Physiological Levels Of Insulin Research Articles

Vascular insulin response is preserved in non‐diabetic patients with coronary artery disease, despite endothelial dysfunction

Background—Patients with coronary artery disease (CAD) are often insulin resistant, a state that predisposes to increased atherosclerosis. Recently, it was suggested that a “vascular insulin resistance” could explain this association, causing endothelial dysfunction and hence atherosclerosis. We therefore studied the vascular insulin response in patients with CAD.Materials and methods—Nine non‐diabetic patients with documented CAD and 31 lean healthy controls were examined. Forearm blood flow was measured by venous occlusion plethysmography. Dose–response studies of acetylcholine (ACh) and sodium nitroprusside (SNP) elicited endothelium‐dependent and ‐independent vasodilation and were repeated during intra‐arterial insulin infusion.Results—Patients were insulin resistant as determined by HOMA index. Insulin infusion resulted in high physiological levels of insulin in the forearm without systemic effects. Patients had a reduced ACh response but insulin infusion increased the ACh response equally in patients and controls (a mean increase of 74 ± 37 vs 57 ± 24%, patients vs controls, p = 0.12). A minor increase of the SNP response was also noted during insulin infusion.Conclusion—Vascular insulin response is intact in non‐diabetic CAD patients in spite of insulin resistance and endothelial dysfunction.

Evidence for lymphatic transport of insulin by topically applied biphasic vesicles.

The cutaneous delivery pathway through the lymphatics of a novel transdermal lipid-based delivery system (biphasic vesicles), which was previously shown to deliver sustained physiological levels of basal insulin in a pain-free manner across the skin, was evaluated in a diabetic rat model. Transdermal patches (one per rat) containing insulin in biphasic vesicles (1-10 mg recombinant human insulin dose) were applied to the shaved abdominal skin of streptozotocin-induced diabetic rats for 73 h. Blood glucose was monitored approximately every 2-10 h using a Lifescan glucose meter. Inguinal lymph node insulin levels were analysed by ELISA. Insulin in the lymph nodes increased in a dose- and time-dependent manner. Maximal transdermal insulin concentrations in the lymph nodes were observed with both 140 IU (5 mg: 43.0 +/- 18.0 microIU mg(-1) (mean +/- s.e.m., n = 4)) and 280 IU (10 mg: 48.0 +/- 19.6 microIU mg(-1) (mean +/- s.e.m., n = 4)) doses of recombinant insulin at t = 73 h. The level of insulin in the lymph nodes after subcutaneous injection of 1 mg insulin at the peak blood glucose response was 35.8 microIU mg(-1) (n = 2), before falling to 0.35 microIU mg(-1) by t = 48 h (n = 2). The lymphatics is involved in the transdermal insulin delivery by biphasic vesicles. This is the first report on the lymphatic transport of a protein after non-invasive topical application on the skin.

Growth hormone, but not insulin, activates STAT5 proteins in adipocytes in vitro and in vivo

STAT 5 proteins are latent transcription factors which have been shown to be activated by growth hormone (GH) in many cell types. However, some recent studies also suggest that STAT 5B is a physiological substrate of the insulin receptor. In our studies, we have shown that physiological levels of insulin do not induce STAT 5 tyrosine phosphorylation or affect the nuclear distribution of STATs 5A or 5B in 3T3-L1 adipocytes. Moreover, we did not observe the activation of STAT 5 in the adipose tissue or skeletal muscle of mice following an acute intraperitoneal injection of insulin. However, acute GH administration, both in vitro and in vivo, resulted in the activation of STAT 5 proteins. In summary, our results indicate that STAT 5 proteins are not activated by physiological levels of insulin in adipose tissue.

Interleukin-6 induces cellular insulin resistance in hepatocytes.

