Suppression Of Free Fatty Acids Research Articles

Insulin sensitivity in African-American children with and without family history of type 2 diabetes.

African-Americans are at increased risk for type 2 diabetes. We have previously demonstrated that African-American children are hyperinsulinemic and insulin resistant compared with their white American peers. The aim of the present investigation was to assess the impact of family history of type 2 diabetes on insulin sensitivity in African-American children. A total of 13 prepubertal healthy children with negative family history (FH-) and 9 with positive family history (FH+) of type 2 diabetes underwent a 3-h hyperinsulinemic (40 mU x m(-2) x min(-1))-euglycemic clamp study to assess insulin sensitivity. The groups were comparable for age, pubertal status, total body adiposity determined by dual-energy X-ray absorptiometry, abdominal adiposity assessed by computed tomography scan at the level of L4-5 lumbar vertebra, and physical fitness measured by maximal oxygen consumption (VO2max). The FH+, compared with the FH-, group had lower insulin-stimulated glucose disposal (10.9+/-1.2 vs. 14.2+/-0.9 mg x kg(-1) x min(-1), P = 0.035) and lower nonoxidative glucose disposal (5.7+/-0.8 vs. 8.3+/-0.6 mg x kg(-1) x min(-1), P = 0.015), with no differences in rates of glucose oxidation, fat oxidation, or insulin-mediated free fatty acid suppression. Fasting hepatic glucose production assessed with [6,6-2H2]glucose and basal rates of glucose and fat oxidation were not different between the two groups. These data suggest that in African-American children, family history of type 2 diabetes is a risk factor for insulin resistance. These children manifest important metabolic alterations, including impaired insulin-stimulated total and nonoxidative glucose disposal early in the first decade of life. We propose that this familial tendency, combined with environmental influences, could lead to type 2 diabetes decades later.

Prolonged elevation of plasma free fatty acids desensitizes the insulin secretory response to glucose in vivo in rats.

Prolonged exposure of pancreatic islets to free fatty acids (FFAs) inhibits glucose-stimulated insulin secretion (GSIS) in vitro. However, FFA inhibition of GSIS has not been clearly demonstrated in vivo. We examined the in vivo effect of prolonged elevation of plasma FFAs on GSIS using a two-step hyperglycemic clamp in rats treated with a 48-h intravenous infusion of either 20% Intralipid plus heparin (INT) (5 microl/min plus heparin, 0.1 U/min; n = 8), oleate (OLE) (1.3 microEq/min; n = 6), saline (SAL) (n = 6), or bovine serum albumin (BSA) (vehicle for OLE; n = 5). Because there was no difference in any of the parameters between BSA and SAL rats, these groups were combined as control rats (CONT) (n = 11). At the end of the 48-h OLE/INT/CONT infusions, after an overnight fast, plasma glucose was clamped for 2 h at 13 mmol/l and for another 2 h at 22 mmol/l. Preclamp plasma FFAs were elevated twofold (P < 0.01) versus CONT with both INT and OLE (NS, INT vs. OLE). Preclamp glucose, insulin, and C-peptide levels were higher in INT than in CONT rats (P < 0.05), suggesting insulin resistance, but they were not different in OLE and CONT rats. The insulin and C-peptide responses to the rise in plasma glucose from basal to 13 mmol/l were lower in OLE (336 +/- 72 pmol/l and 1.2 +/- 0.1 nmol/l, P < 0.01 and P < 0.05, respectively) than in CONT (552 +/- 54 and 1.9 +/- 0.1) rats, but they were not different between CONT and INT rats (648 +/- 150 and 2.0 +/- 0.4). The insulin and C-peptide responses to the rise in plasma glucose from 13 to 22 mmol/l were lower in both INT (1,188 +/- 204 pmol/l and 3.0 +/- 0.3 nmol/l, P < 0.01 and P < 0.001) and OLE (432 +/- 60 and 1.7 +/- 0.2, P < 0.001 vs. CONT or INT) rats than in CONT rats (1,662 +/- 174 and 5.0 +/- 0.6). In summary, 1) both INT and OLE decreased GSIS in vivo in rats, and 2) the impairing effect of INT on GSIS was less than that of OLE, which might be due to the different type of fatty acid (mostly polyunsaturated in INT versus monounsaturated as OLE) and/or to differential effects of INT and OLE on insulin sensitivity. In conclusion, prolonged elevation of plasma FFAs can desensitize the insulin secretory response to glucose in vivo, thus inducing a beta-cell defect that is similar to that found in type 2 diabetes.

