Hepatic and muscle insulin action during late pregnancy in the dog

We evaluated the effects of physiologic increases in insulin on hepatic and peripheral glucose metabolism in nonpregnant (NP) and pregnant (P; 3rd trimester) conscious dogs (n = 9 each) using tracer and arteriovenous difference techniques during a hyperinsulinemic euglycemic clamp. Insulin was initially (-150 to 0 min) infused intraportally at a basal rate. During 0-120 min (Low Insulin), the rate was increased by 0.2 mU x kg(-1) x min(-1), and from 120 to 240 min (High Insulin) insulin was infused at 1.5 mU x kg(-1) x min(-1). Insulin concentrations were significantly higher in NP than P during all periods. Matched subsets (n = 5 NP and 6 P) were identified. In the subsets, insulin was 7 +/- 1, 9 +/- 1, and 28 +/- 3 microU/ml (basal, Low Insulin, and High Insulin, respectively) in NP, and 5 +/- 1, 7 +/- 1, and 27 +/- 3 microU/ml in P. Net hepatic glucose output was suppressed similarly in both subsets (> or =50% with Low Insulin, 100% with High Insulin), as was endogenous glucose rate of appearance. During High Insulin, NP dogs required more glucose (10.8 +/- 1.5 vs. 6.2 +/- 1.0 mg x kg(-1) x min(-1), P < 0.05), and hindlimb (primarily skeletal muscle) glucose uptake tended to be greater in NP than P (18.6 +/- 2.5 mg/min vs. 13.6 +/- 2.0 mg/min, P = 0.06). The normal canine liver remains insulin sensitive during late pregnancy. Differing insulin concentrations in pregnant and nonpregnant women and excessive insulin infusion rates may explain previous findings of hepatic insulin resistance in healthy pregnant women.     

Enteral infusion of glucose at rates approximating EGP enhances glucose disposal but does not cause hypoglycemia

Portal infusion of glucose at rates approximating endogenous glucose production (EGP) causes paradoxical hypoglycemia in wild-type but not GLUT2 null mice, implying activation of a specific portal glucose sensor. To determine whether this occurs in humans, glucose containing [3-3H]glucose was infused intraduodenally at rates of 3.1 mg. kg-1. min-1 (n = 5), 1.55 mg. kg-1. min-1 (n = 9), or 0/0.1 mg. kg-1. min-1 (n = 9) for 7 h in healthy nondiabetic subjects. [6,6-2H2]glucose was infused intravenously to enable simultaneous measurement of EGP, glucose disappearance, and the rate of appearance of the intraduodenally infused glucose. Plasma glucose concentrations fell (P < 0.01) from 90 +/- 1 to 84 +/- 2 mg/dl during the 0/0.1 mg. kg-1. min-1 id infusions but increased (P < 0.001) to 104 +/- 5 and 107 +/- 3 mg/dl, respectively, during the 1.55 and 3.1 mg. kg-1. min-1 id infusions. In contrast, insulin increased (P < 0.05) during the 1.55 and 3.0 mg. kg-1. min-1 infusions, reaching a peak of 10 +/- 2 and 18 +/- 5 micro U/ml, respectively, by 2 h. Insulin concentrations then fell back to concentrations that no longer differed by study end (7 +/- 1 vs. 8 +/- 1 micro U/ml). This resulted in comparable suppression of EGP by study end (0.84 +/- 0.2 and 0.63 +/- 0.1 mg. kg-1. min-1). Glucose disappearance was higher (P < 0.01) during the final hour of the 3.1 than 1.55 mg. kg-1. min-1 id infusion (4.47 +/- 0.2 vs. 2.6 +/- 0.1 mg. kg-1. min-1), likely because of the slightly, but not significantly, higher glucose and insulin concentrations. We conclude that, in contrast to mice, selective portal glucose delivery at rates approximating EGP does not cause hypoglycemia in humans.     

Insulin action during late pregnancy in the conscious dog

Our aim was to assess the magnitude of peripheral insulin resistance and whether changes in hepatic insulin action were evident in a canine model of late (3rd trimester) pregnancy. A 3-h hyperinsulinemic (5 mU.kg(-1).min(-1)) euglycemic clamp was conducted using conscious, 18-h-fasted pregnant (P; n = 6) and nonpregnant (NP; n = 6) female dogs in which catheters for intraportal insulin infusion and assessment of hepatic substrate balances were implanted approximately 17 days before experimentation. Arterial plasma insulin rose from 11 +/- 2 to 192 +/- 24 and 4 +/- 2 to 178 +/- 5 microU/ml in the 3rd h in NP and P, respectively. Glucagon fell equivalently in both groups. Basal net hepatic glucose output was lower in NP (1.9 +/- 0.1 vs. 2.4 +/- 0.2 mg.kg(-1).min(-1), P < 0.05). Hyperinsulinemia completely suppressed hepatic glucose release in both groups (-0.4 +/- 0.2 and -0.1 +/- 0.2 mg.kg(-1).min(-1) in NP and P, respectively). More exogenous glucose was required to maintain euglycemia in NP (15.2 +/- 1.3 vs. 11.5 +/- 1.1 mg.kg(-1).min(-1), P < 0.05). Nonesterified fatty acids fell similarly in both groups. Net hepatic gluconeogenic amino acid uptake with high insulin did not differ in NP and P. Peripheral insulin action is markedly impaired in this canine model of pregnancy, whereas hepatic glucose production is completely suppressed by high circulating insulin levels.     

