5 h after the start of the IGIT, were significantly higher after stress, compared with those on the control day, 10.6 ± 1.5 vs 8.7 ± 1.4 mmol/l, p = 0.01, demonstrating impairment of the insulin sensitivity by mental stress. It is concluded that acute mental stress induces a state of insulin resistance in IDDM patients, which can be demonstrated by an IGIT to appear 1 h after maximal stress and to last more than 5 h.Abbreviations IDDM Insulin-dependent diabetes mellitus - GH growth hormone - IGIT insulin-glucose-infusion test - CWT colour-word test - AUC area under the curve     

Prostaglandin E2 affects both insulin secretion and peripheral insulin sensitivity

The effect of prostaglandin E2 (PGE2) on plasma C-peptide (CP) level and peripheral insulin effectiveness was studied in twenty-five healthy volunteers. Plasma CP levels were studied in 14 volunteers (group C) during a three-hours PGE2 infusion. The same experiment was repeated a week later, with saline infusion. Plasma CP level were measured 15 minutes before the infusion, at the beginning of the experiment (0 min) as well as 15, 30, 60, 120 and 180 minutes after the start of the infusion. Peripheral insulin sensitivity was studied in vivo by means of the artificial endocrine pancreas (Biostator), using the euglycaemic hyperinsulinaemic clamp technique. The healthy volunteers underwent a three-hour experiment with saline (group A) or with saline and PGE2-10 micrograms/min during the third hour of clamping (group B). There was a significant decrease in plasma CP level at the 15 th and 30 th minute of PGE2 infusion in the subjects from group C (p less than 0.05). In the group A there was an increase of 14.2% in the amount of glucose infused during the third hour of clamping as compared to the second (2nd-9.673 +/- 1.680 mg/kg/min, 3rd-11.051 +/- 1.802 mg/kg/min). The amount of glucose infused in the subjects of group B remained the same in the course of clamping (2nd hour-7.938 +/- 2.180 mg/kg/min, 3rd hour-7.932 +/- 2.284 mg/kg/min). The difference in the dynamic changes of the amount of glucose infused between the two groups of volunteers (group A and group B) was significant (F = 5.68, p less than 0.01).(ABSTRACT TRUNCATED AT 250 WORDS)     

In vivo insulin sensitivity in adrenodemedullated rats

Insulin sensitivity and responsiveness were determined in adrenal demedullated rats (ADMX) with euglycemic insulin clamp technique. Adrenal medulla was extirpated bilaterally a week before the study. Catheters were placed at right atrium via right jugular vein for sampling blood and at inferior vena cava via femoral vein for the infusion of insulin and glucose solution. Insulin was infused at rates of 4.4, 8.8, 14.7, 29.3, 88.0, 293.0 mU/kg/min. Blood was collected every five min. during the clamp and glucose infusion rate was modulated to control the blood glucose concentrations at fasting levels. Glucose metabolism was calculated from the amount of glucose infused from 60th to 120th min. during the euglycemic clamp. The results obtained were as follows: 1. Glucose metabolisms of ADMX in each infusion rate of insulin, 4.4, 8.8, 14.7, 29.3, 88.0, 293.0 mU/kg/min were 5.2 +/- 0.5, 12.5 +/- 0.5, 17.6 +/- 1.2, 19.8 +/- 2.3, 29.0 +/- 1.5, and 25.2 +/- 1.9 mg/kg/min, respectively. 2. Glucose metabolisms of control group in each dose were 6.6 +/- 0.4, 9.0 +/- 0.9, 18.5 +/- 1.2, 23.4 +/- 2.4, 24.6 +/- 1.1, and 27.0 +/- 1.3 mg/kg/min, respectively. 3. Significant difference (p less than 0.01) in glucose metabolism between ADMX and control was observed at the insulin infusion rate of 8.8 mU/kg/min which might be equivalent to physiological hyperinsulinemia. 4. There were not any differences in insulin responsiveness between both groups. These results suggest that epinephrine regulates insulin sensitivity under physiological hyperinsulinemic condition via defects of insulin receptors.     

