Carbohydrates, fat, and insulin action

Insulin resistance is a common disorder and is seen in many conditions that are associated with increased risk for cardiovascular disease (eg, obesity, diabetes, hypertension, and cigarette smoking). The role of the diet, irrespective of degree of obesity, in modulating insulin sensitivity is uncertain. An extremely high carbohydrate-fat ratio improves insulin sensitivity whereas more moderate changes (40-60% carbohydrate) produce less convincing results. However, increased fasting concentrations of triglycerides and lower concentrations of high-density-lipoprotein (HDL) cholesterol have frequently been seen with these diets, together with lower concentrations of low-density-lipoprotein (LDL) cholesterol. High-carbohydrate diets based on foods with a low glycemic index combined with a high dietary fiber content should be evaluated. Such diets may produce the desired effects while they prevent unwanted increases in fasting triglyceride concentrations and lower HDL cholesterol.

The cGMP-inhibitable phosphodiesterase modulates glucose transport activation by insulin

To assess the role of the cGMP-inhibitable phosphodiesterase (cGI-PDE) in the action of insulin on glucose transport, adipocytes from young, lean rats were preincubated for 20 min at 37 degrees C with and without OPC 3911, a specific inhibitor of cGI-PDE, and 3-O-methylglucose uptake was measured. Insulin-stimulated glucose transport was impaired by OPC 3911 (approximately 15%) and this impairment became more pronounced in the presence of the degradable cAMP-analogue 8-bromo-cAMP (approximately 45%). This analogue alone did not significantly decrease glucose transport. Furthermore, insulin sensitivity was impaired by the combination of OPC 3911 and 8-bromo-cAMP. Maximal insulin-stimulated glucose transport in adipocytes from aging, obese rats was affected similarly by OPC 3911 and 8-bromo-cAMP, suggesting that cGI-PDE activity is not markedly altered in this insulin-resistant state. In conclusion, cGI-PDE exerts a modulating effect on the stimulatory action of insulin on glucose transport. This effect is particularly pronounced when the cellular cAMP levels are elevated.

Lactate release from the subcutaneous tissue in lean and obese men

Lactate concentration in the subcutaneous interstitial fluid and adipose tissue blood flow (ATBF, ml/100 g.min) were simultaneously measured with the microdialysis technique combined with 133Xe clearance in the abdominal and femoral subcutaneous adipose tissue in nine lean and nine obese men. The studies were performed both in the postabsorptive state and 2 h after an oral glucose load and the results compared to the lactate levels in arterialized venous plasma. After an overnight's fast, arterial lactate was 738 +/- 49 and 894 +/- 69 microM (mean +/- SE) (P < 0.05) in the lean and obese subjects, respectively. The interstitial lactate levels were significantly higher than blood lactate in both subject groups without any regional differences. Abdominal and femoral ATBF was 3.2 +/- 0.6 vs. 2.8 +/- 0.4 and 1.7 +/- 0.3 vs. 2.4 +/- 0.4 ml/100 g.min (P < 0.05) in lean and obese subjects, respectively. Mean apparent lactate release from the abdominal vs. femoral adipose tissue in the fasting state was 10.5 +/- 3.1 vs. 8.6 +/- 2.3 and 6.0 +/- 2.3 vs. 8.5 +/- 2.3 mumol/kg.min (NS) in lean and obese subjects, respectively. Both plasma and interstitial lactate levels increased significantly after an oral glucose load in both subject groups. However, apparent lactate release increased significantly only in the lean group. It is concluded that subcutaneous adipose tissue is a significant source of whole-body lactate release in the postabsorptive state and that this is further enhanced in obese subjects due to their large adipose mass.