Sex differences in African-Americans regarding sensitivity to insulin's glucoregulatory and antilipolytic actions.

The purpose of this study was to determine if there are sex differences in African-Americans regarding the effect of obesity on sensitivity to insulin as a glucoregulatory and antilipolytic hormone. Data from study participants, 127 nondiabetic African-Americans (mean age 32 +/- 4 years), included anthropometric measurements, an oral glucose tolerance test (OGTT), a 2-h euglycemic-hyperinsulinemic clamp, and a fasting triglyceride level. Sensitivity to insulin as a glucoregulatory hormone was determined by M/FFM, where M is the mean glucose infusion rate during the second hour of the clamp and FFM is fat-free mass. Sensitivity to insulin's antilipolytic action was assessed during the OGTT by the percent suppression of free fatty acid (FFA) concentrations between 0 and 120 min. The higher the suppression of FFAs, the greater the sensitivity to insulin's antilipolytic action. The participants were classified by BMI into three groups: nonobese (31 men, 24 women), obese (17 men, 14 women), and severely obese (12 men, 29 women). The women had higher percentages of body fat (P < 0.001), and the men had greater FFM (P < 0.001). The M/FFM values for men versus women in each BMI group were nonobese, 8.8 +/- 2.8 vs. 10.8 +/- 4.4; obese, 7.2 +/- 3.4 vs. 8.5 +/- 3.4; and severely obese, 4.7 +/- 2.1 vs. 6.1 +/- 2.2. The difference between the BMI groups was significant (P < 0.001), as was the difference between men and women (P < 0.01). In addition, there was a significant sex difference in percent suppression of FFAS (P < 0.001). The men and women had similar fasting insulin and FFA concentrations; however, in the men only, the percent suppression of FFA declined with increasing obesity (nonobese, 83 +/- 15%; obese, 73 +/- 18%; and severely obese, 69 +/- 19%; P = 0.02). The women in all three BMI groups had lower FFA levels of 86-88%. Obese African-American men and women are resistant to insulin as a glucoregulatory hormone, but only obese men are resistant to insulin's antilipolytic action; obese African-American women are sensitive to insulin's antilipolytic action. The combined presence of sensitivity to insulin's antilipolytic action with resistance to insulin's glucoregulatory action in obese African-American women may contribute to their high prevalence of obesity and type 2 diabetes.

Characteristics of young offspring of type 2 diabetic parents in a biracial (black-white) community-based sample: The Bogalusa heart study

The impact of race (black-white) and family history of type 2 diabetes mellitus on metabolic characteristics in early life was examined in a community-based sample from Bogalusa, LA. Study subjects included offpsirng of type 2 diabetics (n = 53, 47%) black) and nondiabetics (n = 52, 40% black), with the mean age of each group ranging from 14.2 to 15.6 years. Offspring were given a 1-hour oral glucose tolerance test. Measures of body fatness such as body weight, body-mass index (BMI; weight/height 2), and triceps and subscapular thicknesses were significantly higher only in white offspring of diabetics versus nondiabetics; measures of abdominal fat (waist circumference and waist-to-hip ratio) were significantly higher among offspring of diabetics of both races. Among the measures of glucose homeostasis, basal glucose, insulin, insulin—to-—C-peptide ratio (a measure of hepatic insulin extraction), insulin resistance index (derived from basal glucose and insulin levels), and glucose response after glucose challenge were higher in the offsprig of diabetics of both races. The differences in insulin—to—C-peptide ratio and glucose response remained significant after adjusting for BMI; further, these two variables were independently associated with parental diabetes in both races. Waist-to-hip ratio, glucose response, C-peptide response (a measure of insulin secretion) were lower, and basal insulin—to—C-peptide ratio and postglucose suppression of free fatty acids greater in blacks versus whites, regardless of status of parental diabetes. Black-white differences in postglucose suppression of free fatty acids disappeared after adjusting for BMI. Thus, blacks and whites with parental type 2 diabetes show multiple abnormalities in parameters governing glucose homeostasis early in life, and some of these traits differ between the races, regardless of status of parental diabetes.

Role of free fatty acids and glucagon in the peripheral effect of insulin on glucose production in humans.