Angiotensin II impairs endothelial progenitor cell number and function in vitro and in vivo: implications for vascular regeneration

Endothelial progenitor cells (EPCs) contribute to endothelial regeneration. Angiotensin II (Ang II) through Ang II type 1 receptor (AT(1)-R) activation plays an important role in vascular damage. The effect of Ang II on EPCs and the involved molecular mechanisms are incompletely understood. Stimulation with Ang II decreased the number of cultured human early outgrowth EPCs, which express both AT(1)-R and Ang II type 2 receptor, mediated through AT(1)-R activation and induction of oxidative stress. Ang II redox-dependently induced EPC apoptosis through increased apoptosis signal-regulating kinase 1, c-Jun N-terminal kinase, and p38 mitogen-activated protein kinase phosphorylation; decreased Bcl-2 and increased Bax expression; and activation of caspase 3 but had no effect on the low cell proliferation. In addition, Ang II impaired colony-forming and migratory capacities of early outgrowth EPCs. Ang II infusion diminished numbers and functional capacities of EPCs in wild-type (WT) but not AT(1)a-R knockout mice (AT(1)a(-/-)). Reendothelialization after focal carotid endothelial injury was decreased during Ang II infusion. Salvage of reendothelialization by intravenous application of spleen-derived progenitor cells into Ang II-treated WT mice was pronounced with AT(1)a(-/-) cells compared with WT cells, and transfusion of Ang II-pretreated WT cells into WT mice without Ang II infusion was associated with less reendothelialization. Transplantation of AT(1)a(-/-) bone marrow reduced atherosclerosis development in cholesterol-fed apolipoprotein E-deficient mice compared with transplantation of apolipoprotein E-deficient or WT bone marrow. Randomized treatment of patients with stable coronary artery disease with the AT(1)-R blocker telmisartan significantly increased the number of circulating CD34/KDR-positive EPCs. Ang II through AT(1)-R activation, oxidative stress, and redox-sensitive apoptosis signal-regulating kinase 1-dependent proapoptotic pathways impairs EPCs in vitro and in vivo, resulting in diminished vascular regeneration.     

Aegeline inspired synthesis of novel β3-AR agonist improves insulin sensitivity in vitro and in vivo models of insulin resistance

Background and PurposeIn our drug discovery program of natural product, earlier we have reported Aegeline that is N-acylated-1-amino-2- alcohol, which was isolated from the leaves of Aeglemarmelos showed anti-hyperlipidemic activity for which the QSAR studies predicted the compound to be the β3-AR agonist, but the mechanism of its action was not elucidated. In our present study, we have evaluated the β3-AR activity of novel N-acyl-1-amino-3-arylopropanol synthetic mimics of aegeline and its beneficial effect in insulin resistance. In this study, we have proposed the novel pharmacophore model using reported molecules for antihyperlipidemic activity. The reported pharmacophore features were also compared with the newly developed pharmacophore model for the observed biological activity.Experimental ApproachBased on 3D pharmacophore modeling of known β3AR agonist, we screened 20 synthetic derivatives of Aegeline from the literature. From these, the top scoring compound 10C was used for further studies. The in-slico result was further validated in HEK293T cells co-trransfected with human β3-AR and CRE-Luciferase reporter plasmid for β3-AR activity.The most active compound was selected and β3-AR activity was further validated in white and brown adipocytes differentiated from human mesenchymal stem cells (hMSCs). Insulin resistance model developed in hMSC derived adipocytes was used to study the insulin sensitizing property. 8 week HFD fed C57BL6 mice was given 50 mg/Kg of the selected compound and metabolic phenotyping was done to evaluate its anti-diabetic effect.ResultsAs predicted by in-silico 3D pharmacophore modeling, the compound 10C was found to be the most active and specific β3-AR agonist with EC₅₀ value of 447 nM. The compound 10C activated β3AR pathway, induced lipolysis, fatty acid oxidation and increased oxygen consumption rate (OCR) in human adipocytes. Compound 10C induced expression of brown adipocytes specific markers and reverted chronic insulin induced insulin resistance in white adipocytes. The compound 10C also improved insulin sensitivity and glucose tolerance in 8 week HFD fed C57BL6 mice.ConclusionThis study enlightens the use of in vitro insulin resistance model close to human physiology to elucidates the insulin sensitizing activity of the compound 10C and edifies the use of β3AR agonist as therapeutic interventions for insulin resistance and type 2 diabetes.     

