Body fat mass and the proportion of very large adipocytes in pregnant women are associated with gestational insulin resistance

Background/Objectives:

Pregnancy is accompanied by fat gain and insulin resistance. Changes in adipose tissue morphology and function during pregnancy and factors contributing to gestational insulin resistance are incompletely known. We sought to characterize adipose tissue in trimesters 1 and 3 (T1/T3) in normal weight (NW) and obese pregnant women, and identify adipose tissue-related factors associated with gestational insulin resistance.

Subjects/Methods:

Twenty-two NW and 11 obese women were recruited early in pregnancy for the Pregnancy Obesity Nutrition and Child Health study. Examinations and sampling of blood and abdominal adipose tissue were performed longitudinally in T1/T3 to determine fat mass (air-displacement plethysmography); insulin resistance (homeostasis model assessment of insulin resistance, HOMA-IR); size, number and lipolytic activity of adipocytes; and adipokine release and density of immune cells and blood vessels in adipose tissue.

Results:

Fat mass and HOMA-IR increased similarly between T1 and T3 in the groups; all remained normoglycemic. Adipocyte size increased in NW women. Adipocyte number was not influenced, but proportions of small and large adipocytes changed oppositely in the groups. Lipolytic activity and circulating adipocyte fatty acid-binding protein increased in both groups. Adiponectin release was reduced in NW women. Fat mass and the proportion of very large adipocytes were most strongly associated with T3 HOMA-IR by multivariable linear regression (R²=0.751, P<0.001).

Conclusions:

During pregnancy, adipose tissue morphology and function change comprehensively. NW women accumulated fat in existing adipocytes, accompanied by reduced adiponectin release. In comparison with the NW group, obese women had signs of adipocyte recruitment and maintained adiponectin levels. Body fat and large adipocytes may contribute significantly to gestational insulin resistance.

A maternal diet of fatty fish reduces body fat of offspring compared with a maternal diet of beef and a post-weaning diet of fish improves insulin sensitivity and lipid profile in adult C57BL/6 male mice

AIM

The maternal diet during pregnancy and lactation may affect the long-term health of the offspring. Our aim was to study how a fish or meat diet perinatal and after weaning affects body composition, insulin sensitivity and the profile of n-3 and n-6 polyunsaturated fatty acids (PUFAs) in breast milk, fat depots, skeletal muscle and liver in male adult mice offspring.

METHODS

During gestation and lactation, C57BL/6 dams were fed a herring- or beef-based diet. Half of the pups in each group changed diets after weaning. In offspring, body composition measured by DEXA, plasma lipid profile and insulin sensitivity measured by euglycemic clamp or QUICKI were monitored to adulthood. Analysis of total FAs by GC-MS were performed in the diet, breast milk and in different tissues.

RESULTS

At 9 week of age, offspring of herring-fed dams had less body fat than offspring of beef-fed dams. Mice fed herring after weaning had increased insulin sensitivity at 15 week of age, reduced total plasma cholesterol and triglyceride levels, and compared with beef-fed mice, larger interscapular brown adipose tissue depots. The FA composition of the maternal diet was mirrored in breast milk, and the herring diet significantly affected the FA profile of different tissues, leading to an increased content of n-3 PUFAs.

CONCLUSION

A herring-based maternal diet reduces body fat in the offspring, but the insulin sensitivity, plasma lipids and amount of brown adipose tissue are affected by the offspring's own diet; the herring diet is more beneficial than the beef diet.

Influence of herring (Clupea harengus) and herring fractions on metabolic status in rats fed a high energy diet

AIM

Few dietary studies have looked beyond fish oil to explain the beneficial metabolic effects of a fish-containing diet. Our aim was to study whether addition of herring, or sub-fractions of herring, could counteract negative metabolic effects known to be induced by a high-fat, high-sugar diet.

METHODS

Rats were given six different diets: standard pellets; high energy diet with chicken mince (HiE control); high energy diet with herring mince (HiE herring); and high energy diet with chicken mince and either herring oil (HiE herring oil), herring press juice, PJ (HiE PJ) or herring low molecular weight PJ (HiE LMW-PJ). Factors associated with the metabolic syndrome were measured.

RESULTS

There were no differences in energy intake or body weight between the groups, but animals fed high energy diets had a higher body fat content compared with the pellet group, although not statistically significant in all groups. Mesenteric adipocyte size was smaller in the HiE herring oil group compared with the HiE control. Glucose clamp studies showed that, compared with the pellet group, the HiE control and HiE herring diets, but not the HiE herring oil diet, induced insulin resistance. Addition of herring or herring oil to the high energy diet decreased total cholesterol levels, triacylglycerols and the atherogenic index compared with the HiE control group.

CONCLUSIONS

The results suggest that addition of herring or herring oil counteracts negative effects on blood lipids induced by a high energy diet. The lipid component of herring thus seems to be responsible for these beneficial effects.

Cardiac concentric remodelling induced by non-aromatizable (dihydro-)testosterone is antagonized by oestradiol in ovariectomized rats

Previous studies on the cardiovascular effects of androgens in females, most of them using testosterone treatment, have yielded conflicting results. Testosterone is metabolized into oestradiol (E2) and dihydrotestosterone (DHT) within cardiovascular tissues. The aim of the present study was to explore the cardiovascular effects exerted by E2 and the non-aromatizable androgen DHT and to study possible interactions between these in female rats. Ovariectomized rats were treated with DHT, E2, or DHT+E2 for 6 weeks. DHT increased left-ventricular posterior wall thickness, assessed by echocardiography, whereas left-ventricular dimension, as well as total heart weight and calculated left-ventricular mass, were unchanged. DHT also increased the levels of insulin-like growth factor-I mRNA in the left ventricle. E2 abolished the effect of DHT on left-ventricular remodelling and insulin-like growth factor-I mRNA when the two treatments were given in combination. E2 also reduced androgen receptor mRNA levels in the heart. Neither E2 nor DHT changed blood pressure measured by telemetry. In conclusion, treatment with the endogenous non-aromatizable androgen DHT causes cardiac concentric remodelling in ovariectomized rats, possibly mediated by increased local levels of insulin-like growth factor-I. The effect of DHT on cardiac wall thickness was antagonized by E2, possibly through downregulation of cardiac androgen receptors. These mechanisms may be of importance for the concentric left-ventricular geometric pattern developing in women after menopause.

Effect of exercise on ovarian morphology and expression of nerve growth factor and alpha(1)- and beta(2)-adrenergic receptors in rats with steroid-induced polycystic ovaries

Oestadiol valerate (EV)-induced polycystic ovaries (PCO) in rats cause anovulation and cystic ovarian morphology. Denervation of ovarian sympathetic nerves restores ovulatory disruption. In the present study, we determined whether 5 weeks of voluntary exercise influence ovarian morphology and the expression of sympathetic markers in the EV-induced PCO rat model. The effect of exercise on (i) ovarian morphology; (ii) mRNA and protein expression of nerve growth factor (NGF); and (iii) mRNA and number of ovarian-expressing cells for the NGF receptor (p75 neurotrophin receptor) and the alpha(1a)-, alpha(1b)-, alpha(1d)- and beta(2)-adrenergic receptors (ARs) in rats with EV-induced PCO was evaluated. PCO was induced by a single i.m. injection of EV, and controls were injected with oil alone in adult cycling rats. The rats were divided into four groups: (i) control (oil); (ii) exercise group (oil + exercise); (iii) a PCO group (EV); and (iv) a PCO exercise group (EV + exercise). The exercise and PCO exercise groups ran voluntarily for 5 weeks in computer-monitored wheels placed in the cages where they were housed. The results obtained indicated that ovarian morphology was almost normalised in the PCO exercise group; NGF mRNA and protein concentrations were normalised in the PCO exercise group; high numbers of NGF receptor expressing cells in PCO ovaries were lowered by exercise; and the number of immunopositive cells of the different AR subtypes were all reduced after exercise in the PCO group, except for the alpha(1b)- and beta(2)-AR whereas the mRNA levels were unaffected, indicating transcriptional regulation. In conclusion, our data indicate a beneficial effect of regular exercise, as a modulator of ovarian sympathetic innervation, in the prevention and treatment of human PCOS.