We have shown previously that the greater suppression of endogenous glucose production (GP) with equimolar peripheral vs. portal insulin cannot be detected or is minimally reversed when the insulin-induced suppression of either free fatty acids (FFA) or glucagon alone is prevented. The present experiments were designed to minimize the insulin suppression of both glucagon and FFA in an attempt to further examine the mechanism of insulin's peripheral effect on GP. In nine healthy men, we investigated the effect of limiting the insulin suppression of both FFA and glucagon by infusing heparin (250 U/h), Intralipid 10% (25 ml/h), and glucagon (0.65 ng . kg-1 . min-1) during 1) portal (n = 9), 2) equimolar peripheral (n = 9), and 3) half-dose peripheral insulin delivery (n = 4) by use of our previously published tolbutamide infusion method, with calculation and matching of insulin secretion rate. GP decreased by 57.2 +/- 2. 6% with portal, 39.0 +/- 4.1% with equimolar peripheral, and 31.5 +/- 2.7% with half-dose peripheral insulin delivery (P < 0.001 for portal vs. peripheral and P < 0.001 for portal vs. half-dose peripheral). In contrast, in six control subjects in whom glucagon and FFA were not replaced, GP decreased by 62.6 +/- 2.4% with portal (n = 6), 75.7 +/- 3.0% with peripheral (n = 6), and 56.3 +/- 3.0% with half-dose peripheral (n = 4) insulin delivery (P < 0.01 for portal vs. peripheral and P = not significant for portal vs. half-dose peripheral). In summary, the greater suppression of GP with equimolar peripheral vs. portal insulin is eliminated and markedly reversed if the acute insulin-induced suppression of both plasma FFA and glucagon is minimized. This suggests that the insulin-induced suppression of glucagon and FFA has additive or cooperative effects in mediating the acute extrahepatic effect of insulin on GP.

Inhibition of Lipolysis Stimulates Whole Body Glucose Production and Disposal in Normal Postabsorptive Subjects1

The role played by circulating free fatty acids (FFA) and fat oxidation in the regulation of whole body glucose production and uptake in the basal state is still a matter of debate. This question was analyzed in nine normal overnight fasted volunteers in whom glucose kinetics ([3-3H]glucose infusion) and substrate oxidation rates (indirect calorimetry) were measured during 10.5 h both under placebo conditions and during experimental antilipolysis induced by Acipimox given orally during the last 8 h of the study. During the last 2 h of the tests, the following mean changes (delta) from baseline were recorded in Acipimox vs. placebo studies: delta FFA, -0.26 +/- 0.08 vs. +0.29 +/- 0.06 mmol/L (P < 0.001); delta glucose, -12 +/- 2 vs. -12 +/- 1 mg/dL (P > 0.05); delta glucose production, +16 +/- 5 vs. -15 +/- 3 mg/min (P < 0.001); delta C peptide, -1.11 +/- 0.10 vs. -0.66 +/- 0.10 ng/mL (P < 0.001); delta glucagon, +64 +/- 25 vs. +21 +/- 9 pg/mL (P < 0.05); delta GH, +37 +/- 9 vs. +4 +/- 2 ng/mL (P < 0.007); delta cortisol, +37 +/- 25 vs. -30 +/- 26 ng/mL (P < 0.04). Acipimox inhibited fat oxidation (-18 +/- 4 vs. +19 +/- 4 mg/min; P < 0.001) and enhanced carbohydrate oxidation (+18 +/- 8 vs. -24 +/- 11 mg/min; P < 0.02). Protein catabolism calculated over the 8-h study period was significantly stimulated (+5.7 +/- 2.5 vs. -1.9 +/- 1.7 g/8 h; P < 0.02). During the Acipimox studies, the increased protein breakdown could theoretically account for about 75% of the increased glucose production. Thus, contrary to current opinion, FFA suppression stimulates glucose production and whole body glucose disposal in normal overnight fasted subjects.

Differences in insulin suppression of free fatty acid levels by gender and glucose tolerance status. Relation to plasma triglyceride and apolipoprotein B concentrations. Insulin Resistance Atherosclerosis Study (IRAS) Investigators.