Cellular cyclic AMP levels modulate insulin sensitivity and responsiveness--evidence against a significant role of Gi in insulin signal transduction

Treating rats with pertussis toxin (PTX) both elevated the adipocyte cAMP levels and impaired sensitivity and responsiveness to the antilipolytic effect of insulin in the presence of different beta-adrenergic agonists. However, in the presence of a fixed medium concentration of the degradable cAMP analogue, 8-bromo-cAMP, the effect of insulin was similar in PTX- and control cells. Elevating the cAMP levels in control cells either through different concentrations of the cAMP analogue or addition of adenosine deaminase impaired both insulin sensitivity and responsiveness to a similar extent as that seen in PTX-treated cells. The antilipolytic effect of insulin was exerted through the activation of the cGMP-inhibitable phosphodiesterase (cGI-PDE) as it was dose-dependently impaired by the specific cGI-PDE inhibitor OPC 3911. The results show the importance of the cellular cAMP levels in modulating insulin sensitivity and action. Gi plays a minor role, if any, for the signal transduction of the antilipolytic effect of insulin.

Smoking induces insulin resistance--a potential link with the insulin resistance syndrome

OBJECTIVES

The acute effect of smoking and snuffing on insulin sensitivity was studied in a group of healthy habitual smokers.

DESIGN

The euglycaemic clamp technique was combined with the subcutaneous injection of a bolus (0.1 U kg-1) of fast-acting insulin (Actrapid). Randomized subjects smoked either one cigarette per hour for 6 h, took one bag-packed snuff per hour for 6 h or refrained from nicotine for 48 h before as well as during the clamp.

SUBJECTS

Seven healthy smokers, four females and three males, of normal weight (BMI, mean +/- SEM, 21 +/- 0.7 kg m-2 with a range of 18.6-23.9), aged 31 +/- 2 years (range 24-35 years), who had consumed at least 20 cigarettes per day for at least 5 years were studied. They were recruited through an advertisement in a newspaper.

RESULTS

The steady-state plasma nicotine levels were similar during smoking and snuffing. The insulin and glucose levels were also similar during all three clamps. Smoking, but not snuffing, impaired insulin action (P < 0.05) mainly due to a lower peripheral glucose uptake. The mean growth hormone levels during the 6-h study were more than doubled during smoking (P < 0.01) while no significant differences were seen in the other counter-regulatory hormones.

CONCLUSION

Smoking (also in habitual smokers) acutely impairs insulin action and leads to insulin resistance. Thus, smoking can be of importance for the development of the insulin resistance syndrome associated with risk for cardiovascular disease.

Effect of prolonged hyperglycemia on growth hormone levels and insulin sensitivity in insulin-dependent diabetes mellitus

The aim of the present study was to characterize the effect of a hyperglycemic period (44 hours) on the levels of insulin-antagonistic hormones and insulin sensitivity in seven subjects with well-controlled insulin-dependent diabetes mellitus (IDDM). Hyperglycemia (approximately 15 mmol.L-1) was induced by a glucose infusion while the degree of insulinization was similar to that of the period with near normoglycemia (approximately 6.9 mmol.L-1). Insulin sensitivity was measured with hyperinsulinemic euglycemic clamps performed 4 hours before and after the periods of normoglycemia (control) and hyperglycemia. D-[3-3H]glucose was infused in the second clamp in each study to evaluate glucose production and utilization. Since growth hormone (GH) levels frequently are elevated during poor diabetic control, diurnal GH secretion was measured in blood samples continuously drawn for 24 hours during the euglycemic and hyperglycemic periods. Levels of epinephrine, norepinephrine, cortisol, and nonesterified free fatty acids (NEFA) were similar during the control and hyperglycemic periods and during the clamps. GH levels were also similar, but an abnormal diurnal secretion pattern was present with increased numbers of daytime peaks. Hyperglycemia did not reduce GH secretion in IDDM. Hyperglycemia for 44 hours induced insulin resistance (32% reduction of glucose infusion rate, P < .02). In the control study, a 21% reduction (P = .064, NS) of the glucose disposal rate (Rd) was seen, suggesting that the hospitalization period per se may also reduce insulin sensitivity. In conclusion, a period of hyperglycemia leads to insulin resistance in IDDM patients. This insulin resistance cannot be attributable to increased levels of insulin-antagonistic hormones, although an abnormal secretion pattern for GH was found.(ABSTRACT TRUNCATED AT 250 WORDS)