Glucagon secretion and autonomic signaling during hypoglycemia in late pregnancy

We examined net pancreatic norepinephrine (NE) spillover, pancreatic polypeptide (PP) release, and the decrement in C-peptide to identify factors involved in the blunted counterregulatory glucagon response in pregnancy. Conscious pregnant [pregnant hypoglycemic (Ph); 3rd trimester; n = 8] and nonpregnant [nonpregnant hypoglycemic (NPh); n = 6] dogs were studied during insulin-induced (approximately 12-fold basal insulin concentrations) hypoglycemia (plasma glucose 3.1 mM). Additional dogs were studied during hyperinsulinemic euglycemia [nonpregnant euglycemic (NPe), n = 4; pregnant euglycemic (Pe), n = 5; plasma glucose 6 mM]. Arterial glucagon concentrations declined similarly in NPe and Pe. Areas under the curve (AUCs) of the changes in glucagon and epinephrine were seven- and threefold greater in NPh than Ph (P < 0.05 between groups for both). Glucagon secretion fell below basal in NPe, Pe, and Ph but rose significantly in NPh. C-peptide declined 0.25 +/- 0.06, 0.12 +/- 0.11, 0.28 +/- 0.05, and 0.13 +/- 0.02 ng/ml in NPe, Pe, NPh, and Ph, respectively (P < 0.05, NPh vs. Ph). AUCs of NE spillover were 516 +/- 274, 265 +/- 303, 506 +/- 94, and -63 +/- 79 ng, respectively (P < 0.05, NPh vs. Ph). The AUC of PP release was approximately threefold greater in NPh than Ph (P < 0.05) but not different between euglycemic groups. The current evidence strongly suggests that the blunting of glucagon secretion during insulin-induced hypoglycemia in pregnancy is related to generalized impairment of a number of different signals, including parasympathetic and sympathoadrenal stimuli and altered sensing of circulating and/or intraislet insulin.     

Prior exercise and the response to insulin-induced hypoglycemia in the dog

To test whether hepatic insulin action and the response to an insulin-induced decrement in blood glucose are enhanced in the immediate postexercise state as they are during exercise, dogs had sampling (artery, portal vein, and hepatic vein) catheters and flow probes (portal vein and hepatic artery) implanted 16 days before a study. After 150 min of moderate treadmill exercise or rest, dogs were studied during a 150-min hyperinsulinemic (1 mU.kg(-1).min(-1)) euglycemic (n = 5 exercised and n = 9 sedentary) or hypoglycemic (65 mg/dl; n = 8 exercised and n = 9 sedentary) clamp. Net hepatic glucose output (NHGO) and endogenous glucose appearance (R(a)) and utilization (R(d)) were assessed with arteriovenous and isotopic ([3-(3)H]glucose) methods. Results show that, immediately after prolonged, moderate exercise, in relation to sedentary controls: 1) the glucose infusion rate required to maintain euglycemia, but not hypoglycemia, was higher; 2) R(d) was greater under euglycemic, but not hypoglycemic conditions; 3) NHGO, but not R(a), was suppressed more by a hyperinsulinemic euglycemic clamp, suggesting that hepatic glucose uptake was increased; 4) a decrement in glucose completely reversed the enhanced suppression of NHGO by insulin that followed exercise; and 5) arterial glucagon and cortisol were transiently higher in the presence of a decrement in glucose. In summary, an increase in insulin action that was readily evident under euglycemic conditions after exercise was abolished by moderate hypoglycemia. The means by which the glucoregulatory system is able to overcome the increase in insulin action during moderate hypoglycemia is related not to an increase in R(a) but to a reduction in insulin-stimulated R(d). The primary site of this reduction is the liver.     

