Hepatic insulin resistance during chronic hyperdynamic sepsis.

Disturbances in normal glucose metabolism and homeostasis which manifest as hyperglycemia and glucose intolerance are often observed during clinical sepsis. Skeletal and myocardial muscle as well as whole body insulin resistance have been demonstrated in this laboratory and others during experimental and clinical sepsis. The existence of hepatic insulin resistance in sepsis has yet to be fully elucidated. This study was undertaken to assess hepatic insulin resistance during chronic hyperdynamic sepsis. Animals were randomly assigned to a septic (n = 7), sham (n = 7), or control (n = 7) group. Sepsis was induced in anesthetized dogs via a midline laparotomy whereby a fecal-soaked gauze sponge was placed amid the intestines. Sham animals underwent a laparotomy with mechanical manipulation of the intestines but no fecal implant. Control animals had no previous surgery. Sham and control dogs were pair-fed with the septic dogs. On postoperative day 7, after an overnight fast, animals were anesthetized, intubated, and ventilated. Via a left subcostal laparotomy, electromagnetic flow probes were placed to measure hepatic arterial and portal venous blood flows. Cannulae were placed in femoral, portal, and hepatic veins and femoral artery and used to calculate hepatic outputs of glucose, lactate, and oxygen during a basal period and hyperinsulinemic-euglycemic clamps which used intravenous insulin infusions which ranged from 0.4 to 4,000 mU/min. Mean arterial blood pressure decreased with increasing insulin concentrations in septic animals while no change was seen in control or sham animals. In control and sham animals, net hepatic glucose output (NHGO) decreased in response to increasing insulin levels. Septic animals showed no such inverse relationship and, moreover, showed no change in glucose output response to any insulin infusion, i.e., hepatic insulin unresponsiveness during sepsis. Net hepatic lactate output during basal pre-insulin period during sepsis was negative. This was in contrast to the positive outputs in control and sham animals. Glucose infusion rates (GIR) increased during insulin infusion but were not different between groups at any insulin infusion rate. These data demonstrated a hepatic insulin resistance (unresponsiveness) during chronic hyperdynamic, hypermetabolic sepsis.     

Comparison of hepatic extraction of insulin and glucagon in conscious and anesthetized dogs

Previous studies in anesthetized dogs demonstrated that basal hepatic extraction of insulin and glucagon are approximately 50 and 10-20%, respectively. Because of the stress of anesthesia and surgery, these values may not be relevant to normal physiology. In this study, hepatic extraction of insulin and glucagon were compared in conscious and anesthetized dogs. The conscious dogs had chronically implanted catheters in the portal and hepatic vein and the carotid artery and Doppler flow probes on the portal vein and hepatic artery. The mean basal portal vein insulin (42 +/- 10 and 44 +/- 7 microU/ml, respectively) and glucagon (247 +/- 37 and 219 +/- 20 pg/ml, respectively) concentrations were similar in conscious and anesthetized animals. The mean basal portal vein, but not hepatic artery, plasma flow was significantly increased in conscious dogs (462 +/- 62 vs. 294 +/- 35 ml/min, respectively). Despite the increased portal vein plasma flow in conscious animals, the basal hepatic extractions of insulin (42 +/- 6 vs. 39 +/- 6%, respectively) and glucagon (12 +/- 7 vs. 7 +/- 7%, respectively) were similar in both types of animals. Arginine and cholecystokinin-pancreozymin (CCK-PZ) infusion, which increased the amount of insulin and glucagon presented to the liver in conscious and anesthetized dogs, significantly decreased the hepatic extraction of insulin. Hepatic extraction of glucagon did not change in either group of animals. In contrast, infusion of insulin (1.0 mU/kg X min) and glucagon (4 ng/kg X min) into the portal system did not alter hepatic extraction of insulin even though the amounts of insulin and glucagon presented to that organ were similar to those obtained with arginine and CCK-PZ. The basal arterial glucose level was significantly lower in the conscious dogs but the basal hepatic glucose output was similar in the two groups. The glucose response to the infusion of arginine and CCK-PZ and exogenous hormones was significantly greater in the anesthetized animals.     