Interleukin (IL)-6 is one of several proinflammatory cytokines that have been associated with insulin resistance and type 2 diabetes. A two- to threefold elevation of circulating IL-6 has been observed in these conditions. Nonetheless, little evidence supports a direct role for IL-6 in mediating insulin resistance. Here, we present data that IL-6 can inhibit insulin receptor (IR) signal transduction and insulin action in both primary mouse hepatocytes and the human hepatocarcinoma cell line, HepG2. This inhibition depends on duration of IL-6 exposure, with a maximum effect at 1-1.5 h of pretreatment with IL-6 in both HepG2 cells and primary hepatocytes. The IL-6 effect is characterized by a decreased tyrosine phosphorylation of IR substrate (IRS)-1 and decreased association of the p85 subunit of phosphatidylinositol 3-kinase with IRS-1 in response to physiologic insulin levels. In addition, insulin-dependent activation of Akt, important in mediating insulin's downstream metabolic actions, is markedly inhibited by IL-6 treatment. Finally, a 1.5-h preincubation of primary hepatocytes with IL-6 inhibits insulin-induced glycogen synthesis by 75%. These data suggest that IL-6 plays a direct role in insulin resistance at the cellular level in both primary hepatocytes and HepG2 cell lines and may contribute to insulin resistance and type 2 diabetes.

Metabolic flux analysis of cultured hepatocytes exposed to plasma.

Hepatic metabolism can be investigated using metabolic flux analysis (MFA), which provides a comprehensive overview of the intracellular metabolic flux distribution. The characterization of intermediary metabolism in hepatocytes is important for all biotechnological applications involving liver cells, including the development of bioartificial liver (BAL) devices. During BAL operation, hepatocytes are exposed to plasma or blood from the patient, at which time they are prone to accumulate intracellular lipids and exhibit poor liver-specific functions. In a prior study, we found that preconditioning the primary rat hepatocytes in culture medium containing physiological levels of insulin, as opposed to the typical supraphysiological levels found in standard hepatocyte culture media, reduced lipid accumulation during subsequent plasma exposure. Furthermore, supplementing the plasma with amino acids restored hepatospecific functions. In the current study, we used MFA to quantify the changes in intracellular pathway fluxes of primary rat hepatocytes in response to low-insulin preconditioning and amino acid supplementation. We found that culturing hepatocytes in medium containing lower physiological levels of insulin decreased the clearance of glucose and glycerol with a concomitant decrease in glycolysis. These findings are consistent with the general notion that low insulin, especially in the presence of high glucagon levels, downregulates glycolysis in favor of gluconeogenesis in hepatocytes. The MFA model shows that, during subsequent plasma exposure, low-insulin preconditioning upregulated gluconeogenesis, with lactate as the primary precursor in unsupplemented plasma, with a greater contribution from deaminated amino acids in amino acid-supplemented plasma. Concomitantly, low-insulin preconditioning increased fatty acid oxidation, an effect that was further enhanced by amino acid supplementation to the plasma. The increase in fatty acid oxidation reduced intracellular triglyceride accumulation. Overall, these findings are consistent with the notion that the insulin level in medium culture presets the metabolic machinery of hepatocytes such that it directly impacts on their metabolic behavior during subsequent plasma culture.

Insulin and amino acids independently stimulate skeletal muscle protein synthesis in neonatal pigs.

Infusion of physiological levels of insulin and/or amino acids reproduces the feeding-induced stimulation of muscle protein synthesis in neonates. To determine whether insulin and amino acids independently stimulate skeletal muscle protein synthesis in neonates, insulin secretion was blocked with somatostatin in fasted 7-day-old pigs (n = 8-12/group) while glucose and glucagon were maintained at fasting levels and insulin was infused to simulate either less than fasting, fasting, intermediate, or fed insulin levels. At each dose of insulin, amino acids were clamped at either the fasting or fed level; at the highest insulin dose, amino acids were also reduced to less than fasting levels. Skeletal muscle protein synthesis was measured using a flooding dose of l-[4-(3)H]phenylalanine. Hyperinsulinemia increased protein synthesis in skeletal muscle during hypoaminoacidemia and euaminoacidemia. Hyperaminoacidemia increased muscle protein synthesis during hypoinsulinemia and euinsulinemia. There was a dose-response effect of both insulin and amino acids on muscle protein synthesis. At each insulin dose, hyperaminoacidemia increased muscle protein synthesis. The effects of insulin and amino acids on muscle protein synthesis were largely additive until maximal rates of protein synthesis were achieved. Amino acids enhanced basal protein synthesis rates but did not enhance the sensitivity or responsiveness of muscle protein synthesis to insulin. The results suggest that insulin and amino acids independently stimulate protein synthesis in skeletal muscle of the neonate.