Most discussions of relations of insulin resistance to coronary heart disease risk factors have focused on insulin-stimulated glucose uptake, but insulin suppression of plasma free fatty acid (FFA) levels is also important in lipid and lipoprotein metabolism. To identify groups with impaired insulin suppression of FFAs, we studied a multiethnic cohort of 1521 women and men at four US centers that comprise the Insulin Resistance Atherosclerosis Study (IRAS): 682 with normal glucose tolerance, 352 with impaired glucose tolerance, and 487 with non-insulin-dependent diabetes. The FFA level 2 hours after a 75-gm oral glucose load adjusted for fasting FFAs was used as the measure of insulin suppression. After adjustment for age, center, ethnicity, body mass index, and fasting and 2-hour insulin levels, 2-hour FFA levels were significantly higher in men than women and in persons with impaired glucose tolerance and non-insulin-dependent diabetes mellitus versus normal glucose tolerance. The gender difference was largely accounted for by differences in central obesity as measured by waist-hip ratio; the difference by glucose tolerance status was not affected by central obesity, suggesting a different mechanism. In multivariate regression analyses, 2-hour FFA levels were strongly related to fasting triglyceride and apoB levels, respectively, after adjustment for age, fasting and 2-hour insulin concentrations, and fasting FFA concentrations. In summary, elevated plasma apoB and triglyceride concentrations associated with male gender and with glucose intolerance are partly accounted for by differences in the ability of insulin to suppress FFA concentrations.

Long-term administration of acipimox potentiates growth hormone response to growth hormone-releasing hormone by decreasing serum free fatty acid in obesity

Obesity is associated with an impairment of normal growth hormone (GH) secretion and blunted responses to all stimuli. A high plasma free fatty acid (FFA) level is frequently observed in obesity. FFA participates in the regulation of pituitary GH secretion. To determine whether the derangement of GH secretion in obesity is associated with high plasma FFA levels, tests with GH-releasing hormone (GHRH) and acipimox (ACX), an antilipolytic agent able to decrease FFA, were undertaken in six obese subjects and seven normal control subjects. In addition, the effect of prolonged suppression of FFA level on GH response to GHRH after administration of ACX for 1 month was also examined in each of the obese subjects. The GH response in obese subjects (median, 9.1 μg/L) to GHRH (1–29) (1 μg/kg intravenously [IV]) was significantly blunted as compared with normal control subjects (23.5 μg/L, P < .05). Basal FFA levels were higher in obese subjects (855.2 μEq/L) than in normal control subjects (514.6 μEq/L, P < .05). One-dose ACX (500 mg) decreased FFA levels in both obese and normal subjects: the lowest FFA levels in obese subjects (158.3 μEq/L) 2 to 2.5 hours after ACX were similar to those of normal control subjects (108.7 μEq/L). One-dose ACX potentiated GHRH-stimulated GH response in both obese and normal subjects. GH responses potentiated by ACX in obese subjects (27.1 μg/L) were similar to GH responses to GHRH in normal control subjects, but lower than in normal subjects treated with ACX plus GHRH (58.5 μg/L, P < .05). Thereafter, all of the obese subjects were treated with ACX for 1 month, after which the ACX plus GHRH tests were repeated. After 1 month of acipimox administration in the obese subjects, GH responses (38.8 μg/L) were significantly higher than those of obese subjects treated with GHRH and one-dose ACX plus GHRH ( P < .05). They were similar to GH responses of normal control subjects receiving the one-dose ACX plus GHRH test. In conclusion, in obesity the prolonged suppression of FFA levels induced by long-term administration of ACX potentiated somatotrope responsiveness, likely acting at the pituitary level, suggesting that the duration of FFA suppression had an important relation to the magnitude of GH response.

Gender differences in insulin-induced free fatty acid suppression: studies in an African American population.

This study of African Americans (AA) was designed to investigate gender differences in insulin-induced free fatty acid (FFA) suppression. Sixty AA (34 women, mean age 34 +/- 7.6 years, and 26 men, mean age 30 +/- 2.9 years) participated. All subjects had an oral glucose tolerance test (OGTT). Nineteen women and 18 men also underwent a euglycemic hyperinsulinemic clamp (IC) study. Plasma insulin and FFA concentrations were obtained during both tests at 0, 60, and 120 min. While there was no gender difference in body mass index (P = 0.21), women had greater percent body fat (P < 0.001) calculated by the Siri formula. There was no gender difference in fasting FFA levels, but during the OGTT, women compared to men had significantly greater FFA suppression. Both nonobese and obese women suppressed FFA concentration by 88%, and nonobese and obese men suppressed FFA concentration by 80 and 66% respectively. This gender difference in FFA suppression was significant (P = 0.001) and independent of obesity and insulin concentration. During the IC studies, there were no gender or obesity differences in FFA suppression, with women and men suppressing FFA levels by 87-89% (P = 0.7). Fasting insulin concentrations were higher in obese vs. nonobese (P = 0.03), but fasting FFA concentrations were not different (P = 0.15). For nonobese and obese females, fasting FFA levels were 0.55 +/- 0.24 and 0.44 +/- 0.26 mEq/L, respectively, and for nonobese and obese males, 0.45 +/- 0.2 and 0.35 +/- 0.18 mEq/L, respectively. In women, development of obesity may be enhanced by greater sensitivity to insulin-induced FFA suppression as measured during an OGTT. To detect gender differences in FFA metabolism, the OGTT is superior to the IC. The lack of elevation in fasting FFA levels in obese AA women and men has not been reported in other racial groups and may indicate a greater adipocyte sensitivity to insulin in AA.

