Adenosine reversal of in vivo hepatic responsiveness to insulin

Modulation by adenosine of hepatic responsiveness to insulin was investigated in vivo in 10 healthy mongrel dogs of both sexes by determining net hepatic glucose output (NHGO) in response to insulin during the presence or absence of exogenous adenosine infusion. In addition, two separate series of experiments were performed to study the effect of adenosine (n = 7) or glucagon (n = 5) on NHGO. Basal NHGO, quantitated via the Fick principle, was significantly decreased by insulin infusion (4 U/min; 4.8 +/- 0.6 vs. -1.7 +/- 2.6 mg.kg-1.min-1, P less than 0.05). The addition of an intrahepatic arterial infusion of adenosine (10 mumol/min) during insulin infusion caused glucose output to return to basal levels (insulin, -1.7 +/- 2.6 mg.kg-1.min-1; insulin + adenosine, 3.8 +/- 1.6 mg.kg-1.min-1, P less than 0.05). The addition of intrahepatic arterial saline (control) during insulin infusion had no effect on insulin's action (insulin, -1.0 +/- 1.9 mg.kg-1.min-1; insulin + saline, -1.2 +/- 1.6 mg.kg-1.min-1, P greater than 0.05). Hepatic glucose, lactate, and oxygen deliveries were not affected during either insulin or insulin plus adenosine infusion. Intrahepatic arterial infusion of adenosine alone had no effect on NHGO, whereas intrahepatic arterial infusion of glucagon alone stimulated glucose output approximately fivefold (basal, 2.7 +/- 0.4 mg.kg-1.min-1; glucagon, 15.5 +/- 1.2 mg.kg-1.min-1, P less than 0.01). These results show that adenosine completely reversed the inhibition by insulin of NHGO. These data suggest that adenosine may act as a modulator of insulin action on the liver.     

Insulin-sensitive and insulin-resistant variants in NIDDM

To define the sequence of events that is involved in the pathogenesis of non-insulin-dependent diabetes mellitus (NIDDM), we studied 16 NIDDM individuals (15 of 16 Black patients) with a mean age of 44 yr who had been near normoglycemic for 2-91 mo while off of antidiabetic medicine. With the euglycemic insulin clamp at 100 microU/ml insulin, we defined two populations, one with normal peripheral insulin sensitivity (glucose disposal 7.51 +/- 0.97 mg.kg-1.min-1) and the other with insulin resistance (glucose disposal 3.35 +/- 0.58 mg.kg-1.min-1; P less than .001). The populations did not differ in age, degree of obesity, fasting plasma glucose, glycosylated hemoglobin, clinical presentation, or clinical course. Basal plasma insulin levels were normal in the sensitive group and significantly elevated in the resistant group. Islet cell cytoplasmic antibodies were absent in all patients. Insulin action on the liver was normal in both groups. Basal hepatic glucose production measured with D-[3-3H]glucose was lower in the insulin-resistant group (1.53 +/- 0.11 mg.kg-1.min-1) than in the insulin-sensitive group (1.88 +/- 0.06 mg.kg-1.min-1) or normal control subjects (1.93 +/- 0.05 mg.kg-1.min-1). The decreased basal hepatic glucose production appeared to be secondary to the twofold higher fasting plasma insulin level seen in the insulin-resistant group. The insulin concentration necessary to suppress basal hepatic glucose production by 50% was 29.6 microU/ml in the insulin-sensitive group and 30.5 microU/ml in the insulin-resistant group.(ABSTRACT TRUNCATED AT 250 WORDS)     

Metabolic effects of low-dose insulin therapy on glucose metabolism in diabetic ketoacidosis