Impact of intraportal N(omega)-nitro-L-arginine infusion on hepatic glucose metabolism in total parenteral nutrition-adapted dogs: interaction with infection

During chronic total parenteral nutrition (TPN), liver glucose uptake and lactate release are markedly elevated. However, in the presence of an infection, hepatic glucose uptake and lactate release are reduced. Glucose delivery (the product of liver blood flow and inflowing glucose concentration) is a major determinant of liver glucose uptake. Hepatic blood flow is increased during infection, and increased nitric oxide (NO) biosynthesis is thought to contribute to the increase. Our aim was to determine if the increase in liver blood flow served to limit the infection-induced decrease in hepatic glucose uptake and metabolism. Chronically catheterized conscious dogs received TPN for 5 days at a rate designed to match daily basal energy requirements. On the third day of TPN administration, a sterile (SHAM) or Escherichia coli (E. coli)-containing (INF) fibrin clot was implanted in the peritoneal cavity. Forty-two hours later, somatostatin was infused with intraportal replacement of insulin (10 +/- 2 v 23 +/- 2 microU/mL, SHAM v INF, respectively) and glucagon (22 +/- 4 v 90 +/- 8 pg/mL) to match concentrations observed in sham and infected animals. Tracer and arteriovenous difference techniques were used to assess hepatic glucose metabolism. Following a 120-minute basal sampling period, sham and infected animals received either intraportal saline or N(omega)-nitro-L-arginine (L-NNA; 37 microg x kg(-1) x min(-1)) infusion for 180 minutes. Isoglycemia (120 mg/dL) was maintained with a variable glucose infusion. In the infected group L-NNA infusion decreased hepatic arterial blood flow (23.3 +/- 0.7 to 8.6 +/- 0.5 mL x kg(-1) x min(-1)), but not portal vein blood flow. Neither portal vein nor hepatic artery blood flow were altered by L-NNA infusion in the sham group. Hepatic glucose uptake and lactate metabolism were not altered by L-NNA infusion in either group. In summary, during infection, an increase in NO biosynthesis contributes to the increase in hepatic arterial blood flow, while it exerts no effect on hepatic glucose metabolism.     

Comparison of insulins detemir and glargine: effects on glucose disposal, hepatic glucose release and the central nervous system

Aims: The effects of insulins detemir (Det) and glargine (Glar) on endogenous glucose production (EGP) and net hepatic glucose output (NHGO) were compared. Methods: Arteriovenous difference and tracer ([3‐³H]glucose) techniques were employed during a two‐step hyperinsulinemic euglycaemic clamp in conscious dogs (6 groups, n = 5–6/group). After equilibration and basal sampling (0–120 min), somatostatin was infused and basal glucagon was replaced intraportally. Det or Glar was infused via portal vein (Po), peripheral vein (IV), or bilateral carotid and vertebral arteries (H) at 0.1 and 0.3 mU/kg/min (low Insulin; Glar vs. Det, respectively, 120–420 min) and 4× the low insulin rate (high insulin; 420–540 min). Results: NHGO and EGP were suppressed and glucose R_d and infusion rate were stimulated similarly by Det and Glar at both Low and high insulin with each infusion route. Non‐esterified fatty acid (NEFA) concentrations during low insulin were 202 ± 37 versus 323 ± 75 µM in DetPo and GlarPo (p < 0.05) and 125 ± 39 versus 263 ± 48 µM in DetIV and GlarIV, respectively (p < 0.05). In DetH versus GlarH, pAkt/Akt (1.7 ± 0.2 vs. 1.0 ± 0.2) and pSTAT3/STAT3 (1.4 ± 0.2 vs. 1.0 ± 0.1) were significantly increased in the liver but not in the hypothalamus. Conclusions: Det and Glar have similar net effects on acute regulation of hepatic glucose metabolism in vivo regardless of delivery route. Portal and IV detemir delivery reduces circulating NEFA to a greater extent than glargine, and head detemir infusion enhances molecular signalling in the liver. These findings indicate a need for further examination of Det's central and hepatic effects.     