Short-term infusion of interleukin-6 does not induce insulin resistance in vivo or impair insulin signalling in rats

AIMS/HYPOTHESIS

Interleukin-6 has been implicated in the insulin resistance associated with obesity and impaired glucose tolerance. Previous studies in vitro have shown that IL-6 rapidly (1-2 h) impairs cellular insulin signalling and action through an increased expression of suppressor of cytokine signalling (SOCS)-3. In the present study, IL-6 or saline was infused in rats that were simultaneously in a state of hyperinsulinaemia. Muscle, liver and adipose tissue were excised after 2 h to examine potential effects on insulin signalling or gene expression.

METHODS

The rats were infused with IL-6 or saline during a euglycaemic-hyperinsulinaemic clamp and the glucose infusion rate was measured after 90 to 120 min. Signal transducer and activator of transcription (STAT)3 phosphorylation and insulin-stimulated tyrosine phosphorylation of the insulin receptors and IRS were measured with immunoblotting and gene expression through real-time PCR.

RESULTS

No inhibitory effect of IL-6 on insulin-stimulated whole-body glucose uptake was seen in spite of high circulating levels of IL-6 (0.85+/-0.08 nmol/l). Tyrosine phosphorylation of the insulin receptors and IRS was also unchanged in the liver, skeletal muscles and adipose tissue. However, tyrosine phosphorylation of STAT3 was increased in all tissues, showing that IL-6 signalling was activated. IL-6 mRNA tended to increase, while GLUT4, peroxisome proliferator-activated receptor-gamma coactivator-1 (PGC-1) and adiponectin gene expression were unchanged.

CONCLUSIONS/INTERPRETATION

Infusion of IL-6 for 120 min in rats during euglycaemic-hyperinsulinaemic conditions did not alter the effect of insulin on whole-body glucose homeostasis, plasma adiponectin levels or insulin signalling in target tissues. Thus, the acute effects of IL-6, associated with SOCS-3 induction, do not lead to whole-body insulin resistance. These data further underscore the importance of the chronic, and potentially tissue-specific effects of IL-6 on insulin signalling and action.

Reductions in adipose tissue and skeletal growth in rat adult offspring after prenatal leptin exposure

Leptin is involved in regulating food intake, energy balance and bone formation. Increasing evidence suggests that leptin is also involved in fetal growth and development. The aim of this study was to determine if increased maternal leptin is followed by changes in body composition, skeletal growth or hormonal regulation in the adult rat offspring. Pregnant rats were given injections of either human recombinant leptin (3.5 mg/kg, i.p.) or vehicle on days 8, 10 and 12 of gestation. Both genders of leptin-exposed offspring showed significantly reduced adipose tIssue weight at adult age. Skeletal growth and cortical bone dimensions were significantly reduced. Circulating testosterone levels were significantly increased in female leptin-exposed offspring, and male leptin-exposed offspring had significant testicular enlargement. No significant effects were seen on circulating leptin levels or hypothalamic protein levels of the leptin receptor. The results demonstrate that maternally administered leptin is involved in fetal growth and development, leading to lean offspring with reduced skeletal growth.

Affected skeletal growth but normal bone mineralization in rat offspring after prenatal dexamethasone exposure

Events occurring early in life or prenatally are able to play important roles in the pathogenesis of diseases in adult life. Different sorts of stress or hormonal influences, during particular periods of pregnancy, may result in persisting or transient changes in physiology. Glucocorticoids are used for the treatment of a variety of diseases, to promote organ maturation and to prevent preterm delivery. Glucocorticoids are also known to affect skeletal growth and adult bone metabolism. The aim of the present study was to investigate whether exposure to dexamethasone (Dex) during fetal life has any effect on skeletal growth and/or bone mineral density in adult rat offspring. Pregnant rats were given injections of either Dex (100 micro g/kg) or vehicle on days 9, 11 and 13 of gestation. Dex-exposed male but not female rat offspring showed transient increases in crown-rump length and tibia and femur lengths at 3-6 weeks of age. In contrast, the cortical bone dimensions were altered in 12-week-old female but not male Dex-exposed offspring. The areal bone mineral densities of the long bones and the spine, as determined by dual X-ray absorptiometry, and trabecular as well as cortical volumetric bone mineral density, as measured using peripheral quantitative computerized tomography, were unchanged in both male and female Dex-exposed offspring. In conclusion, prenatal Dex exposure affects skeletal growth in a gender-specific manner, while the mineralization of bones is unaffected in both male and female offspring.

Myocardial insulin-mediated glucose uptake and left ventricular geometry

Whole-body insulin sensitivity has been shown to be impaired in subjects with increased left ventricular relative wall thickness (RWT) and in hypertensive subjects with left ventricular hypertrophy, but the relation between myocardial insulin sensitivity and RWT or left ventricular mass index (LVMI) in normotension is not known. We measured myocardial and skeletal muscle glucose uptake with [18F]fluorodeoxyglucose and positron emission tomography during hyperinsulinemic euglycemic clamp in nine men with wide ranges of echocardiographic RWT and LVMI. The subjects were male, 72-74 years old, normotensive and free from medication or history of heart disease. RWT correlated inversely with skeletal muscle glucose uptake (r = -0.69, p = 0.04), borderline significantly directly with myocardial glucose uptake (r = 0.62, p = 0.07), and directly with the ratio between myocardial and skeletal muscle glucose uptake (r = 0.77, p = 0.02) during hyperinsulinemic euglycemic clamp. LVMI was not related to insulin-mediated myocardial or skeletal muscle glucose uptake or the ratio between myocardial and skeletal muscle glucose uptake. In conclusion, RWT was inversely related to insulin sensitivity in skeletal muscle and borderline significantly directly related to insulin sensitivity in the myocardium in healthy normotensive elderly men, whereas LVMI was not related to myocardial or skeletal muscle insulin sensitivity.

Prenatal cytokine exposure results in obesity and gender-specific programming

Prenatal events appear to program hormonal homeostasis, contributing to the development of somatic disorders at an adult age. The aim of this study was to examine whether maternal exposure to cytokines or to dexamethasone (Dxm) would be followed by hormonal consequences in the offspring at adult age. Pregnant rats were injected on days 8, 10, and 12 of gestation with either human interleukin-6 (IL-6) or tumor necrosis factor-alpha (TNF-alpha) or with Dxm. Control dams were injected with vehicle. All exposed offspring developed increased body weight (P < 0.05--0.001), apparently due to an increase of 30--40% in adipose tissue weight (P < 0.05--0.01). Corticosterone response to stress was increased in the IL-6 group (P < 0.05-0.01). Dxm-treated male rats exhibited blunted Dexamethasone suppression test results. In male rats, insulin sensitivity was decreased after IL-6 exposure (P < 0.01), whereas basal insulin was elevated in the TNF-alpha group (P < 0.01). In female rats, plasma testosterone levels were higher in all exposed groups compared with controls (P < 0.01--0.001), with the exception of Dxm-exposed offspring. Males in the TNF-alpha group showed decreased locomotor activity (P < 0.05), and females in the IL-6 group showed increased locomotor activity (P < 0.05). These results indicate that prenatal exposure to cytokines or Dxm leads to increased fat depots in both genders. In females, cytokine exposure was followed by a state of hyperandrogenicity. The results suggest that prenatal exposure to cytokines or Dxm can induce gender-specific programming of neuroendocrine regulation with consequences in adult life.