The effect of low-dose insulin treatment (5-10 U/h) on hepatic glucose production (HGP) and peripheral glucose disposal was determined in 5 insulin-dependent diabetes mellitus (IDDM) subjects who were admitted with diabetic ketoacidosis (DKA; plasma glucose 598 +/- 50 mg/dl, blood pH 7.20 +/- 0.06, plasma bicarbonate 12 +/- 2 meq/L). Basal hepatic glucose production (4.3 +/- 0.5 mg.kg-1.min-1) in the DKA patients was 1.5- to 2-fold greater (P less than .01) than in controls (2.1 +/- 0.1 mg.kg-1.min-1) and nonketotic IDDM subjects (2.9 +/- 0.3 mg.kg-1.min-1), whereas tissue glucose disposal was significantly reduced (1.7 +/- 0.1 vs. 2.1 +/- 0.1 mg.kg-1.min-1, P less than .05). After the institution of insulin therapy (1 mU.kg-1.min-1), the plasma glucose concentration fell at the rate of 60 +/- 5 mg.dl-1.h-1 to reach a value of 220 +/- 10 mg/dl, which was maintained constant for 2 h (insulin-clamp technique). Blood pH (7.21 +/- 0.06 to 7.35 +/- 0.05) and plasma bicarbonate (12 +/- 3 to 18 +/- 2 meq/L) both increased during insulin therapy (P less than .01). The decline in plasma glucose concentration during insulin therapy primarily resulted from a suppression of HGP (from 4.3 +/- 0.5 to 1.7 +/- 0.2 mg.kg-1.min-1, P less than .01) and to a lesser extent from the stimulation of tissue glucose disposal (1.7 +/- 0.2 to 2.6 +/- 0.3 mg.kg-1.min-1, P less than .01). At this time, urine glucose excretion decreased from 2.6 +/- 0.2 to 0.6 +/- 0.1 mg.kg-1.min-1.(ABSTRACT TRUNCATED AT 250 WORDS)     

Induction of insulin resistance in vivo by amylin and calcitonin gene-related peptide

During hyperinsulinemic glucose-clamp studies, intravenous infusion of calcitonin gene-related peptide (CGRP) in rats antagonized the ability of insulin to stimulate peripheral glucose disposal by 52% (196 +/- 7.2 vs. 105 +/- 10.5 mumol.kg-1.min-1, P less than 0.05) and to inhibit hepatic glucose output by 54% (P less than 0.01). CGRP also inhibited the in vitro effects of insulin to stimulate hexose uptake in cultured BC3H1 myocytes at all insulin concentrations studied. Amylin is a peptide isolated from amyloid deposits in pancreatic islets of type II (non-insulin-dependent) diabetic subjects, is present in normal beta-cells, and bears a striking homology to CGRP. When synthetic human amylin was infused during clamp studies, it inhibited the ability of insulin to stimulate glucose disposal by 56% (96.9 +/- 9.4 vs. 42.4 +/- 5.0 mumol.kg-1.min-1, P less than 0.05) and to suppress hepatic glucose output by 64%. Therefore, amylin and CGRP can cause insulin resistance in vivo and may be implicated in insulin-resistant states such as type II diabetes mellitus.     

Insulin action in obese non-insulin-dependent diabetics and in their isolated adipocytes before and after weight loss

To determine the effects of weight loss on insulin action in patients with non-insulin-dependent diabetes mellitus (NIDDM) and in their isolated adipocytes, we studied nine weight-stabilized Pima Indians [7 females and 2 males; age 39 +/- 3 yr; wt 99.9 +/- 8.2 kg; body fat 39 +/- 2% (means +/- SE)] before and after a 6.7 +/- 1.3-kg weight loss and decrease in fasting plasma glucose from 250 +/- 11 to 148 +/- 15 mg/dl. In vivo insulin action was measured during a 3-insulin-step, hyperglycemic (approximately 310 mg/dl) clamp with somatostatin (250 micrograms/h). At a clamp plasma insulin concentration of 10 microU/ml, glucose disposal rates did not change after weight loss; at approximately 100 microU/ml, glucose disposal rates increased by 21% [from 4.3 +/- 0.2 to 5.3 +/- 0.4 mg X min-1 X kg-1 of fat-free mass (FFM), P less than .01] mostly due to increased carbohydrate oxidation rates (2.0 +/- 0.3 to 2.8 +/- 0.3 mg X min-1 X kg-1 FFM, P less than .02); at 2400 microU/ml, glucose disposal rates increased by 37% (11.4 +/- 0.6 to 15.6 +/- 1.4 mg X min-1 X kg-1 FFM, P less than .02) mostly due to increased nonoxidative carbohydrate disposal rates or storage (7.5 +/- 0.6 to 10.9 +/- 1.3 mg X min-1 X kg-1 FFM, P less than .04). Sensitivity of glucose disposal to insulin in the physiologic range (measured as change in glucose disposal rate per unit change in insulin concentration between clamps at approximately 10 and approximately 100 microU/ml) was very low in these diabetic subjects and did not change after weight loss. Adipocyte cell size, basal and maximal insulin-stimulated glucose transport, and half-maximal rate for transport did not change after weight loss. The data suggest that insulin in the physiologic range has no apparent effect on glucose disposal in patients with NIDDM before or after weight loss. However, a moderate weight loss is associated with enhanced capacity to transport and metabolize glucose in vivo. The discrepancy between in vivo and in vitro results suggests that the adipocyte may not always reflect in vivo insulin action. Diabetes 36:227-36, 1987.     