Insulin oscillations per se do not affect glucose turnover parameters in normal man

To compare the metabolic effects of pulsatile vs. continuous iv insulin infusion, normal men had two glucose-controlled iv glucose infusions using the Biostator for 260 min, during which endogenous pancreatic hormone secretion was inhibited by a somatostatin infusion and glucagon was replaced by continuous glucagon infusion. The two tests were performed at 1-week intervals, during which human insulin was infused either continuously at a constant rate of 0.2 mU kg-1 min-1 or in a pulsatile manner at a rate of 1.3 mU kg-1 min-1 with a switching on/off length of 2/11 min. Blood glucose levels and glucose infusion rates (GIR) were continuously monitored, and glucose turnover was estimated using a [3H]glucose infusion. In both tests, plasma C-peptide dropped markedly, whereas plasma glucagon levels were about twice basal values. Plasma insulin averaged 7 mU liter-1 during continuous infusion and oscillated between 1.5 and 35 mU liter-1 during pulsatile delivery. During the first 30-60 min of both tests, the glucose appearance rate and endogenous glucose production (EGP) increased, resulting in moderate hyperglycemia, which completely suppressed GIR. During the last 65 min, EGP declined, while the glucose disappearance rate and the glucose MCR increased, so that GIR increased progressively to maintain the blood glucose clamped at about 5 mmol liter-1. During this period, no significant differences were found between the two modes of insulin administration for any of the parameters studied. Thus, continuous and pulsatile insulin iv infusion, resulting in physiological peripheral plasma insulin levels, altered the glucose turnover parameters equally, in particular inhibiting EGP, which was stimulated by glucagon during the first part of the study, and stimulating peripheral glucose uptake at the end of the study period.     

The effect of vagal cooling on canine hepatic glucose metabolism in the presence of hyperglycemia of peripheral origin

We examined the role of vagus nerves in the transmission of the portal glucose signal in conscious dogs. At time 0, somatostatin infusion was started along with intraportal insulin and glucagon at 4-fold basal and basal rates, respectively. Glucose was infused via a peripheral vein to create hyperglycemia ( approximately 2 fold basal). At t = 90, hollow coils around the vagus nerves were perfused with -10 degrees C or 37 degrees C solution in the vagally cooled (COOL) and sham-cooled (SHAM) groups, respectively (n = 6 per group). Effectiveness of vagal blockade was demonstrated by increase in heart rate during perfusion in the COOL vs SHAM groups (183 +/- 3 vs 102 +/- 5 beats per minute, respectively) and by prolapse of the third eyelid in the COOL group. Arterial plasma insulin (22 +/- 2 and 24 +/- 3 micro U/mL) and glucagon (37 +/- 5 and 40 +/- 4 pg/mL) concentrations did not change significantly between the first experimental period and the coil perfusion period in either the SHAM or COOL group, respectively. The hepatic glucose load throughout the entire experiment was 46 +/- 1 and 50 +/- 2 mg . kg(-1) . min(-1) in the SHAM and COOL groups, respectively. Net hepatic glucose uptake (NHGU) did not differ in the SHAM and COOL groups before (2.2 +/- 0.5 and 2.9 +/- 0.8 mg . kg(-1) . min(-1), respectively) or during the cooling period (3.0 +/- 0.5 and 3.4 +/- 0.6 mg . kg(-1) . min(-1), respectively). Likewise, net hepatic glucose fractional extraction and nonhepatic glucose uptake and clearance were not different between groups during coil perfusion. Interruption of vagal signaling in the presence of hyperinsulinemia and hyperglycemia resulting from peripheral glucose infusion did not affect NHGU, further supporting our previous suggestion that vagal input to the liver is not a primary determinant of NHGU.     