Dehydroepiandrosterone supplementation improves endothelial function and insulin sensitivity in men

The dehydroepiandrosterone (DHEA) concentration decreases with age. There is evidence that DHEA has a protective effect against age-related disorders, including cardiovascular disease. Accordingly, we examined the effect of DHEA supplementation (25 mg/d) on endothelial function, insulin sensitivity, and fibrinolytic activity in 24 men with hypercholesterolemia (mean age, 54 +/- 1 yr). All subjects were enrolled in a randomized, double-blind study. Flow-mediated dilation of brachial artery after transient occlusion, which was expressed as the percent change from the baseline value of the diameter, increased significantly with DHEA supplementation [DHEA: baseline, 3.9 +/- 0.5%; 4 wk, 6.9 +/- 0.7%; 8 wk, 7.9 +/- 0.6%; 12 wk, 8.4 +/- 0.7% (P < 0.01 vs. baseline for all, by ANOVA); placebo: 4.1 +/- 0.6%, 4.5 +/- 0.5%, 3.9 +/- 0.5%, and 4.4 +/- 0.6% (P < 0.01 for all, by ANOVA)]. There was a significant concurrent reduction in the plasma levels of plasminogen activator inhibitor type 1 during DHEA supplementation [DHEA: 9.1 +/- 2.2, 6.4 +/- 2.3, 5.5 +/- 2.8, and 5.1 +/- 2.0 IU/ml (P < 0.01 vs. baseline, by ANOVA); placebo: 9.0 +/- 2.1, 10.4 +/- 2.2, 9.5 +/- 2.2, and 9.6 +/- 2.1 IU/ml (P < 0.01, by ANOVA)]. DHEA supplementation also decreased steady state plasma glucose [DHEA: baseline, 178.9 +/- 12.2; 12 wk, 132.0 +/- 12.8 mg/dl (P < 0.01, by ANOVA); placebo: 181.0 +/- 13.8 and 179.6 +/- 12.4 mg/dl (P < 0.01, by ANOVA)]. In contrast, steady state plasma insulin did not change during the study in either group. The low dose DHEA supplementation improves vascular endothelial function and insulin sensitivity and decreases the plasminogen activator inhibitor type 1 concentration. These beneficial changes have the potential to attenuate the development of age-related disorders such as cardiovascular disease.     

C-reactive protein impairs hepatic insulin sensitivity and insulin signaling in rats: role of mitogen-activated protein kinases

Plasma C-reactive protein (CRP) concentration is increased in the metabolic syndrome, which consists of a cluster of cardiovascular disease risk factors, including insulin resistance. It is not known, however, whether CRP is merely a marker of accompanying inflammation or whether it contributes causally to insulin resistance. The objective of this study is to investigate the role that CRP may play in the development of insulin resistance. We examined the effect of single-dose intravenous administration of purified human (h)CRP on insulin sensitivity in Sprague-Dawley rats using the euglycemic, hyperinsulinemic clamp technique. hCRP was associated with impaired insulin suppression of endogenous glucose production with no reduction in peripheral tissue glucose uptake, suggesting that hCRP mediated insulin resistance in the liver but not extrahepatic tissues. We further assessed components of the insulin signaling pathway and mitogen-activated protein kinases (MAPKs) in the liver. Liver tissues derived from hCRP-treated rats showed reduced insulin-stimulated insulin receptor substrate (IRS) tyrosine phosphorylation, IRS/phosphatidylinositol 3-kinase (PI3K) association, and Akt phosphorylation, consistent with hCRP-induced impairment of hepatic insulin signaling. Furthermore, hCRP enhanced phosphorylation of extracellular signal-regulated kinase (ERK)1/2 and p38 MAPK as well as IRS-1 Ser(612) . Finally, we observed in primary cultured rat hepatocytes that U0126 (a selective inhibitor of MAPK/ERK kinase1/2) corrected hCRP-induced impairment of insulin signaling. CONCLUSIONS: hCRP plays an active role in inducing hepatic insulin resistance in the rat, at least in part by activating ERK1/2, with downstream impairment in the insulin signaling pathway.     