Transdermal delivery of insulin from a novel biphasic lipid system in diabetic rats.

Noninvasive transdermal insulin delivery could provide diabetic patients with sustained physiological levels of basal insulin in a pain-free manner. We have developed a novel transdermal lipid-based system (Biphasix) suitable for macromolecule delivery across the skin. The objective of this study was to evaluate the pharmacological effects of the Biphasix-insulin delivery system in a diabetic rat model. Transdermal patches (one per animal) containing Biphasix-insulin formulation (10 mg of recombinant human insulin dose) were applied to the shaved abdominal skin of streptozotocin-induced diabetic rats for 48 h. Blood glucose was monitored every 2-4 h using a Lifescan glucose meter. Serum insulin levels were analysed by enzyme-linked immunosorbent assay. A decrease in blood glucose of 43.7 +/- 3.8% (mean +/- SEM, n = 25) was observed compared with initial blood glucose levels. The duration of the response was 51.5 +/- 3.7 h (mean +/- SEM, n = 25). Serum insulin after application of the transdermal Biphasix-insulin patch was 20.08 +/- 5.44 micro IU/mL (mean +/- SEM, n = 13) during the steady state, which was not statistically different from the insulin levels obtained 2 h after subcutaneous injection of 1 mg of recombinant human insulin solution. Insulin bioavailability from the transdermal Biphasix-insulin patches was 21.5 +/- 6.9% (mean +/- SEM, n = 13) based on serum insulin and 39.5 +/- 8.5% (mean +/- SEM, n = 25) based on the pharmacodynamic blood glucose-lowering effects. The Biphasix system successfully delivered insulin transdermally, as evidenced by a significant sustained decrease in blood glucose in diabetic rats, with a corresponding increase in serum insulin. These results support the feasibility of developing a transdermal insulin patch for human applications.

Metabolic pre‐conditioning of cultured cells in physiological levels of insulin: Generating resistance to the lipid‐accumulating effects of plasma in hepatocytes

Metabolic pre‐conditioning of cultured cells in physiological levels of insulin: Generating resistance to the lipid‐accumulating effects of plasma in hepatocytes

Insulin signaling in the transcriptional and posttranscriptional regulation of CYP2E1 expression

Diabetes has been reported to increase the expression of cytochrome P450 (CYP) 2E1 messenger RNA (mRNA) and protein several-fold, and enhanced expression has been associated with elevated ketone bodies. Primary cultured rat hepatocytes were used to explore ketone body and insulin regulation of CYP2E1 expression. Hydroxybutyrate and acetoacetate (AC), alone or in combination, either failed to affect or decreased CYP2E1 mRNA levels by up to 90% relative to untreated hepatocytes. Insulin produced a concentration-dependent decrease in CYP2E1 mRNA levels, and insulin receptor immunoprecipitation showed a correspondence between receptor phosphorylation and the decrease in CYP2E1 mRNA levels at physiologic levels of insulin. Phosphatase inhibitors decreased CYP2E1 mRNA levels by greater than 95%. The phosphatidylinositol 3-kinase (PI3-kinase) inhibitors wortmannin or LY294002 and rapamycin, an inhibitor of p70 S6 kinase phosphorylation, ameliorated the insulin-mediated decrease in CYP2E1 mRNA levels. Geldanamycin, which inhibits Src kinase, also abrogated the insulin-mediated decrease in CYP2E1 mRNA levels. In contrast, the protein kinase C (PKC) inhibitor bisindolylmaleimide, the mitogen-activated protein kinase kinase (MEK) inhibitor PD98059, and the p38 mitogen-activated protein (MAP) kinase inhibitor SB202190 did not affect the insulin-mediated decrease in CYP2E1. CYP2E1 mRNA half-life decreased from ∼48 hours in the absence of insulin to ∼15 hours at 10 nmol/L insulin, and this decrease was prevented by wortmannin. The half-life of CYP2B mRNA was increased by insulin, whereas that of CYP3A was unaffected. Analysis of CYP2E1 gene transcription using heterogeneous nuclear RNA (hnRNA) showed that insulin suppressed CYP2E1 transcription. In conclusion, these data show involvement of transcriptional and posttranscriptional mechanisms in the insulin-mediated regulation of CYP2E1 and implicate PI3-kinase, p70 S6 kinase, and Src kinase in mediating these effects. (HEPATOLOGY .)