Maternal endotoxemia results in obesity and insulin resistance in adult male offspring

Events in utero appear to be important factors contributing to the development of somatic disorders at adult age. The aim of this study was to examine whether maternal immune challenge would be followed at adult age by metabolic and endocrine abnormalities in the offspring. Pregnant rats were given injections of either endotoxin (Escherichia coli lipopolysaccharide; 0.79 mg/kg, ip) or vehicle on days 8, 10, and 12 of gestation. Adult male offspring to lipopolysaccharide-exposed dams were heavier than controls (P < 0.05) and showed increased adipose tissue weights (P < 0.05), elevated food intake (P < 0.05), and increased circulating leptin (P < 0.01). The effect of insulin on glucose uptake was reduced, as measured by an euglycemic hyperinsulinemic clamp technique (P < 0.05). Serum levels of 17beta-estradiol and progesterone were elevated (P < 0.01 and P < 0.05, respectively). Baseline levels of corticosterone were normal, but the corticosterone response to stress was attenuated (P < 0.05), and hippocampal glucocorticoid receptor protein was up-regulated (P < 0.05). Female offspring were uninfluenced, except for increased testosterone levels (P < 0.05), increased baseline corticosterone levels (P < 0.05), and enlargement of heart and adrenals (P < 0.05). The results indicate that maternal endotoxemia leads to obesity, insulin resistance, and high serum levels of leptin in the adult male offspring. This study reports a novel animal model of obesity with features of the metabolic syndrome.

Insulin insensitivity and delayed transcapillary delivery of insulin in oophorectomized rats treated with testosterone

The importance of transcapillary insulin delivery as a regulated step was explored in an insulin resistant rat model. Oophorectomized female rats were exposed to testosterone (OVX + T) for 8 weeks and examined with insulin clamps, muscle microdialysis, and analyses of insulin distribution kinetics. The results were compared with those obtained in sham-operated control rats. After OVX + T, onset of glucose uptake in skeletal muscle was significantly (P < 0.001-0.05) delayed compared with controls as measured by the glucose infusion rate (GIR) during a euglycaemic, hyperinsulinaemic clamp (5 mU kg-1 min-1). The increase in interstitial insulin concentrations was also significantly (P < 0.05) delayed (15-20% lower) in OVX + T treated rats compared with control rats, but to such a small magnitude that this alone could not explain the late onset of the insulin effect. Skeletal muscle capillary density, examined histochemically, was diminished (P < 0.01-0.001) by 20-25% after treatment with OVX + T compared with control animals, as was the peripheral blood flow (P < 0.05) by 40-45%, measured with the microsphere technique. Insulin binding was reduced in proportion to the reduced (P < 0.01) vascular surface area by OVX + T treatment. Transcapillary transport rate of insulin, measured by comparisons of the kinetics of inulin and insulin spaces in muscle with time, tended (ns) to be lower after OVX + T compared with control rats (30-40%) as a reflection of the lower capillary surface area. The data suggest that the delayed onset of insulin action after OVX + T results from combined defects in the muscle cell at a postreceptor level and, to a lesser extent, from retarded transcapillary delivery of insulin.

Estimations of muscle interstitial insulin, glucose, and lactate in type 2 diabetic subjects

Previous measurement of insulin in human muscle has shown that interstitial muscle insulin and glucose concentrations are approximately 30-50% lower than in plasma during hyperinsulinemia in normal subjects. The aims of this study were to measure interstitial muscle insulin and glucose in patients with type 2 diabetes to evaluate whether transcapillary transport is part of the peripheral insulin resistance. Ten patients with type 2 diabetes and ten healthy controls matched for sex, age, and body mass index were investigated. Plasma and interstitial insulin, glucose, and lactate (measured by intramuscular in situ-calibrated microdialysis) in the medial quadriceps femoris muscle were analyzed during a hyperinsulinemic euglycemic clamp. Blood flow in the contralateral calf was measured by vein plethysmography. At steady-state clamping, at 60-120 min, the interstitial insulin concentration was significantly lower than arterial insulin in both groups (409 +/- 86 vs. 1,071 +/- 99 pmol/l, P < 0.05, in controls and 584 +/- 165 vs. 1, 253 +/- 82 pmol/l, P < 0.05, in diabetic subjects, respectively). Interstitial insulin concentrations did not differ significantly between diabetic subjects and controls. Leg blood flow was significantly higher in controls (8.1 +/- 1.2 vs. 4.4 +/- 0.7 ml. 100 g(-1).min(-1) in diabetics, P < 0.05). Calculated glucose uptake was less in diabetic patients compared with controls (7.0 +/- 1.2 vs. 10.8 +/- 1.2 micromol. 100 g(-1).min(-1), P < 0.05, respectively). Arterial and interstitial lactate concentrations were both higher in the control group (1.7 +/- 0.1 vs. 1.2 +/- 0.1, P < 0. 01, and 1.8 +/- 0.1 vs. 1.2 +/- 0.2 mmol/l, P < 0.05, in controls and diabetics, respectively). We conclude that, during hyperinsulinemia, muscle interstitial insulin and glucose concentrations did not differ between patients with type 2 diabetes and healthy controls despite a significantly lower leg blood flow in diabetic subjects. It is suggested that decreased glucose uptake in type 2 diabetes is caused by insulin resistance at the cellular level rather than by a deficient access of insulin and glucose surrounding the muscle cell.

Muscle glucose uptake is effectively activated by ischemia in type 2 diabetic subjects

It has previously been shown that Wortmannin, a phosphatidylinositol 3-kinase inhibitor, inhibits glucose transport activated by insulin but not by ischemia, suggesting the importance of an activating mechanism that bypasses the insulin signal. To evaluate the relevance of this insulin-independent pathway in insulin-resistant subjects, the ability of ischemia to stimulate glucose uptake was investigated in 9 patients with type 2 diabetes and in 9 healthy control subjects (fasting glucose level 9.4 +/- 0.8 vs. 5.1 +/- 0.1 mmol/l, P < 0.001, in type 2 diabetic patients and control subjects, respectively; fasting insulin level insulin 8.1 +/- 2.6 vs. 4.5 +/-0.7 mU/l, P < 0.05, respectively) matched for sex, age, and BMI. Arterial plasma and interstitial concentrations of glucose and lactate (measured by subcutaneous and muscle microdialysis) were recorded in the forearm before, during, and after ischemia induced locally for 20 min. During ischemia, the muscle interstitial glucose concentration decreased significantly from 7.7 +/- 0.6 to 5.4 +/- 0.4 mmol/l (P < 0.01) and from 4.4 +/- 0.3 to 3.6 +/- 0.3 mmol/l (P < 0.05) in type 2 diabetic patients and control subjects, respectively. The arterial-interstitial (A-I) glucose concentration difference was 1.7 +/- 0.6 and 0.7 +/- 0.3 mmol/ at basal, and it increased significantly to 3.5 +/- 0.7 (P < 0.01) and 1.4 +/-0.3 mmol/l (P < 0.05) during ischemia in each group, respectively. Interstitial lactate increased significantly during ischemia from 0.8 +/- 0.1 to 1.1 +/- 0.1 mmol/l (P < 0.05) and from 0.5 +/- 0.1 to 0.9 +/- 0.2 mmol/l (P < 0.05), respectively. The A-I glucose concentration difference was abolished immediately postischemia and regained after approximately 15 min, whereas high interstitial lactate levels remained elevated throughout the study. Subcutaneous interstitial glucose concentrations remained unchanged during ischemia and postischemia in both groups, whereas the interstitial lactate concentration in adipose tissue increased during ischemia from 1.4 +/- 0.2 to 2.0 +/- 0.2 mmol/l (P < 0.05) and from 1.1 +/- 0.1 to 1.8 +/- 0.3 mmol/l (P < 0.05) in type 2 diabetic patients and control subjects, respectively. Plasma glucose and lactate levels were unchanged in both groups during the study period. The results show that in muscle, but not in adipose tissue, glucose uptake is efficiently activated by ischemia in insulin-resistant type 2 diabetic subjects, suggesting the activation of a putative alternative pathway to the insulin signal in muscle cells.