Peroxovanadate but not vanadate exerts insulin-like effects in human adipocytes

Vanadate and peroxovanadate were recently reported to exert maximal or even supramaximal (peroxovanadate) insulin-like effects in rat adipocytes. To evaluate the response in human cells, isolated human adipocytes were exposed to insulin or various concentrations of vanadate (0-10 mmol/l) or peroxovanadate (0-5 mmol/l). Neither vanadate nor peroxovanadate affected 125I-insulin binding and insulin sensitivity. Vanadate exerted no apparent effect on 14C-U-glucose uptake, whereas 0.1 mmol/l peroxovanadate exerted a full insulin-like response (p < 0.001). No additive response was observed by combining either vanadate or peroxovanadate with insulin. Peroxovanadate at 0.1 mmol/l was as effective as insulin in inhibiting isoproterenol-stimulated lipolysis. Neither peroxovanadate nor insulin-inhibited lipolysis stimulated by N6-monobutyryl-cAMP, an analogue which is not hydrolysed by the cAMP-phosphodiesterase. It is concluded that peroxovanadate, but not vanadate, elicits a full insulin-like response in human adipocytes.

Guar gum improves insulin sensitivity, blood lipids, blood pressure, and fibrinolysis in healthy men

A double-blind, placebo-controlled, cross-over study was carried out in 25 healthy, nonobese middle-aged men to test the effect of guar gum on glucose and lipid metabolism, blood pressure, and fibrinolysis. Ten grams guar or placebo granulate was given three times a day for 6 wk with a 2-wk run-in before and a wash-out period after. Decreases in fasting blood glucose (P < 0.001), cholesterol (P < 0.001), triglycerides (P < 0.05), plasminogen activator inhibitor-1 activity (P < 0.01), systolic blood pressure (P < 0.01), and diastolic blood pressure (P < 0.001) were seen during guar treatment when compared with placebo. Insulin sensitivity, measured with the euglycemic-clamp technique, increased (P < 0.01), adipose tissue-glucose uptake measured in vitro increased (P < 0.001), and 24-h urinary excretion of sodium and potassium increased (P < 0.001) during guar treatment. Fasting plasma insulin, renin, aldosterone, and fibrinogen concentrations as well as skeletal-muscle electrolytes, urinary catecholamines, and body weight remained unaltered. These findings support a role for guar in the treatment of the metabolic syndrome in which insulin resistance seems to play a pivotal role.

Cyclic AMP impairs the rapid effect of insulin to enhance cell-surface insulin-binding capacity in rat adipocytes

The aim of this study was to characterize further the interaction between cyclic AMP (cAMP) and insulin binding and action. Rat adipocytes were preincubated at 37 degrees C for 20 min, and after energy depletion with KCN, cell-surface 125I-insulin binding was measured. As recently reported [Eriksson, Lönnroth & Smith (1992) Diabetes 41, 707-714], preincubation with insulin rapidly increased the number of cell-surface insulin binding sites up to approximately 5-fold through recruitment within the plasma membrane. This was completely abolished by the presence of 4 mM-N6-monobutyryl cAMP (a non-hydrolysable cAMP analogue) or 1 microM-isoprenaline, without any apparent change in receptor internalization. Insulin-stimulated receptor tyrosine kinase activity was attenuated by the cAMP analogue only if the exposure of the adipocytes was prolonged to 60 min. The cellular sensitivity to insulin, assessed as 3-O-methylglucose uptake, was markedly decreased by the cAMP analogue, and this could be attributed to the impaired cell-surface binding. However, evidence for post-receptor interactions between cAMP and insulin was also found: an impairment of maximal insulin-stimulated 3-O-methylglucose transport and a delay in the rate of activation of the glucose transport system by insulin. In conclusion, these data demonstrate that beta-adrenergic stimulation and elevated cAMP levels markedly impair the ability of insulin to enhance cell-surface insulin-binding capacity. This novel interaction may be an important mechanism for the cellular insensitivity to insulin produced by cAMP.