Metabolic consequences of sustained suppression of free fatty acids by acipimox in patients with NIDDM

Metabolic consequences of sustained suppression of free fatty acids by acipimox in patients with NIDDM

Dual effects of oleic acid on Ca 2+ mobilization and protein phosphorylation in human platelets in presence or absence of platelet activating factor

This laboratory demonstrated earlier that oleic acid inhibited platelet activating factor (PAF)-induced aggregation and serotonin release of rabbit platelets ( M. Miwa, C. Hill, R. Kumar, J. Sugatani, M. S. Olson, and D. J. Hanahan, 1987, J. Biol. Chem. 262, 527–530). More recently, we reported that oleic acid caused a decrease in phosphatidylinositol-4-phosphate (PIP) and phosphatidylinositol-4,5-bisphosphate (PIP 2), but did not affect the level of inositol-1,4,5-trisphosphate (IP 3), in rabbit platelets ( D. Nunez, J. Randon, C. Gandhi, A. Siafaka-Kapadai, M. S. Olson, and D. J. Hanahan, 1990, J. Biol. Chem. 265, 18330–18838). These results suggested that oleic acid did not stimulate phospholipase C. In contrast, PAF induced a decrease in PIP 2 and an increase in PIP level and IP 3. These effects were shown to be attenuated by oleic acid. In this current study, our experiments show that (a) oleic acid blocked PAF-induced rise in intracellular [Ca 2+] (to provide a mechanism in agreement with our previous experiments which showed that oleic acid inhibited PAF-induced IP 3 rise in platelets) and (b) oleic acid itself induced a gradual rise in [Ca 2+] i, which would provide a mechanism for oleic acid-induced aggregation despite the fact that oleic acid did not cause the production of IP 3 ( Nunez et al., 1990 ). Oleic acid, in a dose-dependent manner, was shown to inhibit PAF-induced Ca 2+ mobilization from intra- and extracellular sources. The inhibition was closely related to the suppressive effect of oleic acid on PAF-induced aggregation. Furthermore, oleic acid inhibited the PAF-stimulated phosphorylation of the 20- and 40-kDa proteins. At concentrations above 20 μ m, oleic acid itself could induce platelet aggregation and Ca 24 mobilization, but the time sequence of these two responses in human platelets was significantly different from those obtained with PAF. Oleic acid alone, at 20 μ m, caused a 1.4-fold increase in the cAMP level in platelets which was followed by a decline to a basal value at higher concentrations of this fatty acid. It seemed clear that elevation of adenylate cyclase activity was not associated with free fatty acid inhibition of platelet activation. Interestingly, both PAF and oleic acid added separately to human platelets induced proteintyrosine phosphorylation, but oleic acid did not cause any inhibition of PAF-induced protein-tyrosine phosphorylation. Thus, these findings show that oleic acid inhibition of PAF-induced aggregation was closely correlated to inhibition of PAF-induced [Ca 2+] i changes and that oleic acid-induced aggregation, which occurred over a different time frame than PAF-aggregation, could be explained by a rise in [Ca 2+] i.

The effect of CP 68,722, a thiozolidinedione derivative, on insulin sensitivity in lean and obese Zucker rats