Systemic and regional free fatty acid metabolism in type 2 diabetes

To determine whether type 2 diabetes mellitus alters systemic and regional free fatty acid ([3H]palmitate) metabolism, 14 nondiabetic (ND) and 14 type 2 diabetic (D) subjects underwent hyperinsulinemic-hyperglycemic (approximately 9.3 mM) clamps. The subjects were matched for age, body mass index, percent body fat, and fat-free mass. D subjects had more (P < 0.05) visceral fat than ND. During somatostatin, replacement growth hormone, and glucagon infusions, insulin was infused to achieve moderate (approximately 75 pmol/l) and high (approximately 150 pmol/l) physiological insulin levels. D subjects had greater (P < 0.02) systemic and regional (splanchnic and leg) palmitate release than ND subjects during both insulin infusion intervals. The relative contributions of splanchnic, leg, and nonsplanchnic upper body regions to systemic palmitate release did not differ between groups, although the last contributed the most (approximately 75%) to systemic palmitate release. Visceral fat area correlated with systemic palmitate flux (r = 0.45, P < 0.03) during both insulin infusions. We conclude that type 2 diabetes is associated with a generalized impairment in insulin suppression of lipolysis compared with equally obese ND individuals.

Assessment of hepatic insulin action in obese type 2 diabetic patients.

Defects in hepatic insulin action in type 2 diabetes and its possible underlying mechanisms were assessed in euglycemic-hyperinsulinemic clamp studies, using improved tracer methods (constant specific activity technique). Ten obese diabetic patients (age 54 years, BMI 29 +/- 0.5 kg/m(2)) and ten matched control subjects were studied at baseline (after an overnight fast) and during insulin infusions of 20- and 40-mU. m(-2). min(-1). In the diabetic patients, plasma glucose levels were normalized overnight before the studies by low-dose insulin infusion. Hepatic sinusoidal insulin levels were estimated, and plasma levels of free fatty acids (FFAs) and glucagon were determined to assess the direct and indirect effects of insulin on hepatic glucose production (HGP) in type 2 diabetes. Baseline rates of HGP (86 +/- 3 vs. 76 +/- 3 mg. m(-2). min(-1), P < 0.05) were slightly elevated in the diabetic patients compared with control subjects, despite much higher hepatic sinusoidal insulin levels (26 +/- 3 vs. 12 +/- 2 mU/l, P < 0.001). Consequently, a marked defect in the direct (hepatic) effect of insulin on HGP appeared to be present at low insulin levels. However, in response to a small increase in baseline hepatic sinusoidal insulin levels of 11 mU/l (26 +/- 3 to 37 +/- 3 mU/l, P < 0.05) in the 20-mU clamp, a marked suppression of HGP was observed in the diabetic patients (86 +/- 3 to 32 +/- 5 mg. m(-2). min(-1), P < 0.001), despite only minimal changes in FFAs (0.33 +/- 0.05 to 0.25 +/- 0.05 mmol/l, NS) and glucagon (14 +/- 1 to 11 +/- 2 pmol/l, P < 0.05) levels, suggesting that the impairment in the direct effect of insulin can be overcome by a small increase in insulin levels. Compared with control subjects, suppression of HGP in the diabetic patients was slightly impaired in the 20-mU clamp (32 +/- 5 vs. 22 +/- 4 mg. m(-2). min(-1), P < 0.05) but not in the 40-mU clamp (25 +/- 2 vs. 21 +/- 3 mg. m(-2). min(-1), NS). In the 20-mU clamp, hepatic sinusoidal insulin levels in the diabetic patients were comparable with control subjects (37 +/- 3 vs. 36 +/- 3 mU/l, NS), whereas both FFA and glucagon levels were higher (i.e., less suppressed) and correlated with the rates of HGP (R = 0.71, P < 0.02; and R = 0.69, P < 0.05, respectively). Thus, at this insulin level impaired indirect (extrahepatic) effects of insulin seemed to prevail. In conclusion, hepatic insulin resistance is present in obese type 2 diabetic patients but is of quantitative significance only at low physiological insulin levels. Defects in both the direct and the indirect effects of insulin on HGP appear to contribute to this resistance.

Insulin-like effects of a physiologic concentration of carnitine on cardiac metabolism.