The inhibitory GTP-binding protein (Gi) regulates the agonistic property of beta-adrenergic ligands in isolated rat adipocytes. Evidence for a priming effect of cyclic AMP

Prenalterol, an allegedly beta 1-selective adrenergic agonist with high intrinsic sympathomimetic activity (ISA), was shown to be weakly lipolytic in rat adipocytes. However, in pertussis-toxin-treated adipocytes, the ISA of prenalterol was markedly increased (from 10-20% to approx. 100% of that of isoprenaline). The cellular sensitivity was also increased (EC50 approx. 60 nM and approx. 3 microM in pertussis-toxin-treated and control cells respectively). A similar effect was seen for other partial agonists such as the beta 2-selective agonist terbutaline and for beta-adrenergic antagonists with some intrinsic activity (metoprolol, pindolol). There was no clear change in sensitivity to isoprenaline's ability to stimulate adenylate cyclase in adipocyte membranes from pertussis-toxin-treated animals but the cyclase activity was increased approx. 4-fold in the presence of 1 microM-GTP. Prenalterol stimulated lipolysis by only small increases in intracellular cyclic AMP (cAMP) levels (less than 10% of that seen with isoprenaline). Basal lipolysis was increased in cells from pertussis-toxin-treated rats and the cellular sensitivity to the non-degradable cAMP analogue, N6-monobutyryl-cAMP, was increased. In control cells, a submaximal concentration of prenalterol (0.1 microM) increased the sensitivity to the cAMP analogues, N6-monobutyryl-cAMP and 8-bromo-cAMP. A low concentration (1 mM) of 8-bromo-cAMP also increased the effect of prenalterol. Similar effects were seen when the phosphodiesterase was inhibited. Thus (1) lipolysis is extremely sensitive to small increases in intracellular cAMP; (2) the degree of activation of adenylate cyclase and thus cAMP formation is the rate-limiting step for the biological response of partial agonists; (3) the inhibitory GTP-binding protein, Gi, is an important modulator ('tissue factor') of the beta-adrenergic agonistic property; (4) low levels of cAMP exert a priming effect on protein kinase A.

Family history of diabetes in relation to different types of obesity and change of obesity during 12-yr period. Results from prospective population study of women in Göteborg, Sweden

OBJECTIVE--To assess the relationship between family history and different types of obesity and change in obesity in a longitudinal population study. RESEARCH DESIGN AND METHODS--A longitudinal population study of 1462 randomly selected women (38-60 yr old) was conducted in Göteborg, Sweden, in 1968-69. The women were restudied after 12 yr. RESULTS--A family history of diabetes in mothers but not fathers showed, in univariate analysis, a significant positive association with obesity expressed as BMI. A family history of diabetes in the mothers was inversely related to body fat distribution expressed as WHR. No other association was observed between family history of diabetes and WHR. The association with BMI was independent of age, WHR, smoking habits, blood glucose, systolic blood pressure, serum cholesterol, serum triglycerides, maternal obesity, and the incidence of diabetes during the 12-yr follow-up period. Twelve years later, in 1980-1981, an independent association still existed between family history for diabetes and BMI measured at that examination, whereas there was no relationship with WHR. Women who had a family history of diabetes increased their BMI significantly more during the 12-yr follow-up compared with the women without a family history of diabetes, whereas there was no difference for the change of WHR. Family history of coronary heart disease and family history of cancer did not correlate to any kind of obesity. CONCLUSIONS--These findings indicate that family history of diabetes is related to overall obesity but not to abdominal adiposity per se.

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

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

Insulin can rapidly increase cell surface insulin binding capacity in rat adipocytes. A novel mechanism related to insulin sensitivity