The effect of a new drug (CP 68,722, Pfizer) on parameters of insulin sensitivity in an established insulin-resistant animal model was examined. Rates of hepatic glucose production (HGP) and peripheral glucose uptake in obese Zucker ( fa fa ) rats treated with a 10-day course of the medication using a two-step (2 and 10 mU/kg/min) euglycemic hyperinsulinemic clamp technique were measured. In addition, changes in substrate concentrations after drug treatment were examined. Basal HGP rates were similar in the lean versus the obese animals (37 ± 3 v 39 ± 3 μmol/kg/min); however, the obese animals had impaired insulin-induced suppression of HGP at both 2 mU/kg/min (36 ± 3 v 23 ± 4 μmol/kg/min) and 10 mU/kg/min (18 ± 5 v 2 ± 1 μmol/kg/min). Insulin stimulation of glucose disposal was also defective in the obese animals (37 ± 2 v 88 ± 7 μmol/kg/min at 2 mU/kg/min and 98 ± 9 v 219 ± 18 μmol/kg/min at 10 mU/kg/min). In addition, obese animals had elevated free fatty acid (FFA) and ketone levels, both of which were resistant to insulin-induced suppression. After drug treatment, few alterations in glucose or lipid metabolism were found in the lean animals. In the obese animals, insulin suppression of HGP was normalized during the higher insulin infusion rate (0 v 18 ± 5 μmol/kg/min at 10 mU/kg/min), and peripheral glucose disposal was enhanced at both steps of the insulin clamp (54 ± 4 v 37 ± 2 μmol/kg/min at 2 mU/kg/min and 134 ± 12 v 98 ± 9 μmol/kg/min at 10 mU/kg/min). Drug treatment lowered both fasting and insulin-stimulated suppression of FFA and ketone concentrations in the obese rat. An unexpected finding was that drug treatment resulted in a marked acceleration of insulin clearance, such that basal insulin levels were reduced, as were insulin levels during the first steps of the insulin clamp. These results suggest that CP 68,722 acts at both the liver and skeletal muscle tissues to enhance insulin sensitivity, and thus may prove to be a useful medication in patients with insulin-resistant diabetes mellitus.

Interaction of exercise and insulin action in humans

To assess the interaction of exercise and insulin action, healthy males were studied with saline infusion (n = 5) or with a hyperinsulinemic euglycemic clamp (0.5, 1.0, 2.0, or 15.0 mU.kg-1.min-1; n = 5 at each dose) during rest (40 min), moderate-intensity cycle exercise (100 min), and recovery (100 min). Metabolism was assessed using isotopic methods and indirect calorimetry. During rest, exercise, and recovery with saline infusion, plasma glucose was unchanged, total glucose utilization (Rd) was 2.4 +/- 0.4, 4.9 +/- 0.2, and 2.6 +/- 0.2 mg.kg-1.min-1, and carbohydrate (CHO) oxidation (OX) was 1.4 +/- 0.3, 10.6 +/- 1.1, and 0.5 +/- 0.2 mg.kg-1.min-1. The glucose infusion, insulin-dependent Rd, and CHO OX increased synergistically when exercise and insulin clamps were combined. Exercise decreased (P less than 0.05) the half-maximal doses (ED50) and increased the maximal responses (Vmax) for insulin-dependent Rd and CHO OX. Estimates of insulin-independent Rd were 1.3 +/- 0.7, 4.1 +/- 1.3, and 1.9 +/- 0.7 mg.kg-1.min-1 and insulin-independent CHO OX were 1.2 +/- 0.9, 10.4 +/- 1.3, and 0.6 +/- 0.3 mg.kg-1.min-1 during rest, exercise, and recovery. Estimates during exercise were greater than those at rest (P less than 0.05). The total suppression of free fatty acids (FFA) and fat OX by insulin were elevated by exercise (P less than 0.05). In summary, exercise and insulin interact synergistically in stimulating Rd and CHO OX.(ABSTRACT TRUNCATED AT 250 WORDS)

Dose-dependent effect of insulin on plasma free fatty acid turnover and oxidation in humans

Methodology for measuring plasma free fatty acid (FFA) turnover/oxidation with [1-14C]palmitate was tested in normal subjects. In study 1, two different approaches (720-min tracer infusion without prime vs. 150-min infusion with NaH14CO3 prime) to achieve steady-state conditions of 14CO2 yielded equivalent rates of plasma FFA turnover/oxidation. In study 2, during staircase NaH14CO3 infusion, calculated rates of 14CO2 appearance agreed closely with NaH14CO3 infusion rates. In study 3, 300-min euglycemic insulin clamp documented that full biological effect of insulin on plasma FFA turnover/oxidation was established within 60-120 min. In study 4, plasma insulin concentration was raised to 14 +/- 2, 23 +/- 2, 38 +/- 2, 72 +/- 5, and 215 +/- 10 microU/ml. A dose-dependent insulin suppression of plasma FFA turnover/oxidation was observed. Plasma FFA concentration correlated positively with plasma FFA turnover/oxidation in basal and insulinized states. Total lipid oxidation (indirect calorimetry) was significantly higher than plasma FFA oxidation in the basal state, suggesting that intracellular lipid stores contributed to whole body lipid oxidation. Hepatic glucose production and total glucose disposal showed the expected dose-dependent suppression and stimulation, respectively, by insulin. In conclusion, insulin regulation of plasma FFA turnover/oxidation is maximally manifest at low physiological plasma insulin concentrations, and in the basal state a significant contribution to whole body lipid oxidation originates from lipid pool(s) that are different from plasma FFA.