Effects of dietary changes on cellular insulin binding and in vivo insulin sensitivity

The effect of a low-sucrose, low-fat diet on insulin sensitivity, insulin binding to monocytes, and insulin secretion in nonketotic diabetic patients was studied. Ten obese diabetics were studied for 1 yr before and during treatment with a 1200–1500-kcal diet, whereas six diabetics of normal weight were studied for 3 mo before and after treatment with a 2200–2400-kcal diet. In the obese group, no change was found in the insulin response to i.v. injection of glucose during treatment (p > 0.1), but the insulin sensitivity was normalized after 1 yr (p < 0.01). The clinical normalization and the improvement of insulin sensitivity were accompanied by a parallel normalization of the binding of insulin to monocytes (p < 0.01). In the group of normal-weight diabetics, both the insulin sensitivity (p < 0.05) and the insulin binding to monocytes (p < 0.05) were normalized after a 3-mo treatment period, but the insulin secretion increased (p < 0.05) without reaching normal values. We conclude that most nonketotic diabetic patients can be controlled by diet treatment alone. The mechanism of action of the low-fat, low-sucrose diet seems for the greatest part to be a normalization of the insulin sensitivity, which is partly caused by a normalization of the cellular insulin binding.     

Obstructive sleep apnoea syndrome impairs insulin sensitivity independently of anthropometric variables

OBJECTIVES: Obstructive sleep apnoea syndrome (OSAS) is strongly associated with obesity and characterized by endocrine and metabolic changes including impairment of insulin sensitivity. The aim of this study was to further clarify the insulin dynamics and glucose metabolism in this condition. DESIGN, PATIENTS AND MEASUREMENTS: We studied 30 obese patients with OSAS [OSA, 21 males, 9 females; age, mean +/- SEM: 53.1 +/- 1.7 years; body mass index (BMI): 38.6 +/- 1.1 kg/m2; waist-to-hip ratio (WHR): 0.99 +/- 0.07; Apnoea/Hypopnoea Index (AHI): 40.5 +/- 5.8 events/h of sleep] by means of overnight polysomnography and oral glucose tolerance testing. Mathematical models were used to assess: (i) whole-body insulin sensitivity index (ISI composite); (ii) hepatic ISI; (iii) the first phase of insulin secretion (DeltaI30'-0'/DeltaG30'-0'). Results were compared with those in 27 weight-matched patients with simple obesity (OB, 12 males, 15 females; age: 48.1 +/- 2.8 years, BMI: 38.5 +/- 1.4 kg/m2, WHR: 0.94 +/- 0.09; AHI: 2.15 +/- 0.5 events/h of sleep) and with 20 normal subjects (NS, 15 females; 5 males, age: 40.4 +/- 2.9 years; BMI: 22.2 +/- 0.6 kg/m2). RESULTS: ISI composite value was significantly lower in OSAS (1.71 +/- 1.41) than in OB (3.08 +/- 0.27) and in NS (6.1 +/- 0.4) even after age-, BMI- and WHR-adjustment. Similarly, hepatic ISI was significantly different among the three groups (OB = 0.25 +/- 0.02, OSAS = 0.16 +/- 0.014 and NS = 0.55 +/- 0.04). Sex did not affect ISI indices. Insulin secretion estimates were not significantly different among the three groups. DISCUSSION: Obese patients with obstructive sleep apnoea syndrome are more insulin resistant than patients with simple obesity independently of the degree and distribution of adiposity. The worsening in insulin sensitivity in obstructive sleep apnoea syndrome patients could reflect the hypoxic state and would account for the increased vascular risk in this condition.     