Effect of insulin on thyroid hormone blood levels in normal and thyroidectomized dogs

Administration of insulin brings about an increase of thyroid hormone blood concentration. This study investigated the possibilities that the elevation is due to increased thyroid secretion or to liberation of hormone from hepatic stores. Monocomponent insulin was administered to intact dogs by jugular perfusion (25 mU/kg/min. for 15 min.) and to thyroidectomized dogs either as single dose i.m. (1 U/kg) or by jugular perfusion (as above). All animals showed at least halving of blood glucose concentration. In normal dogs, circulating labeled thyroid hormone increased with a 2-fold T3 rise. In thyroidectomized dogs, blood T3 as well as T4 remained persistently low, while plasma 125I-T4 disappearance slope was unchanged. When intact animals were held in a glycemic steady state by infusing insulin alone at low dose (0.5 mU/kg/min. for 15 min.) or insulin at the experimental dose (25 mU/kg/min. for 15 min) with simultaneous glucose infusion, blood T3, T4 and labeled PBI showed no changes. These data suggest that the increase of circulating thyroid hormones under insulin administration is due to a stimulation of thyroid secretion. The effect of insulin appears to result from induced hypoglycemia.     

Glyburide increases insulin sensitivity and responsiveness in peripheral tissues of the rat as determined by the glucose clamp technique

The effect of chronic glyburide treatment on insulin sensitivity and responsiveness in vivo in unanesthetized male Sprague-Dawley rats was determined by the hyperinsulinemic-euglycemic clamp technique. Normal animals were surgically prepared for the clamp procedure and then gavaged with glyburide, 2 mg/kg/day, or with normal saline for 6-18 days. Basal plasma glucose concentrations were significantly lower in glyburide-treated animals compared to controls, but basal plasma insulin concentrations were the same. Rates of glucose disposal, calculated before and during insulin infusions of 2 to 40 mU/kg.min with plasma glucose concentration clamped at 125 mg/dl, were significantly greater in the glyburide-treated rats compared to controls. Insulin dose-response curves demonstrate that glyburide treatment increased both insulin sensitivity and responsiveness. Basal hepatic glucose production, estimated by D-[3-3H]Glucose infusion, was significantly greater with glyburide treatment; however the sensitivity of the liver to suppression by insulin infusions of 2 and 4 mU/kg.min was unchanged. These data suggest that the decreased basal plasma glucose concentration observed in rats chronically treated with glyburide is the result of increased glucose disposal in peripheral tissues and not associated with an increase in plasma insulin concentrations or a decrease in hepatic glucose production.     

Blood glucose control and insulin clearance in unrestrained diabetic dogs portally infused with a portable insulin delivery system

Summary Long term glucose control in pancreatectomised dogs has been obtained with portal insulin therapy. When compared to a previous similar study using peripheral infusions, 20% less exogenous insulin was required and peripheral fasting insulin levels were 30% lower. Animals (n = 5) were unrestrained, conscious and carried a programmable insulin pump for 163–224 days. In the post-absorptive state blood glucose was normal (87±5 mg/dl) as was plasma insulin (10±1 mU/l) with porcine insulin infused at a basal rate of 0.36±0.01 mU/kg/min. Following ingestion of a standard mixed meal the infusion rate was increased to 2.47±0.09 mU/kg/ min for 7 1/2 h resulting in post-prandial normalisation of blood glucose. Peripheral plasma insulin levels were twice normal during the post-prandial infusion, but only half those previously reported with peripheral infusions. Insulin clearance rates were 37 ml/kg/min in the basal state and rose significantly post-prandially. In the absence of extra meal-time insulin the clearance rate was unaffected by the resulting post-prandial hyperglycaemia and similar to values observed with insulin infused peripherally at 0.45±0.03 mU/kg/min. No significant increase in the post-prandial rate of insulin clearance relative to the fasting rate was observed with peripherally administered insulin. It was thus concluded that portal insulin replacement in pancreatectomised dogs could normalise both blood glucose and insulin in the fasting state, but post-prandial peripheral insulin levels remained elevated.     