Effects of exercise on insulin distribution and action in testosterone-treated oophorectomized female rats

Administration of testosterone (T) to oophorectomized (Ovx) female rats is followed by severe insulin resistance, localized to postreceptor cellular events in the muscle. In this study, intervention by exercise was introduced to examine whether circulatory adaptations are involved in insulin resistance. Two groups of Ovx rats were studied: one group was given T (Ovx+T); another group had free access to running wheels (Ovx+T+Ex). In addition, one control group (sham operated) was studied. Insulin sensitivity was measured with the euglycemic hyperinsulinemic clamp technique (submaximal) for 150 min. Muscle interstitial glucose and insulin concentrations were measured by microdialysis. The measurements showed that, in Ovx+T rats, the onset of insulin action was significantly (P < 0.05) slower during the first 95 min of the clamp compared with that in Ovx+T+Ex and controls. Muscle interstitial concentrations of insulin but not glucose were lower in both Ovx+T and Ovx+T+Ex rats than in controls throughout the clamp. It was concluded that physical exercise prevented the slow onset of insulin action in Ovx+T rats without changing the distribution time of muscle interstitial insulin. The results indicate that hyperandrogenicity is characterized by delayed muscle insulin action. Physical exercise reverses these defects without any beneficial effect on muscle interstitial insulin concentrations.

Measurement of interstitial insulin in human muscle

Previous measurements in lymph and adipose tissue have indicated that interstitial insulin concentrations are approximately 40% lower than in plasma. Measurements of insulin in human muscle interstitial fluid have not been performed yet. We developed a new external reference technique for calibration of microdialysis catheters in situ. This technique allows correct assessments of interstitial peptide concentrations and was employed to estimate the insulin concentration in medial quadriceps femoris muscle in 11 individuals (age: 37 +/- 3 yr; body mass index: 25.2 +/- 1.2 kg/m2) during a two-step euglycemic hyperinsulinemic clamp. At steady-state insulin and glucose infusion, plasma glucose was 5.9 +/- 0.2 mmol/l, plasma insulin was 155 +/- 17 mU/l, and interstitial muscle insulin was 67 +/- 19 mU/l (n = 9; P < 0.01). At a higher insulin infusion rate, the steady-state plasma insulin concentration was 379 +/- 58 mU/l, and interstitial insulin concentration was 180 +/- 40 mU/l (P < 0.01). The data show for the first time that high physiological and supraphysiological plasma insulin levels give 30-50% lower interstitial concentrations of insulin in the muscle. The importance of capillary delivery as a rate-limiting step for the insulin effect is suggested.

Induction of insulin resistance by glucosamine reduces blood flow but not interstitial levels of either glucose or insulin

To study the effects of a glucosamine infusion on skeletal muscle metabolism, microdialysis was performed in the medial femoral muscle in Sprague-Dawley rats during a euglycemic-hyperinsulinemic clamp (insulin infusion 18 mU x kg(-1) x min(-1)). During steady-state clamping conditions (70 min), an infusion of glucosamine (30 micromol x kg(-1) x min(-1)) or saline was given for 240 min. Blood flow was measured by the microsphere technique at the end of the clamp. An approximately 36% (P < 0.001) reduction in the glucose infusion rate was seen after 170 min in the glucosamine-treated rats compared with control rats. There were no significant differences in interstitial or plasma levels of either insulin or glucose between the two groups. Both interstitial (2.31 +/- 0.18 vs. 1.71 +/- 0.24 mmol/l, P < 0.05) and arterial plasma lactate concentrations (1.29 +/- 0.09 vs. 0.79 +/- 0.09 mmol/l, P < 0.01) were significantly higher in control rats compared with glucosamine-treated rats. Blood flow was significantly reduced in hind limb femoral muscles in the glucosamine-treated rats compared with control rats. The most pronounced reduction in blood flow was seen in the Soleus muscle (27.6 +/- 3.4 vs. 14.7 +/- 2.0 ml x 100 g(-1) x min(-1), P < 0.01). These results demonstrate that induction of insulin resistance by glucosamine results in a reduction of the blood flow rate as well as the uptake of glucose and the production of lactate in skeletal muscle. As a result of the inhibited glucose metabolism, the interstitial glucose concentration was unchanged despite the reduced blood flow after glucosamine administration. The data suggest the importance of regulation of blood flow by nonoxidative metabolism of glucose in resting muscle.

Induction of rat muscle insulin resistance by epinephrine is accompanied by increased interstitial glucose and lactate concentrations

Muscle glucose uptake and lactate release during beta-adrenergic stimulation by epinephrine (epi) and beta-adrenergic blockade by propranolol (prop) were investigated during an euglycaemic hyperinsulinaemic (30 pmol x kg(-1) x min(-1)) with or without added somatostatin (0.1 microg/min; pancreatic) clamp in female rats. To assess the interstitial insulin, glucose and lactate concentrations, microdialysis was done in the medial femoral muscle in both legs. The influence of muscle skeletal blood flow on interstitial insulin, glucose and lactate was examined with the microsphere technique, using 57Co-microspheres. Epinephrine decreased glucose infusion rate by about 75% (p < 0.0001) and increased concentrations of interstitial glucose by about 35% (p < 0.001) and lactate by about 65% (p < 0.01). Plasma insulin concentration increased during beta-adrenergic stimulation by about 25% (p < 0.05) whereas the interstitial insulin concentration was unchanged. Muscle blood flow in the hindlimb was considerably enhanced by about 130%, (p < 0.001) by epinephrine. Infusion of propranolol totally abolished all the above effects induced by epinephrine. The data show that insulin resistance and vasodilation induced by beta-adrenergic stimulation with epinephrine is accompanied by increased interstitial glucose as well as lactate concentrations in muscle. The increased interstitial glucose concentration is the result of a decreased cellular uptake of glucose together with an increased capillary delivery of glucose by vasodilation. It is concluded that the severe cellular resistance to insulin induced by epinephrine could not be overcome either by the increased insulin secretion or by vasodilation.