Growth-stimulatory actions of insulin in vitro and in vivo

Insulin stimulates the growth and proliferation of a variety of somatic cells in culture, and evidence suggests that insulin is also an important regulator of growth in vivo. In cell culture, insulin interacts synergistically with other hormones and growth factors such as platelet-derived growth factor (PDGF), fibroblast growth factor (FGF), epidermal growth factor (EGF), tumor-promoting phorbol esters, and thrombin, to stimulate progression through the cell cycle of cells that have been arrested in G1 by deprivation for serum. In addition, insulin is required by most cells for optimal long term growth in hormone-supplemented serum-free media. In some cells, such as human skin fibroblasts, the growth-promoting effects of insulin appear to be mediated primarily by its low affinity interaction with receptors for insulin-like growth factor I (IGF-I). In other cells, such as hepatocytes, hepatoma cells, adrenocortical tumor cells, mammary carcinoma cells, and F9 embryonal carcinoma cells, insulin appears to stimulate growth by binding to high affinity insulin receptors. The insulin and IGF-I receptor proteins, like the receptor proteins for other growth-promoting hormones such as EGF and PDGF, are closely associated with tyrosine-specific protein kinase activities. The mechanism by which the binding of insulin to its receptor and activation of the receptor-associated tyrosine protein kinase activity control intracellular protein phosphorylation and dephosphorylation reactions, such as the phosphorylation of ribosomal protein S6, is a subject of considerable current interest. The phosphorylation of ribosomal protein S6 may be related mechanistically to the activation by insulin of protein synthesis, and hence the passage of cells through the G1 phase of the cell cycle. Malignant transformation does not generally result in a total loss of the growth requirement of cells for insulin or insulin-like growth factors, although transformation is accompanied in some cases by a qualitative reduction in the insulin/IGF requirement. Abnormalities in insulin production or sensitivity in vivo are accompanied by abnormalities in growth; thus, insulin appears to be an important regulator of growth in vivo. Some of the growth-promoting effects of insulin in vivo may be attributable to direct action of insulin, while other effects may be caused by the regulatory effect of insulin on somatomedin production, and possibly on somatomedin action.     

Insulin resistance in the aged: a quantitative evaluation of in vivo insulin sensitivity and in vitro glucose transport

It has recently been made clear that reduced sensitivity to exogenous insulin can be demonstrated in the course of aging. This phenomenon has been further investigated with the aid of sophisticated techniques, such as the euglycemic clamp, which, when coupled with the measurement of hepatic glucose production, showed that "impaired tissue sensitivity to insulin is the primary factor responsible for the decrease of glucose tolerance in advancing age." Nevertheless, this study did not establish whether such impairment reflects reduced sensitivity (receptor deficiency) or reduced response (postreceptor or receptor plus postreceptor defect), as shown in other diseases. Evidence in favor of the view that receptor deficiency is responsible can be seen in our observation of an approximately 50% reduction in receptors in a study of insulin binding on isolated human fat cells. Two aspects of this question appeared to require further investigation: tissue sensitivity to receptor-saturating insulin concentration (euglycemic clamp at about 1000 microU/mL plasma insulin), and the glucose transport system coupled to the receptor. A decrease in receptors alone should shift the insulin sensitivity curve to the right, both in vivo (euglycemic clamp) and in vitro (glucose transport), with no reduction of the maximum effect. A solution to this question is proposed in the light of a study conducted on young volunteers and subjects over 65 years old.     

Vanadate increases cell surface insulin binding and improves insulin sensitivity in both normal and insulin-resistant rat adipocytes

Summary The aim of this study was to elucidate the acute effects of vanadate on cell surface insulin binding and insulin sensitivity in rat adipocytes. The cells were preincubated at 37° for 20 min followed by energy depletion with potassium cyanide, extensive washing and ¹²⁵I-insulin binding. The presence of vanadate or insulin during the preincubation period dose-dependently enhanced ¹²⁵I-insulin binding to normal adipocytes (maximally 4–5-fold) through an increased number of binding sites without any change in receptor affinity. Submaximal concentrations of vanadate added together with insulin enhanced the cellular sensitivity to the effect of insulin to stimulate 3-O-methylglucose transport. Vanadate, but not insulin, was also capable of increasing insulin binding as well as insulin sensitivity in insulin-resistant cells (treatment with N⁶-monobutyryl cAMP or amiloride and adipocytes from obese, aging rats). There was a correlation between the effect of vanadate to augment insulin binding and its ability to enhance cellular insulin sensitivity. Thus, the data suggest that short-term vanadate treatment improves insulin sensitivity through enhanced receptor binding and that this occurs in both normal and insulin-resistant cells.