Role of glucagon suppression on gluconeogenesis during insulin treatment of the conscious diabetic dog

Summary In seven insulin-deficient (<3 mU/l) pancreatectomised dogs, the direct and glucagon-related indirect effects of intraportal insulin infusion (350 U/kg-min; 12±1 mU/l) on glucose production were determined. Insulin was infused for 300 min during which time the plasma glucagon concentration was allowed to fall (314±94 to 180±63 ng/l) for 150 min before being replaced by an infusion intraportally at 2.6ng/kg-min (323±61 ng/l) for the remaining 150 min. Glucose production and gluconeogenesis were determined using arterio-venous difference and tracer techniques. Insulin infusion shut off net hepatic glucose output and caused the plasma glucose, blood glycerol and plasma non-esterified fatty acid levels to fall. It caused the hepatic fractional extraction of alanine (0.41 ±0.10 to 0.21±0.06) and lactate (0.32 ±0.09 to 0.04 ±0.03) to fall which increased their concentrations. When glucagon was replaced, all of these changes were fully or partly reversed with the exception of the changes in glycerol and nonesterified fatty acids. Indeed, 70% of the fall in hepatic glucose production and virtually 100% of the changes in lactate and alanine metabolism produced by basal insulin infusion were mediated by a fall in glucagon. However, the fall in hepatic uptake of glycerol was unaffected by changes in glucagon and thus gluconeogenesis from this substrate was inhibited by insulin per se probably as a result of reduced lipolysis. The latter effect of insulin may explain the incomplete restoration of hepatic glucose production when hyperglucagonaemia was re-established during insulin infusion.     

Dose-response relationship of insulin to glucose fluxes in the awake and unrestrained mouse.

The purpose of the study was to use the hyperinsulinemic-euglycemic clamp technique to generate insulin dose-response curves for insulin suppression of endogenous glucose output (EGO) and stimulation of the glucose disposal rate (GDR) in conscious unstressed mice. Five groups of male ICR (Institute for Cancer Research) mice were studied (N = 43). The animals underwent surgery for implantation of a jugular vein catheter 2 to 3 days before the clamp and were fasted 6 hours before the study. Each group was clamped at a different insulin infusion rate of 0, 2.5, 10, or 20 mU/kg/min. 3H-3-glucose was infused for measurement of the glucose turnover rate (rate of appearance [Ra]). Blood samples were collected by milking a severed tail-tip. EGO was calculated as the difference between the Ra and glucose infusion rate (GIR), and the glucose clearance rate (GCR) as the GDR divided by the plasma glucose concentration. From the curves generated, half-maximal EGO and GCR were obtained at a plasma insulin concentration of 20 to 30 microU/mL, which was achieved at an insulin infusion rate of about 4 to 5 mU/kg/min. Maximal suppression of EGO and stimulation of the GCR occurred at an insulin infusion rate of 10 mU/kg/min. The establishment of normative curves for insulin-stimulated glucose metabolism in conscious mice facilitates the evaluation of glucose metabolism in a variety of mouse models of insulin resistance.     

Loss of insulin‐mediated microvascular perfusion in skeletal muscle is associated with the development of insulin resistance

Aim: The aetiology of the development of type 2 diabetes remains unresolved. In the present study, we assessed whether an impairment of insulin‐mediated microvascular perfusion occurs early in the onset of insulin resistance. Materials and methods: Hooded Wistar rats were fed either a normal diet (ND) or a high‐fat diet (HFD) for 4 weeks. Anaesthetized animals were subjected to an isoglycaemic hyperinsulinaemic clamp (3 or 10 mU/min/kg × 2 h), and measurements were made of glucose infusion rate (GIR), hindleg glucose uptake, muscle glucose uptake by 2‐deoxy‐d ‐glucose (R′g), glucose appearance (Ra), glucose disappearance (Rd), femoral blood flow (FBF) and hindleg 1‐methylxanthine disappearance (1‐MXD, an index of microvascular perfusion). Results: Compared with ND‐fed animal, HFD feeding led to a mild increase in fasting plasma glucose and plasma insulin, without an increase in total body weight. During the clamps, HFD rats showed an impairment of insulin‐mediated action on GIR, hindleg glucose uptake, R′g, Ra, Rd and FBF, with a greater loss of insulin responsiveness at 3 mU/min/kg than at 10 mU/min/kg. The HFD also impaired insulin‐mediated microvascular perfusion as assessed by 1‐MXD. Interestingly, 1‐MXD was the only measurement that remained unresponsive to the higher dose of 10 mU/min/kg insulin. Conclusions: We conclude that the early stage of insulin resistance is characterized by an impairment of the insulin‐mediated microvascular responses in skeletal muscle. This is likely to cause greater whole body insulin resistance by limiting the delivery of hormones and nutrients to muscle.     