Pharmacologic (millimolar) levels of carnitine have been reported to increase myocardial glucose oxidation, but whether physiologically relevant concentrations of carnitine affect cardiac metabolism is not known. We employed the isolated, perfused rat heart to compare the effects of physiologic levels of carnitine (50 microM) and insulin (75 mU/l [0.5 nM]) on the following metabolic processes: (1) glycolysis (release of 3H2O from 5-3H-glucose); (2) oxidation of glucose and pyruvate (production of 14CO2 from U-14C-glucose, 1-14C-glucose, 3,4-14C-glucose, 1-14C-pyruvate, and 2-14C-pyruvate); and (3) oxidation of palmitate (release of 3H2O from 9,10-3H-palmitate). We found that addition of carnitine (50 microM) to a perfusate containing both glucose (10 mM) and palmitate (0.5 mM) stimulated glycolytic flux by 20%, nearly doubled the rate of glucose oxidation, and inhibited palmitate oxidation by 20%. These actions of carnitine were uniformly similar to those of insulin. When carnitine and insulin were administered together, their effects on the oxidation of glucose and palmitate, but not on glycolysis, were additive. When pyruvate (1 mM) was substituted for glucose, neither carnitine nor insulin influenced the rate of oxidation of pyruvate or palmitate. In combination, however, carnitine and insulin sharply suppressed pyruvate oxidation (75%) and doubled the rate of palmitate oxidation. None of the responses to carnitine or insulin was affected by varying the isotopic labeling of glucose or pyruvate. The results show that carnitine, at normal blood levels, exerts insulin-like effects on myocardial fuel utilization. They also suggest that plasma carnitine in vivo may interact with insulin both additively and permissively on the metabolism of carbohydrates and fatty acids.

No effect of insulin on glucose blood-brain barrier transport and cerebral metabolism in humans.

The effect of hyperinsulinemia on glucose blood-brain barrier (BBB) transport and cerebral metabolism (CMRglc) was studied using the intravenous double-indicator method and positron emission tomography using [18F]fluorodeoxyglucose as tracer (PET-FDG). Sixteen normal healthy control subjects (25 +/- 4 years old) were studied twice during a euglycemic and a euglycemic-hyperinsulinemic condition. Our hypothesis was that high physiologic levels of insulin did not affect the BBB transport or net metabolism of glucose. During insulin infusion, arterial plasma insulin levels increased from 48.5 to 499.4 pmol/l. The permeability-surface area products for glucose and FDG BBB transport obtained with the double-indicator method remained constant during hyperinsulinemia. Similarly using PET-FDG, no changes were observed in the unidirectional clearance of FDG from blood to brain. k2* (FDG transport from brain to blood) increased significantly by 15 and 18% (gray and white matter, respectively), and k4* (dephosphorylation of FDG) increased by 18%. The increase in k2* may be caused by insulin inducing a decrease in the available FDG brain pool. The increase in k4* may be related to an increased loss of labeled products during insulin fusion. Irrespective of these changes, CMRglc remained unchanged in all brain regions. We conclude that hyperinsulinemia within the normal physiologic range does not affect BBB glucose transport or net cerebral glucose metabolism.

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.

Furosemide decreases the sensitivity of glucose transport to insulin in skeletal muscle in vitro.

The effects of the diuretic furosemide on the sensitivity of glucose disposal to insulin were investigated in rat soleus muscle in vitro. At basal levels of insulin, the rates of 3-O-methylglucose transport, 2-deoxyglucose phosphorylation and lactate formation were not affected significantly by furosemide (0.5 mmol/l). However, furosemide significantly decreased these rates at physiological and maximal levels of insulin. The contents of 2-deoxyglucose and glucose 6-phosphate in the presence of furosemide were not significantly different from those in control muscles at all levels of insulin studied. It is concluded that furosemide decreases the sensitivity of glucose utilization to insulin in skeletal muscle by directly inhibiting the glucose transport process.

Relationship between insulin's haemodynamic effects and insulin-mediated glucose uptake.