Minimal influence of blood flow on interstitial glucose and lactate-normal and insulin-resistant muscle

To study the regulation of the interstitial glucose concentration in skeletal muscle, nine control subjects and nine older and overweight non-insulin-dependent diabetes mellitus (NIDDM) subjects with extreme insulin resistance were investigated with microdialysis in the medial femoral muscle before and during a euglycemic insulin clamp. After an overnight fast, arterial plasma glucose concentration was 4.9 +/- 0.1 and 8.5 +/- 0.6 mmol/l (P < 0.001), respectively. The arterial-interstitial concentration ([a-i]) differences of glucose and lactate were 0.43 +/- 0.16 (P < 0.05) and -0.13 +/- 0.05 mmol/l, respectively, in normal subjects. In NIDDM subjects, [a-i] differences for glucose and lactate were nonsignificant. Muscle blood flow was similar in controls and NIDDM subjects. During the glucose clamp, the glucose [a-i] differences increased and the lactate [a-i] differences decreased significantly in both groups. The glucose 170 infusion rate was 8.0 +/- 0.77 vs. 3.2 +/- 0.51 mg.kg-1.min-1 (P < 0.001), and blood flow was 9.9 +/- 1.6 vs. 6.7 +/- 0.9 ml.100 g-1.min-1 (P < 0.05) in controls and NIDDM subjects, respectively. These results show that 1) the capillary wall is rate limiting for muscle glucose uptake and lactate release in control subjects but not in postabsorptive hyperglycemic insulin-resistant subjects, 2) vasodilation during insulin infusion does not prevent the increase in [a-i] difference of glucose in normal subjects, and 3) in severely insulin-resistant muscle, the [a-i] difference of glucose is not extended despite lack of vasodilation.

Imprinting of female offspring with testosterone results in insulin resistance and changes in body fat distribution at adult age in rats

In women, a relative hyperandrogenicity is statistically associated with insulin resistance and centralization of body fat, which are predictors for the development of non-insulin-dependent diabetes mellitus. The aim of this study was to evaluate the effect of androgenization of newborn female rats on insulin sensitivity at adult age. To mimic the neonatal androgen peak normally observed in male rats, female pups were administered one high dose of testosterone (T) subcutaneously within 3 h after birth. They were then given back to their mothers and followed to adult age. At the end of the week 9, tail samples were taken, showing no differences in fasting plasma concentrations of glucose, lactate, insulin, or free fatty acids between T-treated rats and controls. Plasma concentrations of T and progesterone were significantly lower in the T-treated rats, whereas no differences were found in the levels of corticosterone, estradiol, insulin-like growth factor I, or ACTH. After 10 wk, insulin sensitivity was studied with hyperglycemic and euglycemic hyperinsulinemic (5 mU insulin/kg/min) clamp techniques. The T-treated rats showed insulin resistance with both techniques, which was overcome with time and increasing insulin concentrations during the clamp measurements. The T-treated rats were also heavier and had increased relative weights of skeletal muscles and the spleen. Parametrial, retroperitoneal, and inguinal adipose tissues decreased in weight while mesenteric adipose tissue tended to increase, resulting in an approximately 30-50% larger mesenteric than other adipose tissues. It is concluded that neonatal T imprinting of female rats is followed by insulin resistance, changes in adipose tissue distribution, and an enlarged lean mass, without elevation of circulating T. Similar changes are seen in adult female rats or women receiving T.

Interstitial muscle insulin and glucose levels in normal and insulin-resistant Zucker rats

To study interstitial insulin and glucose concentrations, microdialysis was performed in the medial femoral muscles in normal SD rats as well as in insulin-resistant obese Zucker rats during a euglycemic insulin clamp. [14C]inulin was given (0.1 mCi/rat) as a constant subcutaneous infusion 24 h before the insulin clamp. Insulin infusion rates were 5-8 mU x kg(-1) x min(-1) (low rate) for 140 min and 10-20 mU x kg(-1) x min(-1) (high rate) for another 100 min. The relationship between insulin and [14C]inulin dialysate recoveries was evaluated in vivo and in vitro in plasma to calculate interstitial insulin concentration. Relative microdialysis recovery of interstitial insulin in vivo was 3.0 +/- 0.3% (mean +/- SE, n = 68). In normal SD rats, plasma and interstitial insulin concentrations were identical when plasma insulin was < or =250 mU/ml, whereas interstitial insulin was lower when plasma insulin was > or =350 mU/ml. Half-maximal glucose infusion rate was achieved in the presence of plasma and interstitial insulin concentrations of approximately 140 mU/ml, whereas maximal glucose disposal was seen at interstitial insulin concentrations of approximately 325 mU/ml, corresponding to approximately 500 mU/ml in plasma. In electrically stimulated and contracting (1 Hz) normal muscle with markedly increased blood flow, the dialysate insulin concentration was significantly higher at high rates, but not at low rates, of insulin infusion. In insulin-resistant obese Zucker rats, the interstitial insulin concentration was similar to that in plasma, even at pharmacological concentrations. The glucose infusion rate was significantly lower in the obese Zucker rats at both insulin infusion rates than in the lean animals. The glucose content in dialysates from skeletal muscle was equal in both obese and lean rats during the low insulin infusion rate. During the high insulin infusion rate, dialysate glucose concentrations decreased significantly in both groups but were significantly higher in the obese Zucker rats. The data suggest that transport of insulin and glucose diffusion across the capillary wall are rate limiting for insulin as well as for glucose metabolism in muscle in normal rats. This does not appear to be the case in the insulin-resistant obese Zucker rats, where the reduced insulin responsiveness in muscle is due to muscular cellular defects rather than an inhibited transcapillary delivery of insulin.

Growth hormone improves cardiac function in rats with experimental myocardial infarction

Accumulating evidence suggests from experimental and clinical studies beneficial effects of growth hormone (GH) on contractility, although concomitant cardiac hypertrophy, generally considered to be a cardiovascular risk factor, has also been reported. In the present study, we combine a rat model with impaired cardiac performance after myocardial infarction (MI) with echocardiographic evaluation of GH effects on cardiac structure and function. We have used a rat model with ligation of the left coronary artery in normal, growing male rats resulting in subsequent impaired cardiac performance. After 6 weeks' recovery, blind transthoracic echocardiography was performed to determine infarction size, cardiac geometry and performance. Rats with no signs of myocardial infarction were excluded from the study. After randomization, the rats were treated with daily s.c. injections of saline (n = 8) or recombinant human growth hormone (rhGH) (n = 6) at a dose of approximately 1 mg kg-1 body weight for 1 week. A new blind echocardiography examination was performed after treatment demonstrating a 13% increase in ejection fraction (EF) and a 50% increase in cardiac index in GH-treated rats compared with control rats (P < 0.01). Moreover, GH caused a significant decrease in end-systolic volume. There were no significant changes in left ventricular (LV) or interventricular wall thickness, LV dimensions, heart rate or diastolic function. No effects were seen on LV weight, cardiac insulin-like growth factor (IGF) I, IGF-I receptor and GH receptor mRNA content. GH in a physiological dose improves systolic function in an experimental model of heart failure without signs of hypertrophy, suggesting a potential role as a therapeutic agent in the treatment of heart failure and merits further investigation.