Effect of training on insulin sensitivity of glucose uptake and lipolysis in human adipose tissue

Training increases insulin sensitivity of both whole body and muscle in humans. To investigate whether training also increases insulin sensitivity of adipose tissue, we performed a three-step hyperinsulinemic, euglycemic clamp in eight endurance-trained (T) and eight sedentary (S) young men [insulin infusion rates: 10,000 (step I), 20,000 (step II), and 150,000 (step III) microU x min(-1) x m(-2)]. Glucose and glycerol concentrations were measured in arterial blood and also by microdialysis in interstitial fluid in periumbilical, subcutaneous adipose tissue and in quadriceps femoris muscle (glucose only). Adipose tissue blood flow was measured by (133)Xe washout. In the basal state, adipose tissue blood flow tended to be higher in T compared with S subjects, and in both groups blood flow was constant during the clamp. The change from basal in arterial-interstitial glucose concentration difference was increased in T during the clamp but not in S subjects in both adipose tissue and muscle [adipose tissue: step I (n = 8), 0.48 +/- 0.18 mM (T), 0.23 +/- 0.11 mM (S); step II (n = 8), 0.19 +/- 0.09 (T), -0.09 +/- 0.24 (S); step III (n = 5), 0.47 +/- 0.24 (T), 0.06 +/- 0.28 (S); (T: P < 0.001, S: P > 0.05); muscle: step I (n = 4), 1. 40 +/- 0.46 (T), 0.31 +/- 0.21 (S); step II (n = 4), 1.14 +/- 0.54 (T), -0.08 +/- 0.14 (S); step III (n = 4), 1.23 +/- 0.34 (T), 0.24 +/- 0.09 (S); (T: P < 0.01, S: P > 0.05)]. Interstitial glycerol concentration decreased faster in T than in S subjects [half-time: T, 44 +/- 9 min (n = 7); S, 102 +/- 23 min (n = 5); P < 0.05]. In conclusion, training enhances insulin sensitivity of glucose uptake in subcutaneous adipose tissue and in skeletal muscle. Furthermore, interstitial glycerol data suggest that training also increases insulin sensitivity of lipolysis in subcutaneous adipose tissue. Insulin per se does not influence subcutaneous adipose tissue blood flow.     

Accretion of visceral fat and hepatic insulin resistance in pregnant rats

Insulin resistance (IR) is a hallmark of pregnancy. Because increased visceral fat (VF) is associated with IR in nonpregnant states, we reasoned that fat accretion might be important in the development of IR during pregnancy. To determine whether VF depots increase in pregnancy and whether VF contributes to IR, we studied three groups of 6-mo-old female Sprague-Dawley rats: 1) nonpregnant sham-operated rats (Nonpreg; n = 6), 2) pregnant sham-operated rats (Preg; n = 6), and 3) pregnant rats in which VF was surgically removed 1 mo before mating (PVF-; n = 6). VF doubled by day 19 of pregnancy (Nonpreg 5.1 +/- 0.3, Preg 10.0 +/- 1.0 g, P < 0.01), and PVF- had similar amounts of VF compared with Nonpreg (PVF- 4.6 +/- 0.8 g). Insulin sensitivity was measured by hyperinsulinemic-euglycemic clamp in late gestation in chronically catheterized unstressed rats. Glucose IR (mg.kg(-1).min(-1)) was highest in Nonpreg (19.4 +/- 2.0), lowest in Preg (11.1 +/- 1.4), and intermediate in PVF- (14.7 +/- 0.6; P < 0.001 between all groups). During the clamp, Nonpreg had greater hepatic insulin sensitivity than Preg [hepatic glucose production (HGP): Nonpreg 4.5 +/- 1.3, Preg 9.3 +/- 0.5 mg.kg(-1).min(-1); P < 0.001]. With decreased VF, hepatic insulin sensitivity was similar to nonpregnant levels in PVF- (HGP 4.9 +/- 0.8 mg.kg(-1).min(-1)). Both pregnant groups had lower peripheral glucose uptake compared with Nonpreg. In parallel with hepatic insulin sensitivity, hepatic triglyceride content was increased in pregnancy (Nonpreg 1.9 +/- 0.4 vs. Preg 3.2 +/- 0.3 mg/g) and decreased with removal of VF (PVF- 1.3 +/- 0.4 mg/g; P < 0.05). Accretion of visceral fat is an important component in the development of hepatic IR in pregnancy, and accumulation of hepatic triglycerides is a mechanism by which visceral fat may modulate insulin action in pregnancy.     