Effect of imidapril, an angiotensin-converting enzyme inhibitor, on fructose-induced insulin resistance in rats

The effect of imidapril, an angiotensin-converting enzyme (ACE) inhibitor, on insulin resistance was studied in high-fructose-fed rats. A sequential hyperinsulinemic euglycemic clamp procedure (insulin infusion rates: 3 and 30 mU/kg BW/min) was employed in 15 high-fructose-fed rats and 10 normal chow-fed rats under the awake condition. Five of the high-fructose-fed and five of the normal chow-fed rats, respectively, were continuously given imidapril (5 mg/kg BW/min) or saline during the two-step euglycemic clamp study. Furthermore, both imidapril and L-NMMA were infused in another 5 high-fructose-fed rats during the low-dose insulin clamp. Glucose infusion rate (GIR) was regarded as an index of the whole-body insulin action. In the low-dose insulin infusion, the high-fructose feeding resulted in a marked decrease in GIR (p<0.05). Imidapril infusion significantly raised the GIRs in the high-fructose-fed rats (p<0.05). There was no significant difference in GIRs between the chow-fed rats and the imidapril-infused rats with high-fructose diet. In the high-fructose-fed rats, L-NMMA abolished the increase in GIR induced by imidapril (p<0.05). Imidapril did not significantly change the GIRs in the chow-fed rats. In the high-dose insulin infusion, no significant difference in GIR was found among the chow-fed rats, the chow-fed rats given imidapril, the high-fructose-fed rats, and the high-fructose-fed rats given imidapril. These results suggest that, in insulin-resistant rats induced by the high-fructose feeding, an ACE inhibitor, such as imidapril, can improve the whole-body insulin-mediated glucose disposal and that this effect of imidapril is essentially linked to increased activation of NO-pathway.     

The acute effect of metformin on glucose production in the conscious dog is primarily attributable to inhibition of glycogenolysis

Although metformin has been used worldwide to treat type 2 diabetes for several decades, its mechanism of action on glucose homeostasis remains controversial. To further assess the effect of metformin on glucose metabolism, 10 42-hour-fasted conscious dogs were studied in the absence ([Con] n = 5) and presence ([Met] n = 5) of a portal infusion of metformin (0.15 mg x kg(-1) x min(-1)) over 300 minutes. Hepatic glucose production was measured by both arteriovenous-difference and tracer methods. All dogs were maintained on a pancreatic clamp and in a euglycemic state to ensure that any changes in glucose metabolism would result directly from the effects of metformin. The arterial metformin level was 21 +/- 3 microg/mL during the test period. Net hepatic glucose output (NHGO) decreased in Met dogs from 1.9 +/- 0.2 to 0.7 +/- 0.1 mg x kg(-1) x min(-1) (P < .05). NHGO remained unchanged in Con dogs (1.7 +/- 0.3 to 1.5 +/- 0.3 mg x kg(-1)min(-1)). Tracer-determined glucose production paralleled NHGO. The net hepatic glycogenolytic rate decreased from 1.0 +/- 0.2 to -0.3 +/- 0.2 mg x kg(-1) x min(-1) (P < .05) in Met dogs, but remained unchanged in Con dogs (0.8 +/- 0.2 to 0.8 +/- 0.3 mg x kg(-1) x min(-1)). No significant change in gluconeogenic flux was found in eitherthe Metgroup (1.2 +/- 0.3 to 1.3 +/- 0.3 mg x kg(-1) x min(-1)) or the Con group (1.3 +/- 0.4 to 1.0 +/- 0.3 mg x kg(-1) x min(-1)). No significant changes were observed in glucose utilization or glucose clearance in either group. In conclusion, in the normal fasted dog, (1) the primary acute effect of metformin on glucose metabolism was an inhibition of hepatic glucose production and not a stimulation of glucose utilization; and (2) the inhibition of glucose production was attributable to a decrease in hepatic glycogenolysis and not to an alteration in gluconeogenic flux.     