Measurement of interstitial muscle glucose and lactate concentrations during an oral glucose tolerance test

To study the relationship between blood flow rate and muscle metabolism, muscle microdialysis was performed in nine human subjects (5 females and 4 males) after an oral glucose load (75 g). Two microdialysis probes were inserted into the medial femoral muscle for estimation of glucose and lactate concentrations in the interstitial fluid, and the muscle blood flow was measured concomitantly with strain-gauge plethysmography. After subjects fasted overnight, their glucose concentration in arterial plasma and interstitial fluid was 4.6 +/- 0.13 vs. 3.8 +/- 0.23 mmol/l (P < 0.05), and the corresponding lactate concentrations were 0.60 +/- 0.07 vs. 0.83 +/- 0.07 mmol/l (P < 0.05). Muscle blood flow was 5.2 +/- 0.7 and 7.5 +/- 1.4 ml.100 g-1.min-1 (P < 0.05) at 0 and 90 min after oral glucose, respectively. The arterial-interstitial concentration differences of glucose increased after oral glucose [at 0 min 0.73 +/- 0.24 vs. 2.19 +/- 0.60 mmol/l at 90 min (P < 0.001)]. The corresponding values for lactate were -0.23 +/- 0.10 at 0 min vs.-0.26 +/- 0.18 mmol/l at 90 min (not significant). The data show that 1) the capillary wall is partly rate limiting for glucose uptake, and 2) after oral glucose, the glucose concentration gradient over the capillary wall increases despite a limited increase in blood flow rate, which then mediates approximately 10-20% of total enhancement of glucose uptake in muscle.

The effects of hyperinsulinaemia on myocardial mass, blood pressure regulation and central haemodynamics in rats

Left ventricular hypertrophy is a condition with high mortality. An association with insulin resistance and hyperinsulinaemia has recently been suggested. The aim of this study was to examine the effects of isolated hyperinsulinaemia on cardiac weight and haemodynamic regulation. Rats were exposed to hyperinsulinaemia for 7 weeks after adrenalectomy with corticosterone substitution and continuous infusion of propranolol to control counter-regulatory mechanism (n = 15) (AIP group). Hypoglycaemia was prevented by glucose in the drinking water. Hyperinsulinaemic (AIP) rats were heavier and had increased relative masses of the myocardium (left ventricle 17% and right ventricle 20%), kidneys and adipose tissues in comparison with normoinsulinaemic adrenalectomized, corticosterone- and propranolol-treated controls (AP) (n = 10). Blood pressure in the insulin-exposed animals, measured weekly by the tail-cuff method in conscious rats, was not different from (AP) controls over 5 weeks, but increased in the sixth week. At the end of the seventh experimental week, blood pressure measured intra-arterially was also found to be elevated. Heart rate was not changed but total peripheral resistance was about twice that of controls (P < 0.001). Cardiac output and stroke volume was 30-40% lower in the AIP rats (P < 0.05). It is concluded that exposure to elevated insulin levels with control of counter-regulating mechanisms from beta-adrenergic mechanisms and adrenals is not immediately followed by blood pressure elevation. It is, therefore, suggested that early onset of blood pressure elevation after insulin exposure might be caused by insulin counter-regulatory events, causing both insulin resistance and blood pressure elevation. The long-term adaptations may involve a direct influence by insulin as a 'trophic factor' on myocardial and on peripheral resistance levels, followed by increased blood pressure, decreased cardiac and stroke volume.

Physical fitness, muscle morphology, and insulin-stimulated limb blood flow in normal subjects

The response of limb blood flow to insulin is highly variable even in normal subjects. We examined whether physical fitness or differences in muscle morphology contribute to this variation. Maximal aerobic power, muscle fiber composition and capillarization, and the response of forearm glucose extraction and blood flow to a sequential hyperinsulinemic euglycemic clamp (serum insulin 374 +/- 10, 816 +/- 23, and 2,768 +/- 78 pmol/l) were determined in 16 normal males (age 25 +/- 1 yr, body mass index 24 +/- 1 kg/m2). Maximal aerobic power correlated positively with the proportion of type I fibers (r = 0.67, P < 0.01) and negatively with the proportion of type IIb fibers (r = -0.73, P < 0.01). Fiber composition but not blood flow correlated significantly with forearm and whole body glucose uptake. All doses of insulin significantly increased forearm blood flow, maximally by 123 +/- 21%. The ratio of capillaries per fiber was significantly correlated with basal and insulin-stimulated blood flow (0.58- 0.76, P < 0.05-0.01). Mean arterial blood pressure and the insulin-induced increase in blood flow were inversely correlated (r = -0.59, P < 0.05). We conclude that variation in glucose extraction is significantly determined by muscle fiber composition, whereas variation in insulin-stimulated blood flow is closely associated with muscle capillarization.

Mechanisms behind insulin resistance in rat skeletal muscle after oophorectomy and additional testosterone treatment

The absence of female sex hormones, as well as testosterone treatment of oophorectomized (OVX) female rats has been demonstrated to result in decreased whole-body insulin-mediated glucose uptake. The cellular mechanism behind this insulin resistance and the role of low levels of female sex hormones as a risk factor for development of peripheral insulin resistance are not yet fully clarified. We assessed the protein expression of GLUT4 and glycogen synthase, as well as insulin-induced translocation of GLUT4 to the plasma membrane, in soleus skeletal muscle from control rats, OVX rats, and OVX rats treated for 8 weeks with testosterone (OVX + T). Whole-body insulin-mediated glucose uptake assessed by the hyperinsulinemic-euglycemic clamp procedure was 25% lower in OVX rats (P < 0.001) and addition of testosterone treatment further decreased insulin-mediated glucose uptake in OVX + T rats by 48% (P < 0.001) compared with controls. GLUT4 protein expression in soleus muscles was unaltered in the OVX and OVX + T rats compared with controls. Insulin induced a 3.7-fold increase (P < 0.05) in the plasma membrane content of GLUT4 in soleus muscle from control rats, whereas plasma membrane content of GLUT4 in soleus muscle from OVX or OVX + T rats was unaltered in response to insulin. Glycogen synthase protein expression in muscle homogenates was decreased by 25% in the OVX group (P < 0.05) and by 37% in the OVX + T group (P < 0.05) when compared with the control group. Insulin receptor and tyrosine kinase activities in the basal and insulin-stimulated states did not differ between the OVX and OVX + T rats. In conclusion, the absence of female sex hormones appears to decrease insulin-mediated whole-body glucose uptake via an impaired insulin-stimulated translocation of GLUT4 to the plasma membrane and by decreased protein expression of glycogen synthase. Testosterone treatment further impairs whole-body insulin-mediated glucose uptake, presumably by additional impairment of glycogen synthase expression.

Extracellular distribution of insulin in muscle of rats exposed to long-term hyperinsulinaemia

The importance of increased capillary density for the regulation of insulin sensitivity by transcapillary delivery of insulin to muscle cells in insulin-exposed rats was investigated by direct microdialysis measurements of interstitial (125I] insulin concentrations in the femoral muscle during an euglycaemic hyperinsulinaemic clamp. In insulin-exposed rats plasma insulin was approximately 25% (P < 0.05) higher than that in control animals during the first 100 min and reached their maximal concentrations after 100 min. After a nitroprusside infusion given at 100 min both groups had similar concentrations of insulin in plasma as well as in muscle interstitial fluid. However, mean glucose infusion rate during the first clamp hour was 20.5 +/- 2.3 and 12.6 +/- 5.2 mg kg-1 min-1 (P < 0.05) in insulin-exposed and control animals, respectively. During the second clamp hour the corresponding figures were 21.1 +/- 2.4 and 13.9 +/- 2.6 (P < 0.05). It may be concluded that capillarization and/or nitroprusside affected plasma insulin concentrations without altering either the interstitial insulin levels or the insulin effect on glucose consumption. The data suggest that the elevated insulin sensitivity after chronic insulin exposure is dependent on other than transcapillary transport events and demonstrate the different kinetics for insulin distribution in plasma and in the interstitial fluid.