Familial longevity is marked by enhanced insulin sensitivity

Insulin resistance is a risk factor for various age-related diseases. In the Leiden Longevity study, we recruited long-lived siblings and their offspring. Previously, we showed that, compared to controls, the offspring of long-lived siblings had a better glucose tolerance. Here, we compared groups of offspring from long-lived siblings and controls for the relation between insulin and glucose in nonfasted serum (n = 1848 subjects) and for quantitation of insulin action using a two-step hyperinsulinemic-euglycemic clamp (n = 24 subjects). Groups of offspring and controls were similar with regard to sex distribution, age, and body mass index. We observed a positive bi-phasic linear relationship between ln (insulin) levels and nonfasted glucose with a steeper slope from 10.7mU L(-1) insulin onwards in controls compared to offspring (P = 0.02). During the clamp study, higher glucose infusion rate was required to maintain euglycemia during high-dose insulin infusion (P = 0.036) in offspring, reflecting higher whole-body insulin sensitivity. After adjustment for sex, age, and fat mass, the insulin-mediated glucose disposal rate (GDR) was higher in offspring than controls (42.5 ± 2.7 vs. 33.2 ± 2.7 micromol kg(-1) min(-1) , mean ± SE, P = 0.025). The insulin-mediated suppression of endogenous glucose production and lipolysis did not differ between groups (all P > 0.05). Furthermore, GDR was significantly correlated with the mean age of death of the parents. In conclusion, offspring from long-lived siblings are marked by enhanced peripheral glucose disposal. Future research will focus on identifying the underlying biomolecular mechanisms, with the aim to promote health in old age.     

Antecedent short-term central nervous system administration of estrogen and progesterone alters counterregulatory responses to hypoglycemia in conscious male rats

The aim of this study was to test the hypothesis that antecedent short-term administration of estradiol or progesterone into the central nervous system (CNS) reduces levels of neuroendocrine counterregulatory hormones during subsequent hypoglycemia. Conscious unrestrained male Sprague-Dawley rats were studied during randomized 2-day experiments. Day 1 consisted of an 8-h lateral ventricle infusion of estradiol (1 mug/mul; n = 9), progesterone (1 mug/mul; n = 9), or saline (0.2 mul/min; n = 10). On day 2, a 2-h hyperinsulinemic (30 pmol.kg(-1).min(-1)) hypoglycemic (2.9 +/- 0.2 mM) clamp was performed on all rats. Central administration of estradiol on day 1 resulted in significantly lower plasma epinephrine levels during hypoglycemia compared with saline, whereas central administration of progesterone resulted in increased levels of plasma norepinephrine and decreased levels of corticosterone both at baseline and during hypoglycemia. Glucagon responses during hypoglycemia were unaffected by prior administration of estradiol or progesterone. Endogenous glucose production following day 1 estradiol was significantly lower during day 2 hypoglycemia, and consequently, the glucose infusion rate to maintain the glycemia was significantly greater after estradiol administration compared with saline. These data suggest that 1) CNS administration of both female reproductive hormones can have rapid effects in modulating levels of counterregulatory hormones during subsequent hypoglycemia in conscious male rats, 2) forebrain administration of reproductive hormones can significantly reduce pituitary adrenal and sympathetic nervous system drive during hypoglycemia, 3) reproductive steroid hormones produce differential effects on sympathetic nervous system activity during hypoglycemia, and 4) reduction of epinephrine resulted in significantly blunted metabolic counterregulatory responses during hypoglycemia.     

Role of the hepatic sympathetic nerves in the regulation of net hepatic glucose uptake and the mediation of the portal glucose signal

Portal glucose delivery enhances net hepatic glucose uptake (NHGU) relative to peripheral glucose delivery. We hypothesize that the sympathetic nervous system normally restrains NHGU, and portal glucose delivery relieves the inhibition. Two groups of 42-h-fasted conscious dogs were studied using arteriovenous difference techniques. Denervated dogs (DEN; n=10) underwent selective sympathetic denervation by cutting the nerves at the celiac nerve bundle near the common hepatic artery; control dogs (CON; n=10) underwent a sham procedure. After a 140-min basal period, somatostatin was given along with basal intraportal infusions of insulin and glucagon. Glucose was infused peripherally to double the hepatic glucose load (HGL) for 90 min (P1). In P2, glucose was infused intraportally (3-4 mg.kg(-1).min(-1)), and the peripheral glucose infusion was reduced to maintain the HGL for 90 min. This was followed by 90 min (P3) in which portal glucose infusion was terminated and peripheral glucose infusion was increased to maintain the HGL. P1 and P3 were averaged as the peripheral glucose infusion period (PE). The average HGLs (mg.kg(-1).min(-1)) in CON and DEN were 55+/-3 and 54+/-4 in the peripheral periods and 55+/-3 and 55+/-4 in P2, respectively. The arterial insulin and glucagon levels remained basal in both groups. NHGU (mg.kg(-1).min(-1)) in CON averaged 1.7+/-0.3 during PE and increased to 2.9+/-0.3 during P2. NHGU (mg.kg(-1).min(-1)) was greater in DEN than CON (P<0.05) during PE (2.9+/-0.4) and failed to increase significantly (3.2+/-0.2) during P2 (not significant vs. CON). Selective sympathetic denervation increased NHGU during hyperglycemia but significantly blunted the response to portal glucose delivery.     