To elucidate the acute effect of insulin on its receptor, rat adipocytes were preincubated with insulin, washed with KCN to inhibit receptor cycling, and 125I-labeled insulin binding was measured. Preincubating cells from young insulin-sensitive rats with insulin increased cell surface binding up to approximately fourfold without changing apparent receptor affinity. This effect was rapid (t1/2 less than 5 min) and had a similar dose-response relationship as the effect on glucose transport. It was also energy dependent because preincubation with KCN completely abolished the effect of subsequent insulin exposure. The increased binding capacity was not recovered after cell solubilization or in partially purified receptors or isolated plasma membranes. Cells pretreated with insulin were less sensitive to the ability of trypsin to remove cell surface receptors, suggesting a conformational change of the receptors. This was also supported by the finding that the polyclonal binding in insulin-treated but not in control cells. Vanadate mimicked the effect of insulin to increase insulin binding, whereas concanavalin A, vasopressin, phorbol esters, or the adenosine analogue phenyl isopropyl adenosine was without effect. Insulin-resistant adipocytes from obese rats displayed no increase in cell surface binding after insulin treatment, despite normal tyrosine kinase activity in response to insulin. Thus, both insulin and vanadate elicit a rapid effect to markedly increase the number of cell surface insulin binding sites in intact rat adipocytes. This appears to occur independently of protein kinase C and the inhibitory GTP binding protein (Gi). Furthermore, the effect of insulin could not be demonstrated in insulin-resistant cells, suggesting that this mechanism may be of importance for the regulation of insulin sensitivity.

Glycerol production in subcutaneous adipose tissue in lean and obese humans

To estimate the regional subcutaneous glycerol production rate in normal and obese humans, the venous arterialized plasma glycerol, interstitial glycerol in the subcutaneous adipose tissue together with adipose tissue blood flow (ATBF, ml/100 g.min) were measured in the postabsorptive state and for 2 h after ingestion of 100 g of oral glucose. Eight lean and eight obese men with normal oral glucose tolerance tests were investigated with the subcutaneous microdialysis technique and 133Xe clearance. In the postabsorptive state, the interstitial glycerol concentrations in lean and obese subjects were 170 +/- 21 vs. 282 +/- 28 microM (P less than 0.01) and 156 +/- 23 vs. 225 +/- 12 microM (P less than 0.05) in the abdominal and femoral subcutaneous adipose tissue, respectively. The corresponding arterial glycerol levels were 54 +/- 4 vs. 75 +/- 14 microM (NS). Abdominal ATBF was greater in lean subjects (3.2 +/- 0.6 vs. 1.6 +/- 0.3; P less than 0.05), whereas femoral ATBF was similar in both groups (2.7 +/- 0.4 vs. 2.4 +/- 0.7). Estimated mean local glycerol release (mumol/100 g.min) was similar in the lean and obese group (0.16 +/- 0.03 vs. 0.20 +/- 0.05 and 0.18 +/- 0.02 vs. 0.17 +/- 0.04) in the abdominal and femoral site, respectively. We conclude that glycerol production from the subcutaneous tissue is increased in obesity, irrespective of adipose tissue distribution. This enhancement is due to the increased adipose tissue mass.

Insulin-antagonistic effects of pulsatile and continuous glucagon infusions in man--a comparison with the effect of adrenaline

The insulin-antagonistic effects of pulsatile (3 min pulses every 20 min) and continuous glucagon infusions were studied over 4 h with the euglycemic clamp technique in healthy subjects. Comparisons were made to the effect of a continuous adrenaline infusion. Glucose production and utilization were evaluated with D-3-3H-glucose and somatostatin was used in all studies to inhibit the endogenous release of insulin and glucagon. The amount of glucagon given during the pulsatile infusions (27% of that during continuous infusion) was adjusted so that the peak glucagon levels were the same as during the continuous infusion (372 +/- 22 and 365 +/- 20 ng/L, respectively). The insulin-antagonistic effects of pulsatile and continuous glucagon infusions were similar during the first hour and imparied the insulin effect with 44 +/- 8 and 47 +/- 6%, respectively. However, when infused continuously, the effect of glucagon declined rapidly, whereas the effect of a pulsatile infusion decreased more slowly and was evident for 3 h. Raising the glucagon level 4-fold restored the insulin-antagonistic effect again suggesting that the cells had become desensitized. In contrast, the insulin-antagonistic effect of adrenaline was persistent throughout the 4 h of the study and impaired insulin action with 54 +/- 2%. The effects of pulsatile and continuous glucagon infusions were entirely due to the stimulation of glucose production while that of adrenaline mainly was due to inhibition of peripheral glucose uptake. In conclusion, the acute stimulatory effect of glucagon on glucose production is transient but it is better maintained when given as intermittent pulses rather than as a continuous infusion. In contrast, the insulin-antagonistic effect of adrenaline on glucose uptake is persistent for at least 4 h.