Free–Fatty Acid Inhibition of Insulin Binding, Degradation, and Action in Isolated Rat Hepatocytes

The effect of free fatty acids (FFAs) on insulin binding and action was investigated in isolated rat hepatocytes. Oleic acid (0.4 mM) added to the cells rapidly (within 45 min) reduced insulin binding and degradation (each by 45%; P less than 0.001, n = 7) without changing the apparent receptor affinity. The effect was concentration dependent; a half-maximal inhibitory effect occurred at 0.150 +/- 0.050 mM (mean +/- SE). Oleic acid exerted no effect on insulin binding in energy-depleted (KCN-treated) cells. Oleic, palmitic, stearic, palmitoleic, and eicosapentaenoic acids were equally effective in reducing insulin binding. FFA did not change insulin binding to partially purified insulin receptors, thus excluding a direct effect on the insulin receptor. Furthermore, binding to partially purified receptors from solubilized cells pretreated with 0.2 mM oleic acid was not changed, indicating the effect of FFA in intact cells is on the rate of receptor internalization and/or recycling. Concomitant with the effect on insulin binding, oleic acid elicited a concentration-dependent reduction in nonstimulated cellular [14C]aminoisobutyric acid uptake (AIB; 29 +/- 8%, P less than 0.05) and decreased the maximal effect of insulin (39 +/- 7%, P less than 0.05). Thus, in a concentration-dependent manner, different fatty acids can reduce the number of binding sites for insulin and the degradation of insulin by isolated liver cells. Basal and insulin-stimulated AIB transport was reduced, suggesting the presence of postbinding perturbations. These data suggest that FFA exerts an important modulating effect on insulin action in the liver.

Free-fatty acid inhibition of insulin binding, degradation, and action in isolated rat hepatocytes

Free-fatty acid inhibition of insulin binding, degradation, and action in isolated rat hepatocytes

Defective Free-Fatty Acid and Oxidative Glucose Metabolism in IDDM During Hypoglycemia: Influence of Glycemic Control

To examine the impact of diabetes and its treatment on plasma free-fatty acid (FFA) and oxidative fuel metabolism during hypoglycemia, we combined indirect calorimetry with [3-3H]glucose during a 4-h low-dose insulin infusion (plasma insulin approximately 2-fold above basal) in six poorly controlled and nine well-controlled insulin-dependent diabetes mellitus (IDDM) patients and in six healthy subjects. Diabetic subjects received insulin overnight to maintain euglycemia before study. Although free-insulin levels and counterregulatory hormone responses were similar, the plasma glucose fall was more pronounced in well-controlled diabetic subjects. In well-controlled diabetic and healthy subjects, the small increment in insulin rapidly suppressed plasma FFA and fat oxidation by approximately 50% and stimulated carbohydrate oxidation by approximately 80%. In contrast, plasma FFA levels did not fall in poorly controlled diabetic subjects, and glucose oxidation was not stimulated. To determine whether this resistance to the antilipolytic effect of insulin occurs in the absence of hypoglycemic counterregulation, we used a sequential low-dose euglycemic insulin clamp (0.2, 0.3, and 0.5 mU.kg-1.min-1). In healthy subjects, plasma FFA was nearly maximally suppressed at the lowest insulin dose. In contrast, plasma FFA remained persistently elevated in poorly controlled diabetic subjects at each insulin dose. However, the insulin dose-response curve for suppression of plasma FFA was near normal in well-controlled subjects. We conclude that poorly controlled IDDM diabetic patients are resistant to the antilipolytic effects of insulin and show impaired stimulation of glucose oxidation during insulin-induced hypoglycemia. Amelioration of these defects in well-controlled patients may be another factor contributing to the higher risk of hypoglycemia during intensified insulin therapy.

Oxidative fuel metabolism during mild hypoglycemia: critical role of free fatty acids.