Simultaneous NMR microdialysis study of brain glucose metabolism in relation to fasting or exercise in the rat.

To study the impact of exercise or fasting and of subsequent glucose supplementation on glucose metabolism in rats, a spectrophotometric method was used to determine peripheral blood glucose; a technique associating (1)H-NMR spectroscopy and cortical microdialysis was also used to observe intra- plus extracellular and extracellular brain glucose variations, respectively. Compared with control animals (204 +/- 19 microM in dialysate, n = 10), exercise increased brain extracellular glucose levels to 274 +/- 22 microM (n = 8; P < 0.05), whereas fasting induced a drop in glucose levels down to 140 +/- 9 microM (n = 8; P < 0.05). After fasting, glucose supplemented by infusion increased glycemia from 7.4 +/- 0.4 to 19.9 +/- 0.8 mM (n = 10; P < 0.001), as well as extracellular and extra- plus intracellular brain glucose to 263 +/- 20% (n = 8; P < 0.001) and 342 +/- 28% (n = 8; P < 0.001), respectively, over basal for that group. After exercise, a similar infusion increased glycemia from 7. 3 +/- 0.3 to 16.8 +/- 1.1 mM (n = 10; P < 0.001), as well as extracellular and extra- plus intracellular brain glucose to 178 +/- 19% (n = 8; P < 0.001) and 244 +/- 20% (n = 8; P < 0.001), respectively, over basal for that group. These results confirmed the existence of a link between glucose level variations in peripheral and cerebral areas but also showed that exercise increased extracellular brain glucose levels despite peripheral hypoglycemia, suggesting a specific regulation mechanism of cerebral glucose metabolism during exercise.     

Brain region-dependent effects of dexamethasone on counterregulatory responses to hypoglycemia in conscious rats

The aim of this study was to determine whether activation of central type II glucocorticoid receptors can blunt autonomic nervous system counterregulatory responses to subsequent hypoglycemia. Sixty conscious unrestrained Sprague-Dawley rats were studied during 2-day experiments. Day 1 consisted of either two episodes of clamped 2-h hyperinsulinemic (30 pmol x kg(-1) x min(-1)) hypoglycemia (2.8 +/- 0.1 mM; n = 12), hyperinsulinemic euglycemia (6.2 +/- 0.1 mM; n = 12), hyperinsulinemic euglycemia plus simultaneous lateral cerebroventricular infusion of saline (24 microl/h; n = 8), or hyperinsulinemic euglycemia plus either lateral cerebral ventricular infusion (n = 8; LV-DEX group), fourth cerebral ventricular (n = 10; 4V-DEX group), or peripheral (n = 10; P-DEX group) infusion of dexamethasone (5 microg/h), a specific type II glucocorticoid receptor analog. For all groups, day 2 consisted of a 2-h hyperinsulinemic (30 pmol x kg(-1) x min(-1)) or hypoglycemic (2.9 +/- 0.2 mM) clamp. The hypoglycemic group had blunted epinephrine, glucagon, and endogenous glucose production in response to subsequent hypoglycemia. Consequently, the glucose infusion rate to maintain the glucose levels was significantly greater in this group vs. all other groups. The LV-DEX group did not have blunted counterregulatory responses to subsequent hypoglycemia, but the P-DEX and 4V-DEX groups had significantly lower epinephrine and norepinephrine responses to hypoglycemia compared with all other groups. In summary, peripheral and fourth cerebral ventricular but not lateral cerebral ventricular infusion of dexamethasone led to significant blunting of autonomic counterregulatory responses to subsequent hypoglycemia. These data suggest that prior activation of type II glucocorticoid receptors within the hindbrain plays a major role in blunting autonomic nervous system counterregulatory responses to subsequent hypoglycemia in the conscious rat.     

Rapid effects of insulin and glucose on the hepatic incorporation of gluconeogenic substrates into glyceride glycerol and glycogen

1. The hepatic utilization of gluconeogenic substrates was investigated shortly after portal infusion of either insulin or glucose in fasted rats. 2. After 20 min of insulin infusion blood glucose concentration decreased. However, neither glucose generation from precursors such as alanine or pyruvate nor their incorporation into fatty acids was modified. Under these conditions, insulin rapidly increased the incorporation of gluconeogenic substrates into the hepatic glyceride glycerol fraction. Insulin treatment led to a decrease in substrate incorporation into liver glycogen. 3. After 20 min of portal glucose infusion both plasma insulin and glucose concentrations increased and the incorporation of pyruvate into hepatic glyceride glycerol and into glycogen was also stimulated. 4. A close relationship was observed between blood glucose concentrations and the level of incorporation of gluconeogenic substrates into liver glycogen. 5. In conclusion, during fasting insulin stimulates the incorporation of gluconeogenic substrates into the glycerol moiety of hepatic glycerides, which may be the preferential mechanism through which fatty acid esterification is accomplished during refeeding. This effect of insulin is rapid and detected even before other classical modifications induced by the hormone such as gluconeogenesis inhibition or lipogenesis activation. Furthermore, the effect is not related to insulin-induced hypoglycemia since glucose infusion mimics insulin action on glyceride glycerol synthesis.     