Insulin resistance and hyperinsulinaemia are associated with hypertension. The relationship between insulin's metabolic and haemodynamic actions has not been fully elucidated however. We investigated, using the euglycaemic clamp technique, the relationship between insulin-mediated glucose uptake and insulin-induced changes in leg blood flow and cardiac index in 13 healthy subjects. As insulin's effects on blood flow are time dependent, studies were performed during a 4-h insulin (50 mU kg-1 h-1) infusion period. Mean arterial pressure during insulin infusion increased (82.9 +/- 6.7 to 89.8 +/- 7.7 mmHg; P < 0.001), whereas heart rate was unaltered. Leg blood flow gradually increased from 1.09 +/- 0.57 to 1.47 +/- 0.67 mL min-1 dL-1 during the second hour, and to 1.65 +/- 0.68 mL min-1 dL-1 during the fourth hour of the clamp (P = 0.01). Stroke volume index increased from 56.5 +/- 13.3 to 63.7 +/- 16.3 mL m-2 (P = 0.004) and cardiac index from 3.42 +/- 1.02 to 3.73 +/- 1.05 L min-1 m-2 (P = 0.04). The insulin-mediated glucose disposal and the increases in leg blood flow were not correlated during the second hour (r = 0.21, P = 0.51) but showed a strong correlation during the fourth hour of the clamp (r = 0.88, P < 0.001). Insulin-mediated glucose disposal was not correlated with the increases in cardiac index. Thus, insulin-mediated muscle blood flow may be an important contributor to glucose uptake during sustained exogenous hyperinsulinaemia aiming at physiological insulin levels.

Evidence for an Inhibitory Effect of Physiological Levels of Insulin on the Growth Hormone (GH) Response to GH-Releasing Hormone in Healthy Subjects

It has been previously reported that in healthy subjects, the acute reduction of free fatty acids (FFA) levels by acipimox enhances the GH response to GHRH. In the present study, the GH response to GHRH was evaluated during acute blockade of lipolysis obtained either by acipimox or by insulin at different infusion rates. Six healthy subjects (four men and two women, 25.8 +/- 1.9 yrs old, mean +/- SE) underwent three GHRH tests (50 micrograms iv, at 1300 h) during: 1) iv 0.9% NaCl infusion (1200-1500 h) after oral acipimox administration (250 mg) at 0700 h and at 1100 h; 2) 0.1 mU.kg-1.min-1 euglycemic insulin clamp (1200-1500 h) after oral acipimox administration (250 mg at 0700 h and at 1100 h); 3) 0.4 mU.kg-1.min-1 euglycemic insulin clamp (1200-1500 h) after oral placebo administration (at 0700 and 1100 h). Serum insulin (immunoreactive insulin) levels were significantly different in the three tests (12 +/- 2, 100 +/- 10, 194 +/- 19 pmol/L, P < 0.06), plasma FFA were low and similar (0.04 +/- 0.003, 0.02 +/- 0.005, 0.02 +/- 0.003, not significant), and the GH response to GHRH was progressively lower (4871 +/- 1286, 2414 +/- 626, 1076 +/- 207 micrograms/L 120 min), although only test 3 was significantly different from test 1 (P < 0.05). Pooling the three tests together, a significant negative regression was observed between mean serum immunoreactive insulin levels and the GH response to GHRH (r = -0.629, P < 0.01). Our results indicate that in healthy subjects, acipimox and hyperinsulinemia produce a similar decrease in FFA levels and that at similar low FFA, the GH response to GHRH is lower during insulin infusion than after acipimox. These data suggest that insulin exerts a negative effect on GH release. Because the insulin levels able to reduce the GH response to GHRH are commonly observed during the day, for instance during the postprandial period, we conclude that the insulin negative effect on GH release may have physiological relevance.

Skeletal muscle triglyceride levels are inversely related to insulin action.