Rapid formation of capillary endothelial cells in rat skeletal muscle after exposure to insulin

Research has suggested a role for insulin delivery through capillaries in muscle in the regulation of insulin sensitivity. Therefore, the formation and turn-over of capillary endothelial cells in muscle were studied in relation to exposure to moderately elevated insulin concentrations with or without concomitant increase of corticosterone concentrations. Female rats were exposed to a moderate, physiological hyperinsulinaemia (approximately 450 pmol/l) for 24 h 48 h, 3 days, 7 days and 7 weeks. Propranolol was used to inhibit elevated adrenergic activity. In one insulin-exposed group, corticosterone secretion was controlled by adrenalectomy with substitution of corticosterone to maintain normal concentrations, while another group was left with adrenal corticosterone secretion intact. Rats were exposed to insulin with controlled, non-elevated corticosterone concentrations after adrenalectomy and corticosterone substitution; compared to controls, the number of mitoses in capillary endothelial cells in the soleus and extensor digitorum longus muscle were approximately doubled after 24 h, reaching a maximum, about fivefold higher than controls, after 3 days. After 7 weeks of insulin exposure there were no longer any significant differences between control and insulin-exposed rats. The number of capillaries per unit muscle surface area was moderately (10-15%) but significantly increased at 7 days (only the extensor digitorum longus muscle) and 7 weeks (the extensor digitorum longus and the soleus muscles). In rats exposed to insulin, with intact adrenals, endogenous corticosterone production resulted in concentrations about threefold higher than in rats adrenalectomized with subsequent corticosterone substitution. In these rats the increase in mitoses in capillary endothelium was totally abolished. The results of this study suggest that exposure to insulin in this rat model is followed by a dramatic short-term increase in the formation of new capillary endothelial cells in muscle. It is also suggested that this growth factor-like effect of insulin is abolished by corticosterone. It is suggested that insulin and corticosterone exert opposite effects on the capillary network in muscles, which might be important for the insulin supply to this tissue, and hence for regulation of insulin sensitivity.

The effects of long-term hyperinsulinaemia on insulin sensitivity in rats

The effects of long-term exposure (7 wk) to hyperinsulinaemia on insulin sensitivity were studied in female rats. The rats were made hyperinsulinaemic by implantation of osmotic minipumps that were changed once a week. Elevated adrenergic activity and secretion of glucocorticoids were controlled by another minipump with propranolol and adrenalectomy with corticosterone substitution, respectively. This resulted in hyperinsulinaemia and moderate hypoglycaemia, the latter probably counteracted by overeating and increased glucagon secretion, as indicated by increased body weight and lower liver glycogen contents, respectively. Euglycaemic, hyperinsulinaemic clamp measurements showed a significantly higher glucose disposal rate (P < 0.05) in the hyperinsulinaemic rats 18.8 +/- 1.1 mg kg-1 min-1 compared with the control groups 14.6 +/- 0.4 and 15.4 +/- 0.9 mg kg-1 min-1. Insulin stimulation of 2-deoxyglucose as well as glycogen synthesis was measured in the extensor digitorum longus muscle, the red and white part of the gastrocnemius, the soleus muscle, the liver and in parametrial, retroperitoneal, and inguinal adipose tissue. No differences were found between the groups in the insulin response of the 2-deoxyglucose uptake. Glycogen synthesis was significantly elevated in all muscles in the insulin treated compared with the control rats but no differences were found in the liver. Capillary density was significantly elevated per unit muscle surface area in the soleus and extensor digitorum longus muscles of the insulin-exposed rats.(ABSTRACT TRUNCATED AT 250 WORDS)

The effects of oestrogen and progesterone on insulin sensitivity in female rats

The effects of 17-beta-oestradiol (E2) and progesterone (P) on insulin sensitivity were determined in oophorectomized (OVX) rats by the euglycaemic hyperinsulinaemic clamp technique combined with measurements of insulin-stimulated 2-deoxy-D-glucose (2-DOG) transport and glycogen synthesis in white and red parts of the gastrocnemius, the extensor digitorum longus and soleus muscles as well as in the liver (only glycogen synthesis). OVX was followed by insulin resistance in the clamp measurements. This was paralleled by a decreased insulin-stimulated content of 2-DOG in muscles, an index of glucose transport. Glycogen synthesis in muscle was also decreased, although to less extent. E2, alone or in combination with P, restored this to values of intact controls, while P alone was followed by insulin resistance. Liver glycogen synthesis was also decreased by OVX but this required combination of E2 and P to be fully restored. It was concluded that particularly E2 plays an important role in the maintenance of normal insulin sensitivity while P alone seems to be followed by insulin resistance, both effects apparently mainly by regulation of glucose uptake in muscle. E2 + P may be of importance for maintenance of normal glycogen synthesis in the liver.

Effects of hyperinsulinemia on muscle fiber composition and capitalization in rats

Statistical studies repeatedly have shown an association between systemic insulin resistance and a preponderance of highly glycolytic, relatively insulin-insensitive muscle fibers as well as a low density of muscle capillaries. The nature of the relationship between these observations is, however, not clear. Female rats were made hyperinsulinemic for 7 days by implantation of osmotic minipumps. Elevated adrenergic activity and secretion of glucocorticoids were controlled by another minipump with propranolol and adrenalectomy was controlled with glucocorticoid substitution. This resulted in hyperinsulinemia and moderate hypoglycemia, the latter probably counteracted by overeating and increased glucagon secretion, as indicated by increased body weight and lower liver glycogen contents, respectively. Systemic insulin sensitivity was increased and measured with a hyperinsulinemic-euglycemic clamp technique. This was paralleled by an elevated glucose utilization estimated as uptake of 2-deoxyglucose in parametrial, retroperitoneal, and inguinal adipose tissues and the soleus and extensor digitorum longus muscles. Glycogen synthesis was also elevated in the soleus muscle. Muscle fiber composition changed with hyperinsulinemia and elevated 2-deoxyglucose uptake toward more fast-twitch, type II, particularly type IIb fibers, whereas the proportion of slow-twitch, type I fibers, diminished. Capillary density was elevated per unit muscle surface area as well as per muscle fiber. This was paralleled by increased insulin sensitivity systemically and in muscles. These results suggest that muscle fiber composition alterations may be a consequence rather than a cause of hyperinsulinemia and that capillarization rather than fiber composition is of importance for insulin sensitivity in muscle.

The effects of testosterone on insulin sensitivity in male rats

In order to examine the effects of testosterone (T) on insulin sensitivity, male rats were castrated or sham-operated, and exposed to low or high doses of T to substitute normal or to produce high serum T concentrations. Insulin sensitivity was followed by euglycaemic, hyperinsulinaemic glucose clamp measurements. An index of insulin-stimulated glucose transport was obtained in the white gastrocnemius (WG), extensor digitorum longus (EDL), red gastrocnemius (RG) and soleus (SOL) muscles after a bolus dose of [2-3H]deoxyglucose (2-DOG) when steady state was obtained in the clamp measurements. Glycogen synthesis was followed similarly with [U-14C]glucose as a labelled precursor after isolation of glycogen in the muscles mentioned, and in the liver. Castration and high T were followed by a marked insulin resistance in the clamp measurements. This was paralleled by a diminished insulin stimulation of glucose incorporation into glycogen down to about 50% of control values, apparently equally pronounced in all muscles but not found in liver glycogen synthesis. 2-DOG uptake was diminished by castration in the WG and RG muscles but was unaffected by high doses of T. Substitution of castrated rats with a low dose of T, restoring their serum T concentrations to the normal range, completely abolished these perturbations of insulin sensitivity. It is concluded that T is an important regulator of muscular insulin sensitivity, which seems to be highest in a 'window' of normal serum T concentrations.