Clinical characteristics of OGTT-derived hepatic- and muscle insulin resistance in healthy young men

[Purpose]

Insulin inhibits glucose release in the liver but increases glucose absorption in muscles. When insulin cannot properly control glucose, it negatively affects glucose metabolism and, furthermore, contributes to the onset of metabolic syndrome and chronic disease. Therefore, this study''s goal is to understand the clinical characteristics of hepatic insulin resistance and muscle insulin sensitivity in healthy young men.

[Methods]

Twenty-eight healthy young men (age 23.3 ± 0.5; mean ± SE) participated in this study. Liver function and blood lipids were measured by blood sampling from brachial vein after participants fasted the previous day. Hepatic insulin resistance and muscle insulin sensitivity were evaluated using two-hour OGTT along with surrogate index related to insulin sensitivity. The VO_2max was evaluated using cycle ergometer. Systemic insulin sensitivity was evaluated using two-hour euglycemic hyperinsulinemic clamp method.

[Results]

Hepatic insulin resistance showed a significant correlation with body fat (r = 0.609, p < 0.05). Also, hepatic insulin resistance showed a significant correlation with GOT (r = 0.467), GPT (r = 0.434), and γ-GTP (r = 0.375), reflecting liver functions, as well as showing a significant correlation with hs-CRP (r = 0.492, p < 0.05). On the other hand, muscle insulin sensitivity had no correlation with neither body fat nor liver function index (p > 0.05), and among surrogate indexes, it showed a significant correlation with Avignon (r = -0.493) and Matsuda index (r = -0.577). Glucose infusion rate, using the clamp method, showed a significant correlation with muscle insulin sensitivity (r = 0.448, p < 0.05). The VO_2max had a significant correlation with hepatic insulin resistance (r = -0.435, p < 0.05) and muscle insulin sensitivity (r = 0.474, p < 0.05), respectively.

[Conclusion]

For young men in their 20''s, the OGTT-based hepatic insulin sensitivity was an indicator of hepatic function and body fat but muscle insulin sensitivity was related to peripheral insulin sensitivity. Also, for young men, higher VO_2max indicated lower hepatic insulin resistance and higher muscle insulin sensitivity.     

Free fatty acid-induced hepatic insulin resistance: a potential role for protein kinase C-delta

The mechanisms of the impairment in hepatic glucose metabolism induced by free fatty acids (FFAs) and the importance of FFA oxidation in these mechanisms remain unclear. FFA-induced peripheral insulin resistance has been linked to membrane translocation of novel protein kinase C (PKC) isoforms, but the role of PKC in hepatic insulin resistance has not been assessed. To investigate the biochemical pathways that are induced by FFA in the liver and their relation to glucose metabolism in vivo, we determined endogenous glucose production (EGP), the hepatic content of citrate (product of acetyl-CoA derived from FFA oxidation and oxaloacetate), and hepatic PKC isoform translocation after 2 and 7 h Intralipid + heparin (IH) or SAL in rats. Experiments were performed in the basal state and during hyperinsulinemic clamps (insulin infusion rate, 5 mU. kg(-1). min(-1)). IH increased EGP in the basal state (P < 0.001) and during hyperinsulinemia (P < 0.001) at 2 and 7 h. Also, 7-h infusion of IH induced resistance to the suppressive effect of insulin on EGP (P < 0.05). Glycerol infusion (resulting in plasma glycerol levels similar to IH infusion) did not have any effect on EGP. IH increased hepatic citrate content by twofold, independent of the insulin levels and the duration of IH infusion. IH induced hepatic PKC-delta translocation from the cytosolic to membrane fraction in all groups. PKC-delta translocation was greater at 7 compared with 2 h (P < 0.05). In conclusion, 1) increased FFA oxidation may contribute to the FFA-induced increase in EGP in the basal state and during hyperinsulinemia but is not associated with FFA-induced hepatic insulin resistance, and 2) the progressive insulin resistance induced by FFA in the liver is associated with a progressive increase in hepatic PKC-delta translocation.     