Blood glucose regulation using an open-loop insulin delivery system in pancreatectomized dogs given glucose infusions

Summary This study characterizes the glycaemic and insulin responses of a group of 5 anaesthetized dogs to a portal glucose infusion of 10 mg/kg/min before and after pancreatectomy. Insulin was administered intraportally to the pancreatectomized dogs according to a simple preprogrammed waveform composed of a constant basal rate of 0.35±0.02 mU/kg/min which was increased to 2.00mU/kg/min at the time of the 60 minute glucose challenge. When this square waveform was applied the glycaemic response was similar to that seen in the normal controls in the baseline and challenge periods. Blood glucose concentration differed significantly (p<0.05) only from 20 to 100 minutes after the end of the challenge when it was higher by 20±1 mg/dl. Insulin levels were not significantly different from controls. It may be concluded that normoglycaemia and normoinsulinaemia can be maintained by a simple constant rate of portal insulin delivery while the blood glucose response to a glucose infusion can be ostensibly normalized without hyperinsulinaemia simply by enhancing insulin delivery during the challenge. The feasibility of this approach implies that with further development of the preprogrammed waveforms and with a greater understanding of their characteristics portable insulin delivery systems may be realized which accomodate more physiological challenges. The portal route for insulin delivery may however be necessary if peripheral hyperinsulinism is inappropriate.     

Insulin sensitivity in zinc-depleted rats: assessment with the euglycaemic hyperinsulinic clamp technique

The present knowledge about zinc deficiency and insulin-sensitivity is not yet established. Using three groups of rats fed zinc-depleted diet (ZD) zinc adequate diets, either Pair Fed or ad libitum for a six weeks period, we measured the glucose turn over by the euglycaemic hyperinsulinaemic clamp technique coupled with tritiated glucose as tracer. The basal hepatic glucose production (HGP) and insulinaemia were lower in zinc-depleted rats. At a low rate of insulin infusion (0.6 mU/min/rat) the zinc-depleted rats did not show any difference in hepatic insulin sensitivity compared with the pair-fed animals. At high level of insulin rate (3 mU/min/rat; 9 mU/min/rat), the zinc-depleted rats exhibited a lower glucose uptake compared to the two control groups (Pair-fed and Ad libitum animals). This peripheral insulin resistance is therefore related to a modification of insulin receptors, or post receptors events in zinc deficiency.     

Pharmacokinetic approach to the estimation of hepatic removal of insulin

We simultaneously measured hepatic insulin removal invasively and estimated hepatic clearance and extraction of insulin pharmacokinetically from cardiac output and peripheral plasma concentrations (relatively) noninvasively. The invasive methods involved continuous electromagnetic measurements of portal venous and hepatic arterial blood flow and simultaneous intermittent sampling of blood from the portal and hepatic veins and femoral artery for assay of insulin concentrations. The noninvasive method assumed that hepatic plasma flow is proportional to cardiac output and that hepatic clearance is a constant fraction of total body clearance of insulin. In anesthetized dogs (n = 6), endogenous insulin was suppressed with somatostatin (800 ng/kg/min) while biosynthetic human insulin (0.25, 0.50, and 1.00 mU/kg/min) was infused to steady state during three consecutive 90-min periods. Insulin concentrations were directly proportional to the infusion rate (p less than 0.01). Hepatic blood flow accounted for 20 +/- 2% of cardiac output. Measured hepatic clearance accounted for 51 +/- 5% of total body clearance of insulin and correlated with the pharmacokinetic estimates (p less than 0.01); the estimates of hepatic clearance ranged from 91 to 114% of the measured values. We conclude that this pharmacokinetic approach, which requires only samples of peripheral blood and estimates of hepatic blood flow, may be used to study the hepatic removal of insulin relatively noninvasively.     