In animal studies, increased amounts of triglyceride associated with skeletal muscle (mTG) correlate with reduced skeletal muscle and whole body insulin action. The aim of this study was to test this relationship in humans. Subjects were 38 nondiabetic male Pima Indians (mean age 28 +/- 1 years). Insulin sensitivity at physiological (M) and supraphysiological (MZ) insulin levels was assessed by the euglycemic clamp. Lipid and carbohydrate oxidation were determined by indirect calorimetry before and during insulin administration. mTG was determined in vastus lateralis muscles obtained by percutaneous biopsy. Percentage of body fat (mean 29 +/- 1%, range 14-44%) was measured by underwater weighing. In simple regressions, negative relationships were found between mTG (mean 5.4 +/- 0.3 micromol/g, range 1.3-1.9 micromol/g) and log10M (r = -0.53, P < or = 0.001), MZ (r = -0.44, P = 0.006), and nonoxidative glucose disposal (r = -0.48 and -0.47 at physiological and supraphysiological insulin levels, respectively, both P = 0.005) but not glucose or lipid oxidation. mTG was not related to any measure of adiposity. In multiple regressions, measures of insulin resistance (log10M, MZ, log10[fasting insulin]) were significantly related to mTG independent of all measures of obesity (percentage of body fat, BMI, waist-to-thigh ratio). In turn, all measures of obesity were related to the insulin resistance measures independent of mTG. The obesity measures and mTG accounted for similar proportions of the variance in insulin resistance in these relationships. The results suggest that in this human population, as in animal models, skeletal muscle insulin sensitivity is strongly influenced by local supplies of triglycerides, as well as by remote depots and circulating lipids. The mechanism(s) underlying the relationship between mTG and insulin action on skeletal muscle glycogen synthesis may be central to an understanding of insulin resistance.

Selective, Physiological Transport of Insulin Across the Blood-Brain Barrier: Novel Demonstration by Species-Specific Radioimmunoassays

Banks, W. A., J. B. Jaspan and A. J. Kastin. Selective, physiological transport of insulin across the blood-brain barrier: Novel demonstration by species-specific radioimmunoassays. Peptides 18(8) 1257–1262; 1997.—Insulin in blood is thought to cross the blood-brain barrier (BBB) to act within the brain to help control appetite. We examined the ability of blood-borne insulin to cross the BBB. Human insulin was infused for 48 h subcutaneously at several doses into mice and the amount of human and murine insulin in serum and brain measured with species-specific radioimmunnoassays. For the exogenous human insulin, both brain and blood concentrations increased with increasing doses of infused insulin, whereas for the endogenous murine insulin, brain and blood concentrations decreased. Since the mouse cannot make human insulin, blood was the only source for the human insulin in brain, demonstrating that insulin does indeed cross the BBB. The relationship between the concentrations of human insulin in brain and blood was nonlinear, showing that passage is by a saturable mechanism. Partial saturation of the transporter occurred at euglycemic concentrations of serum insulin. Thus, insulin enters the brain by a saturable transport system that is operational primarily at physiological levels of serum insulin. Published by Elsevier Science Inc.

In situ regulation of lipolysis by insulin and norepinephrine: A microdialysis study during euglycemic-hyperinsulinemic clamp

Lipolytic responsiveness of subcutaneous and epididymal adipose tissue to norepinephrine (NE) was measured by microdialysis before and during a euglycemic-hyperinsulinemic clamp in male Sprague-Dawley rats (280 ± 7 g, n = 8). Microdialysis probes were perfused with standard Krebs-Ringer buffer without (basal condition [BC]) or with NE 10 −6 mol/L to determine basal and stimulated rates of lipolysis. The dialysate concentration of glycerol was measured (lipolytic index). NE infusion resulted in 3.0- and 4.2-fold increases in glycerol release in abdominal subcutaneous and epididymal adipose tissues, respectively. A euglycemic-hyperinsulinemic clamp at 6 mU/kg · min increased by ninefold the insulinemia (120 ± 9 U/L). Hyperinsulinemia suppressed basal glycerol release by 57% and 42% in subcutaneous and epididymal adipose depots, respectively (BC + I). Lipolytic responses to NE infusion during a euglycemic-hyperinsulinemic clamp (NE + I) were reduced by 45% and 33% in subcutaneous and epididymal adipose tissues, respectively, as compared with BC. Under BC, the lipolytic response to NE was greater in epididymal than in subcutaneous adipose tissue. Physiological levels of insulin regulated basal lipolysis and counteracted adrenergic stimulation of lipolysis to a similar extent in both superficial (subcutaneous) and intraabdominal (epididymal) adipose tissues. Our findings show that lipolysis is more responsive to NE in epididymal than in subcutaneous adipose tissue. The antilipolytic effects of insulin are similar in both superficial and deep intraabdominal adipose tissues. Furthermore, physiological plasma insulin levels cannot fully antagonize the lipolytic effects of NE.