Insulin regulation of steroidogenic activity in rat culture luteal cells

The effect of insulin on the function of rat luteal cells in monolayer culture was examined. Cells were obtained from PMSG-hCG primed immature rats and further cultured in serum free medium with or without insulin. The hormone produced an increase of progesterone production and maximal stimulation was achieved at 0.2 nM of insulin (100% stimulation). This effect was enhanced by addition of methyl-isobutyl-xantine (MIX 0.1 mM) to the culture medium. However, the stimulation produced by LH was not augmented by the presence of insulin. The conversion of progesterone into 20 alpha-hydroxy-progesterone was also enhanced after insulin treatment. Luteal cells were also cultured in the presence of 25-hydroxy-cholesterol (10 micrograms/ml). In these conditions insulin produced a 2-fold increase in progesterone production. Aromatase activity was assessed by adding androstenedione (0.25 microM) as substrate. Insulin produced a 14-fold stimulation of estradiol production after 24 h of culture. Insulin action was tested in short time incubations of luteal cells in a glucose free medium, in these experiments the hormone was able to induce a significant increase in progesterone and 20 alpha-hydroxy-progesterone production. These data suggest that luteal cell function is regulated by insulin and that this hormone has a direct effect on the steroidogenic process.     

Hepatic portal venous delivery of a nitric oxide synthase inhibitor enhances net hepatic glucose uptake

Hepatic portal venous infusion of nitric oxide synthase (NOS) inhibitors causes muscle insulin resistance, but the effects on hepatic glucose disposition are unknown. Conscious dogs underwent a hyperinsulinemic (4-fold basal) hyperglycemic (hepatic glucose load 2-fold basal) clamp, with assessment of liver metabolism by arteriovenous difference methods. After 90 min (P1), dogs were divided into two groups: control (receiving intraportal saline infusion; n = 8) and LN [receiving N(G)-nitro-L-arginine methyl ester (L-NAME), a nonspecific NOS inhibitor; n = 11] intraportally at 0.3 mg x kg(-1) x min(-1) for 90 min (P2). During the final 60 min of study (P3), L-NAME was discontinued, and five LN dogs received the NO donor SIN-1 intraportally at 6 mug x kg(-1) x min(-1) while six received saline (LN/SIN-1 and LN/SAL, respectively). Net hepatic fractional glucose extraction (NHFE) in control dogs was 0.034 +/- 0.016, 0.039 +/- 0.015, and 0.056 +/- 0.019 during P1, P2, and P3, respectively. NHFE in LN was 0.045 +/- 0.009 and 0.111 +/- 0.007 during P1 and P2, respectively (P < 0.05 vs. control during P2), and 0.087 +/- 0.009 and 0.122 +/- 0.016 (P < 0.05) during P3 in LN/SIN-1 and LN/SAL, respectively. During P2, arterial glucose was 204 +/- 5 vs. 138 +/- 11 mg/dl (P < 0.05) in LN vs. control to compensate for L-NAME's effect on blood flow. Therefore, another group (LNlow; n = 4) was studied in the same manner as LN/SAL, except that arterial glucose was clamped at the same concentrations as in control. NHFE in LNlow was 0.052 +/- 0.008, 0.093 +/- 0.023, and 0.122 +/- 0.021 during P1, P2, and P3, respectively (P < 0.05 vs. control during P2 and P3), with no significant difference in glucose infusion rates. Thus, NOS inhibition enhanced NHFE, an effect partially reversed by SIN-1.     

Differential effects of pharmacological liver X receptor activation on hepatic and peripheral insulin sensitivity in lean and ob/ob mice

Liver X receptor (LXR) agonists have been proposed to act as anti-diabetic drugs. However, pharmacological LXR activation leads to severe hepatic steatosis, a condition usually associated with insulin resistance and type 2 diabetes mellitus. To address this apparent contradiction, lean and ob/ob mice were treated with the LXR agonist GW-3965 for 10 days. Insulin sensitivity was assessed by hyperinsulinemic-euglycemic clamp studies. Hepatic glucose production (HGP) and metabolic clearance rate (MCR) of glucose were determined with stable isotope techniques. Blood glucose and hepatic and whole body insulin sensitivity remained unaffected upon treatment in lean mice, despite increased hepatic triglyceride contents (61.7 +/- 7.2 vs. 12.1 +/- 2.0 nmol/mg liver, P < 0.05). In ob/ob mice, LXR activation resulted in lower blood glucose levels and significantly improved whole body insulin sensitivity. GW-3965 treatment did not affect HGP under normo- and hyperinsulinemic conditions, despite increased hepatic triglyceride contents (221 +/- 13 vs. 176 +/- 19 nmol/mg liver, P < 0.05). Clamped MCR increased upon GW-3965 treatment (18.2 +/- 1.0 vs. 14.3 +/- 1.4 ml x kg(-1) x min(-1), P = 0.05). LXR activation increased white adipose tissue mRNA levels of Glut4, Acc1 and Fasin ob/ob mice only. In conclusion, LXR-induced blood glucose lowering in ob/ob mice was attributable to increased peripheral glucose uptake and metabolism, physiologically reflected in a slightly improved insulin sensitivity. Remarkably, steatosis associated with LXR activation did not affect hepatic insulin sensitivity.     