Effect of free fatty acids on insulin receptor binding and tyrosine kinase activity in hepatocytes isolated from lean and obese rats

We demonstrated previously that high physiological concentrations of free fatty acids (FFA) rapidly decrease insulin binding, degradation, and action in isolated rat hepatocytes. In this study, hepatocytes from lean and obese Sprague-Dawley rats (Alab, Stockholm) were preincubated with or without 0.4 mM oleic acid, and the effect on insulin binding and tyrosine kinase activity was measured. In the absence of exogenous FFA, insulin binding was reduced in hepatocytes from obese compared with lean rats (mean +/- SE reduction 44 +/- 7%, n = 8, P less than 0.01). Furthermore, the inhibitory effect of oleic acid added to hepatocytes from lean rats (n = 8; 40 +/- 9%, P less than 0.01) was not seen in cells from obese rats. Treating obese rats with Etomoxir, a carnitine palmitoyl transferase I inhibitor, increased insulin binding to isolated hepatocytes by 41 +/- 13% (n = 5, P less than 0.05). There was no difference in total binding to partially purified insulin receptors from solubilized hepatocytes from lean and obese rats, whether cells were or were not preincubated with oleic acid. Tyrosine kinase activity of partially purified receptors from basal or insulin-stimulated cells was not affected by either obesity, treatment with Etomoxir, or preincubating the cells with oleic acid. Thus, both obesity and elevated ambient FFA levels are associated with impaired insulin cell surface binding to isolated hepatocytes, possibly through an effect of lipid oxidation on the internalization/recycling of the insulin-receptor complex without any perturbation of the receptor tyrosine kinase activity. The data suggest that the reduced insulin binding to hepatocytes from obese rats is due to elevated ambient FFA levels.

Fatty acids in the portal vein of the rat regulate hepatic insulin clearance

The effects of FFA on hepatic insulin clearance were studied in the in situ perfused rat liver. Clearance decreased with increasing body weight (age) of the rats. When FFA were added to the perfusate a 40% reduction of hepatic removal of insulin was found over the normal, physiological range (less than 1,000 mumol/liter), less pronounced in heavier rats. When perfusion was started with high concentrations of FFA, inhibition was rapidly reversible, a phenomenon again blunted in heavier rats. In contrast to FFA, different glucose concentrations in the perfusate did not affect the hepatic insulin uptake in the presence of FFA within physiological concentrations. Thus, hepatic clearance of insulin is proportional to rat weight (age) and portal FFA concentrations. Other studies have recently shown that fatty acids inhibit insulin binding, degradation, and function in isolated rat hepatocytes, and that hepatic clearance is inversely dependent on hepatic triglyceride concentrations, both inhibitions reversible by prevention of fatty acid oxidation. It is suggested that the diminished hepatic clearance of insulin in heavier (older) rats is at least partly due to their relative obesity and increased hepatic triglyceride contents. This effect as well as that of portal FFA is probably mediated via fatty acid oxidation in the liver. This mechanism may have implications for the regulation of hepatic metabolism, and peripheral insulin concentrations.

Measurements by microdialysis of free tissue concentrations of propranolol

To determine the free concentration of a drug (propranolol) in the interstitial space in humans in vivo, seven male students were investigated by microdialysis of the periumbilical subcutaneous tissue. The microdialysis catheters were calibrated in vivo and the propranolol concentration was determined by high-performance liquid chromatography. Ten hours after intake of 80 mg of propranolol, the total plasma and free interstitial propranolol concentrations were 80 +/- 43 and 7 +/- 2 nM, respectively. After a second dose, maximum concentration was reached after 80 +/- 10 min and 98 +/- 12 min, in plasma, and the concentrations in the interstitial water were 594 +/- 138 and 27 +/- 7 nM, respectively. In a second study, microdialysis was performed on the left ventricular wall in six pigs receiving an intravenous injection of 5 mg of propranolol followed by a constant propranolol infusion for 40 min (5 mg propranolol per h). The maximum concentrations of propranolol were 97 +/- 29 and 6 +/- 2 nM in plasma and in interstitial water, respectively. The data suggest that microdialysis is a useful tool for recording the free concentrations of a drug in the interstitial space.