Tissue uptake of insulin and inulin in red and white skeletal muscle in vivo

By employing a tissue uptake technique, the kinetics of the plasma-interstitial equilibration of radiolabeled insulin, inulin, and albumin were followed in four muscles of differing capillarity in anesthetized rats. The soleus muscle (SOL), and the red portion of the gastrocnemius muscle (RG), as well as the extensor digitorum longus muscle (EDL) and the white portion of the gastrocnemius muscle (WG) were investigated. After constant intravenous tracer infusions and repeated plasma sampling under euglycemic clamp conditions, animals were killed at varying time intervals and the muscles mentioned above were dissected out. The radioactivity of tracer per gram of tissue in each muscle divided by the plasma activity of tracer per milliliter of plasma, i.e., "the plasma equivalent space" of tracer, thus could be followed as a function of time. From this function the permeability-surface area (PS) of inulin as well as the distribution volumes at time 0 (V0) of inulin and insulin and their equilibrium distribution volumes (VE) were assessed. The PS for inulin (in ml.min-1.100 g muscle-1) was 0.52 +/- 0.10 (mean +/- SE) in WG, increasing with more red fibers to 1.37 +/- 0.18 in SOL. Also the inulin interstitial distribution volume (at blood-tissue tracer equilibrium; VE) increased in this order (in ml/100 g) from 7.30 +/- 0.91 in WG to 12.93 +/- 0.89 in SOL. The V0 for insulin was found to be approximately fivefold larger than the plasma volume in each muscle sample, indicating a high degree of binding of insulin to structures within the vascular compartment, conceivably to the vascular endothelium.(ABSTRACT TRUNCATED AT 250 WORDS)

Effects of short-term testosterone exposure on insulin sensitivity of muscles in female rats

The effects of testosterone on insulin sensitivity were studied in oophorectomized female rats. Euglycemic, hyperinsulinemic clamp measurement showed a marked decrease of insulin sensitivity after 48 but not 24 h of testosterone exposure, which was overcome at high insulin concentrations. Insulin stimulation of 2-deoxyglucose uptake as well as glycogen synthesis was measured in the white and red parts of the gastrocnemius, the extensor digitorum longus, and soleus muscles as well as in the liver (only glycogen synthesis). After 24 h of treatment, inhibition of both 2-deoxyglucose uptake and glycogen synthesis was found in the most insulin-sensitive muscles. After 48 h of insulin stimulation, glycogen synthesis was inhibited in all examined individual muscles (white and red parts of gastrocnemius, extensor digitorum longus, and soleus) as was the activity of the insulin-sensitive part of glycogen synthase in muscle. Inhibition of insulin-stimulated 2-deoxyglucose uptake again affected the most insulin-sensitive muscles. There was a slight but significant change of muscle fiber composition toward less long-chain myosin and more short-chain myosin-containing fibers. Serum cortisol, plasma free fatty acids, and blood glycerol did not change. It is concluded that testosterone administration in moderate doses to oophorectomized female rats is followed by a rapid deterioration of insulin sensitivity in muscle, mediated mainly by perturbations of the insulin receptor-glycogen synthesis systems apparently coinciding with changes in muscle morphology.

The effects of cortisol on insulin sensitivity in muscle

The effects of cortisol on insulin sensitivity were examined in rats with the euglycaemic, hyperinsulinaemic clamp technique. Uptake of 2-deoxyglucose and incorporation of glucose into glycogen was followed in the white gastrocnemius, extensor digitorum longus, red gastrocnemius and soleus muscles as well as the liver (only glycogen synthesis). Maximal velocity and fractional velocity of the insulin-sensitive part of glycogen synthase (FV %) was measured in the muscles, as well as muscle fibre composition and capillary density. After 24 h exposure to cortisol, insulin sensitivity was diminished in the clamp measurements. This was paralleled by a decrease in glycogen synthesis in the most insulin-sensitive red gastrocnemius and Soleus muscles, but not in the white gastrocnemius or extensor digitorum longus muscles or the liver, and no effect was seen on 2-deoxyglucose uptake in muscles. FV % was markedly inhibited in all muscles. After 48 h exposure to cortisol, glycogen synthesis was markedly inhibited in all muscles, and 2-deoxyglucose uptake in all except the least insulin-sensitive muscle, WG. No changes in muscle morphology were found. These results suggest that the insulin resistance caused by cortisol is elicited in a stepwise manner, starting with an inhibition in the glycogen synthesis system in insulin-sensitive muscles, later including all muscles as well as 2-deoxyglucose uptake. This occurs without changes in morphology.

Effects of testosterone on muscle insulin sensitivity and morphology in female rats

Intact or oophorectomized (OVX) female rats were given moderate doses of testosterone for 12 wk. Insulin-stimulated glucose transport with submaximal insulin concentrations was studied with the euglycemic clamp technique. Glycogen synthesis and 2-deoxy-D-glucose uptake were measured during the clamp in the extensor digitorum longus, white and red portions of the gastrocnemius, and in the soleus muscles by tracer technique. Testosterone treatment resulted in elevations of circulating testosterone, increased plasma insulin concentrations, and a marked decrease in insulin-stimulated glucose transport. In control animals, glycogen synthesis and 2-deoxy-D-glucose transport increased with increasing concentrations of type 1 fibers. Testosterone inhibited glycogen synthesis and 2-deoxy-D-glucose transport to approximately 50% in all muscles except 2-deoxy-D-glucose transport in intact rats. Glycogen synthesis in the liver was not affected. Testosterone administration also resulted in changes in muscle morphology. The relative number of type 1 fibers decreased, whereas type 2 fibers increased. This was most pronounced in red muscles. There was also a decrease in capillary density after testosterone treatment. It was concluded that testosterone administered to female rats is followed by marked insulin resistance. This is correlated to alterations in muscle morphology with fewer type 1 fibers and a lower degree of capillarization, which are both known to be characteristics of insulin-insensitive muscles.

Testosterone treatment of ovariectomized rats: effects on lipolysis regulation in adipocytes

The effects of testosterone treatment (2 mg every 14 days, for three months) on adipocyte lipolysis of intact (250-300 g body weight) and ovariectomized female rats were studied. Testosterone treatment of intact rats had no effect. Ovariectomy was followed by an increase in fat cell size and a decrease of lipolysis stimulated by isoproterenol, norepinephrine, epinephrine, forskolin, cAMP and isobutylmethylxantine. The number of beta-adrenergic receptors was reduced. There was, however, no change in the antilipolytic effects of UK 14,304 (alpha 2-adrenergic agonist), nicotinic acid, N6-phenylisopropyladenosine or insulin. Testosterone treatment of ovariectomized rats restored the number of beta-adrenoceptors and lipolysis stimulated by cAMP and isobutylmethylxantine, but not lipolysis stimulated by catecholamines and forskolin, suggesting a remaining defect in the catalytic unit of adenylate cyclase. These results indicate that ovariectomy is followed by a profound derangement of the lipolytic pathway at several levels, from beta-adrenoceptors number to the triglyceride lipase activity. This is partially restored by treatment with testosterone, which, however, has no effect on intact female rats. This study emphasizes the importance of ovarian integrity for the lipolytic regulation and the inability of testosterone to replace ovarian function in this regard or to affect lipolysis in intact female rats.