Effect of follicle size on in vitro production of steroids and insulin-like growth factor (IGF)-I,IGF-II,and the IGF-binding proteins by equine ovarian granulosa cells

Little is known regarding the hormonal regulation of granulosa cell steroidogenesis and the ovarian insulin-like growth factor (IGF) system in the mare. The objectives of this study were to determine, first, if estradiol, insulin, and/or FSH affect steroid production by equine granulosa cells (experiment 1) and, second, if the components of the IGF system are produced by equine granulosa cells in culture as well as whether estradiol, insulin, and/or FSH affects IGF and/or IGF-binding protein (IGFBP) production by equine granulosa cells (experiment 2). Granulosa cells from small (6-15 mm), medium (16-25 mm), and large (25-48 mm) follicles were collected from cyclic mares (n = 14), cultured for 2 days in medium containing 10% fetal calf serum, washed, and then treated for an additional 2 days in serum-free medium with or without added hormones. In experiment 1, large-follicle granulosa cells produced less progesterone and more estradiol than did medium- and/or small-follicle granulosa cells (P < 0.05). Progesterone production was inhibited (P < 0.05) by FSH and insulin in small- and medium- but not in large-follicle granulosa cells; estradiol was without effect. Insulin increased (P < 0.05) estradiol production in small- and medium-follicle granulosa cells but had no effect in large-follicle granulosa cells. In experiment 2, IGF-I production was inhibited (P < 0.05) by insulin across all follicle sizes but was not affected by estradiol or FSH. Granulosa cells of medium and large follicles produced more IGF-II than did granulosa cells of small follicles (P < 0.05). Insulin and FSH inhibited (P < 0.05) IGF-II production by granulosa cells of large and medium but not of small follicles; estradiol was without effect. Only IGFBP-2 and -5 were produced by equine granulosa cells. Production of IGFBP-2 was less (P < 0.10) in granulosa cells of large versus those of small and medium follicles, whereas medium-follicle granulosa cells produced more (P < 0.05) IGFBP-5 than did small- or large-follicle granulosa cells. Averaged across follicle sizes, estradiol increased (P < 0.05) IGFBP-2 production, FSH increased (P < 0.10) IGFBP-2 and -5 production, and insulin was without effect. These results indicate that IGF-I, IGF-II, IGFBP-2, and IGFBP-5 are produced by equine granulosa cells and that insulin, FSH, and estradiol play a role in the regulation of steroidogenesis and the IGF system of equine granulosa cells.     

Effect of lowering postprandial hyperglycemia on insulin secretion in older people with impaired glucose tolerance

Glucose tolerance declines with age, resulting in a high prevalence of diabetes and impaired glucose tolerance (IGT) in the older population. Hyperglycemia per se can lead to impaired beta-cell function (glucose toxicity). We tested the role of glucose toxicity in age-related beta-cell dysfunction in older people (65 +/- 8 yr) with IGT treated with the alpha-glucosidase inhibitor acarbose (n = 14) or placebo (n = 13) for 6 wk in a randomized, double-blind study. Baseline and posttreatment studies included 1) an oral glucose tolerance test (OGTT), 2) 1-h postprandial glucose monitoring, 3) a frequently sampled intravenous glucose tolerance test (insulin sensitivity, or S(I)), and 4) glucose ramp clamp (insulin secretion rates, or ISR), in which a variable glucose infusion increases plasma glucose from 5 to 10 mM. The treatment groups had similar baseline body mass index; fasting, 2-h OGTT, and 1-h postprandial glucose levels; and S(I). In these carefully matched older people with IGT, both fasting (5.7 +/- 0.2 vs. 6.3 +/- 0.2 mM, P = 0.002) and 1-h postprandial glucose levels (6.9 +/- 0.3 vs. 8.2 +/- 0.4 mM, P = 0.02) were significantly lower in the acarbose than in the placebo group. Despite this reduction of chronic hyperglycemia in the acarbose vs. placebo group, measures of insulin secretion (ISR area under the curve: 728 +/- 55 vs. 835 +/- 81 pmol/kg, P = 0.9) and acute insulin response to intravenous glucose (329 +/- 67 vs. 301 +/- 54 pM, P = 0.4) remained unchanged and impaired. Thus short-term improvement of chronic hyperglycemia does not reverse beta-cell dysfunction in older people with IGT.