Characterization of the insulin-antagonistic effect of growth hormone in man

The insulin-antagonistic effect of growth hormone was characterized by infusing the hormone at three different infusion rates (6, 12 or 24 mU.kg-1.min-1) for one h in 11 healthy subjects. The insulin effect was measured with the euglycaemic clamp technique combined with D-(3-3H)-glucose infusion to evaluate glucose production and utilization. A control study with NaCl (154 mmol.l-1) infusion was also performed. The insulin levels during the clamps were similar in all studies (36 +/- 0.2 mU.l-1). Peak growth hormone levels were reached at 60 min (growth hormone 6 mU.kg-1.h-1: 31 +/- 5; growth hormone 12 mU.kg-1.h-1: 52 +/- 4 and growth hormone 24 mU.kg-1.h-1; 102 +/- 8 mU.l-1). The insulin-antagonistic effect of growth hormone started after approximately 2 h, was maximal after 4-5 h (approximately 39% inhibition of glucose infusion rate between control and growth hormone 24 mU.kg-1.h-1) and lasted for 6-7 h after peak levels. The resistance was due to a less pronounced insulin effect both to inhibit glucose production and to stimulate glucose utilization. Growth hormone infusion of 12 mU.kg-1.h-1 induced a similar insulin-antagonistic effect as the higher infusion rate whereas 6 mU.kg-1.h-1 induced a smaller response with a duration of 1 h between 3-4 h after peak levels of growth hormone. The present study demonstrates that growth hormone levels similar to those frequently seen in Type 1 (insulin-dependent) diabetic patients during poor metabolic control or hypoglycaemia, have pronounced insulin-antagonistic effects.(ABSTRACT TRUNCATED AT 250 WORDS)

Prevention of inhibitory effect of free fatty acids on insulin binding and action in isolated rat hepatocytes by etomoxir

We recently demonstrated a marked inhibitory effect of high physiological concentrations of free fatty acids (FFAs) on insulin binding, degradation, and action in isolated rat hepatocytes. To elucidate the mechanism, male rats were treated for 3 days with saline (control) or etomoxir (ethyl 2-[6-(p-chlorophenoxy)hexyl]-glycidate), a prodrug, which in vivo is converted to a specific competitive inhibitor of carnitine palmitoyltransferase, and thus, lipid oxidation. Oleic acid (0.4 mM) reduced both 125-I-labeled insulin binding and insulin-stimulated [14C]aminoisobutyric acid transport approximately 40% in cells from control animals. However, this FFA concentration was without effect in cells from etomoxir-treated animals. Etomoxir increased EC50 for the inhibitory effect of oleic acid on insulin binding approximately threefold. The data indicate that the mitochondrial oxidation of fatty acids may be important for their inhibitory effect on insulin binding and action in isolated rat hepatocytes.

Amiloride inhibits insulin sensitivity and responsiveness in rat adipocytes through different mechanisms

To investigate the mechanisms by which amiloride inhibits insulin action rat adipocytes were treated with insulin and with amiloride added before or after energy depleting the cells with 2 mM KCN. Amiloride decreased the insulin response on 3-0-methylglucose transport, IGF-II- and insulin binding in both intact and energy depleted cells. In contrast, the sensitivity to insulin was inhibited by amiloride only when it was added before KCN. The effect of amiloride on insulin sensitivity was probably exerted through the impaired activation of the insulin receptor tyrosine kinase and the decreased insulin binding. However, insulin responsiveness was probably impaired through a direct effect on the plasma membrane proteins. In contrast to a recent report with pituitary cells, amiloride did not affect the activation of the inhibitory GTP-binding protein (Gi) in rat adipocytes.