Characterization of the late posthypoglycemic insulin resistance in insulin-dependent diabetes mellitus

The insulin effect (6.5 to 7.5 hours) following hypoglycemia was studied with the euglycemic clamp technique in eight patients with insulin-dependent diabeteses mellitus (IDDM). The results were compared with a control study with the same insulin infusion, but where hypoglycemia was prevented by a glucose infusion. Glucose production (Ra) and utilization (Rd) were evaluated with D-(3-3H) glucose infusion. Hypoglycemia (glucose nadir, 1.5 +/- 0.1 mmol/L) caused a marked increase in cortisol and growth hormone, whereas the release of adrenaline and, in particular, glucagon was low. The plasma free insulin levels were similar in the studies, including during the clamp periods. The glucose infusion rates (GIR) were significantly lower after the hypoglycemia as compared with the control study (control, 2.4 +/- 0.3; hypoglycemia, 1.5 +/- 0.3 mg/kg x min; P less than .05). Thus, hypoglycemia induces prolonged insulin resistance. The posthypoglycemic insulin resistance during a moderate hyperinsulinemic (approximately 30 mU/L) clamp was mainly due to a decreased insulin effect on glucose utilization (control, 2.9 +/- 0.2; hypoglycemia, 2.2 +/- 0.2 mg/kg x min; P less than .02), whereas the insulin effect on glucose production was not significantly different after hypoglycemia.     

Effect of acute hyperglycemia on plasma triglyceride concentration and triglyceride secretion rate in non-fasted rats

The effect of an intravenous infusion of glucose on plasma triglyceride (TG) concentration in fed rats was determined in order to partially elucidate the mechanism of diabetes-induced hypertriglyceridemia. Glucose infused at 8 mg/kg per min caused the plasma TG concentration to be elevated significantly when compared to controls infused with saline alone. In rats which were euglycemic (clamped, insulin infused at 2.5 mU/kg per min), plasma TG concentration remained constant throughout the glucose infusion period (8 mg/kg per min). Hyperglycemic rats infused with insulin (2.5 mU/kg per min) as well as with glucose (16 mg/kg per min) were also hypertriglyceridemic. Infusion of insulin alone did not change the concentration of plasma TG over a 150 min period. Glucose was also infused (8 mg/kg per min) with somatostatin (1 micrograms/kg per min) to block endogenous production of insulin. Somatostatin infusion did not suppress glucose-induced hypertriglyceridemia. For all treatments, the net change in TG concentration was found to positively correlate with the net change in plasma glucose concentration at 150 min after the infusions (r = 0.83, P less than 0.001). The higher TG concentration in the glucose infused, hyperglycemic clamp and glucose plus somatostatin groups reflected an increased rate of TG secretion, in the presence of a lower concentration of plasma free fatty acids. These results suggest that in a non-fasted state, acute hyperglycemia increases plasma TG by stimulating hepatic TG secretion, in a manner which is independent of either plasma insulin or free fatty acids levels.     

Meal-induced enhancement in insulin sensitivity is not triggered by hyperinsulinemia in rats

Several reports confirmed the phenomenon of postprandial increase in whole-body insulin sensitivity. Although the initial step of this process is unknown, the pivotal role of postprandial hyperinsulinemia has strongly been suggested. The aim of the present study was to determine whether hyperinsulinemia per se induces insulin sensitization in healthy male Wistar rats. Rapid insulin sensitivity test (RIST) were performed in fasted, anesthetized rats before and during stable hyperinsulinemia achieved by hyperinsulinemic euglycemic glucose clamping (HEGC) with insulin infused either through the jugular vein (systemic HEGC) or into the portal circulation (portal HEGC) at a rate of 3 mU/(kg min). Insulin sensitivity expressed by the rapid insulin sensitivity (RIST) index (in milligrams per kilogram) was characterized by the total amount of glucose needed to maintain prestudy blood glucose level succeeding an intravenous bolus infusion of 50 mU/kg insulin over 5 minutes. In fasted animals, the RIST index was 37.4 ± 3.1 mg/kg. When hyperinsulinemia mimicking the postprandial state was achieved by systemic HEGC, the RIST index (39.7 ± 10.6 mg/kg) showed no significant changes as compared with the pre-HEGC values. Hyperinsulinemia achieved by portal insulin infusion also failed to modify the RIST index (35.7 ± 4.3 mg/kg). The results demonstrate that acute hyperinsulinemia, no matter how induced, does not yield any sensitization to the hypoglycemic effect of insulin.