We examined oxidative fuel metabolism (indirect calorimetry) during mild insulin-induced hypoglycemia in normal subjects. Four groups of studies (4 h each) were performed: 1) insulin alone (0.3 mU.kg-1.min-1, 2) insulin plus heparin, 3) insulin plus propranolol, and 4) insulin plus propranolol plus heparin (starting at 75 min). In all groups, insulin rose threefold (approximately 20 microU/ml), whereas glucose concentrations fell by approximately 20 mg/dl (to 65-70 mg/dl) due to a transient decrease in glucose production; glucose uptake did not change. During insulin alone, carbohydrate oxidation rose markedly during the 1st 60-90 min (by 85%, P less than 0.05) as circulating free fatty acids (FFA) and fat oxidation declined. Subsequently, FFA rose and carbohydrate oxidation declined toward base line. The latter changes were completely abolished by propranolol and restored when propranolol-induced suppression of FFA was overcome by addition of heparin. Heparin, by preventing the insulin-induced fall in FFA, also blocked the early rise in carbohydrate oxidation. We conclude that small increments in insulin markedly stimulate carbohydrate oxidation without increasing glucose uptake. This effect is due to the exquisite sensitivity of lipolysis to insulin and is overcome by catecholamine release during hypoglycemia. Mild hyperinsulinemia promotes glucose metabolism primarily through indirect effects on FFA.

Free fatty acid inhibition of the insulin induction of glucose-6-phosphate dehydrogenase in rat hepatocyte monolayers.

Rat hepatocytes in monolayer culture were utilized to determine if the decrease in glucose-6-phosphate dehydrogenase (G6PD) activity resulting from the ingestion of fat can be mimicked by the addition of fatty acids to a chemically, hormonally defined medium. G6PD activity in cultured hepatocytes was induced several-fold by insulin. Dexamethasone or T3 did not amplify the insulin induction of G6PD. Glucose alone increased G6PD activity in cultured hepatocytes from fasted donors by nearly 500%. Insulin in combination with glucose induced G6PD an additional two-fold. The increase in G6PD activity caused by glucose was greater in hepatocytes isolated from 72 hr-fasted rats as compared to fed donor rats. Such a response was reminiscent of the "overshoot" phenomenon in which G6PD activity is induced well above the normal level by fasting-refeeding rats a high glucose diet. Addition of linoleate to the medium resulted in a significant suppression of insulin's ability to induce G6PD, but linoleate had no effect on the induction of G6PD activity by glucose alone. A shift to the right in the insulin-response curve for the induction of G6PD also was detected for the induction of malic enzyme and acetyl-CoA carboxylase. Arachidonate (0.25 mM) was a significantly more effective inhibitor of the insulin action than linoleate was. Apparently rat hepatocytes in monolayer culture can be utilized as a model to investigate the molecular mechanism by which fatty acids inhibit the production of lipogenic enzymes. In part, this mechanism of fatty acid inhibition involves desensitization of hepatocytes to the lipogenic action of insulin.

Preservation of glucose tolerance and insulin secretory response to repeated glucose loads by the feeding of minimal glucose during prolonged fasting

The facilitation of glucose disposal (Staub-Traugott effect) and potentiation of serum insulin (IRI) concentration normally occurring after closely spaced intravenous glucose loads, are known to disappear after prolonged starvation. To study the effects of minimal amounts of glucose during fasting upon the insulin response and disposal of repeated intravenous glucose tolerance tests, obese volunteers were fasted for a mean of 25 ± 2 days, while receiving either 8 or 16 gm of oral glucose every 6 hr, and compared to totally fasted subjects without glucose supplementation. Weight loss rate and the fall in basal IRI and glucose levels were similar to those of totally fasted subjects. However, the Staub-Traugott effect and insulin secretory dynamics after stimulation by repetitive intravenous glucose loading were preserved by this glucose modified fast, while baseline serum glucagon levels (IRG) were significantly lower, and the basal IRI/IRG ratios were thus unchanged from the fed state. IRG and free fatty acid suppression were similar in the fed and glucose modified fasted states. Lactic acid levels increased as expected after the repeated glucose injections in the fed state, but failed to do so after the prolonged modified fast until the second and third repetitive glucose loads, in which a significant rise coincided with accelerated glucose disposal. It is suggested that minimal amounts of carbohydrate during fasting preserve the insulin potentiating action of glucose, preferentially sparing a delayed releasable pool of insulin, while protecting the glucose utilization mechanisms, including increased glycolysis, responsible for the Staub-Traugott effect.