Increased basal glucose production and utilization in nondiabetic first-degree relatives of patients with NIDDM

To characterize the abnormalities in basal glucose homeostasis in people who are at increased risk for non-insulin-dependent diabetes mellitus (NIDDM), we measured the rates of basal hepatic glucose output (HGO), glucose disappearance, and metabolic clearance of glucose (MCR) in 27 nondiabetic first-degree relatives of NIDDM patients and 16 age-, sex-, and weight-matched healthy control subjects with no family history of NIDDM. Mean fasting plasma glucose was significantly lower (P less than 0.05) in control subjects (mean +/- SE 77 +/- 2 mg/dl) than in relatives (84 +/- 2 mg/dl). Mean basal insulin levels were not significantly different between relatives and control subjects (10.0 +/- 1.5 vs. 7.7 +/- 1.0 microU/ml). Mean basal HGO was significantly lower in control subjects compared with relatives (1.83 +/- 0.07 vs. 2.20 +/- 0.10 mg.kg-1.min-1, P less than 0.05). Mean MCR was similar in relatives (2.58 +/- 0.12 mg.kg-1.min-1) and control subjects (2.35 +/- 0.09 mg.kg-1.min-1). In summary, this study demonstrates that basal hepatic glucose production and glucose utilization are increased in glucose-tolerant first-degree relatives compared with healthy control subjects. We conclude that impaired basal hepatic glucose regulation rather than glucose disposal is present as an early defect in glucose-tolerant first-degree relatives of NIDDM patients.     

Metformin normalizes nonoxidative glucose metabolism in insulin-resistant normoglycemic first-degree relatives of patients with NIDDM.

Many first-degree relatives of patients with non-insulin-dependent diabetes mellitus (NIDDM) are characterized by insulin resistance. Because metformin improves peripheral insulin sensitivity, we examined the acute effect of metformin and placebo on glucose and lipid metabolism in nine insulin-resistant first-degree relatives of NIDDM patients with the euglycemic insulin-clamp technique combined with indirect calorimetry and infusion of [3-3H]glucose. Either placebo or 500 mg metformin was taken in random order twice the day before and once 1 h before the clamp. Nine healthy individuals without family history of diabetes served as control subjects. Basal plasma glucose was normal and did not differ between the metformin and the placebo study (4.8 +/- 0.2 vs. 5.0 +/- 0.2 mM) and neither did basal hepatic glucose production (10.59 +/- 0.54 vs. 10.21 +/- 0.80 mumol.kg-1.min-1). Insulin-stimulated glucose disposal was significantly increased by 25% after metformin compared with placebo (26.67 +/- 2.87 vs. 21.31 +/- 1.73 mumol.kg-1.min-1, P less than 0.05). The enhancement in glucose utilization was primarily due to normalization of nonoxidative glucose disposal (from 8.02 +/- 1.35 to 15.07 +/- 2.69 mumol.kg-1.min-1, P less than 0.01, vs. 15.65 +/- 2.72 mumol.kg-1.min-1 in control subjects). In contrast, glucose oxidation during the clamp was slightly lower after metformin compared with both placebo (11.59 +/- 0.83 vs. 13.30 +/- 1.00 mumol.kg-1.min-1, P = 0.06) and healthy control subjects (15.68 +/- 1.38 mumol.kg-1.min-1, P less than 0.05). We conclude that acutely administered metformin improves peripheral insulin sensitivity in insulin-resistant normoglycemic individuals primarily by stimulating the nonoxidative pathway of glucose metabolism.     

In vivo insulin resistance induced by amylin primarily through inhibition of insulin-stimulated glycogen synthesis in skeletal muscle

We examined the in vivo mechanisms of amylin-induced resistance in concious rats (n = 18). During 180-min euglycemic insulin-clamp (21.5 pmol.kg-1.min-1) studies, amylin (50, 200, or 500 pmol.kg-1.min-1; plasma concentration from 3 x 10(-10) to 9 x 10(-9) M) infusion determined a 19-27% reduction in glucose uptake (117.8 +/- 7.0 vs. 145.8 +/- 11.0, 107.1 +/- 9.2 vs. 145.1 +/- 6.7, and 105.0 +/- 7.2 vs. 144.4 +/- 7.0 mumol.kg-1.min-1 at 50, 200, or 500 pmol.kg-1.min-1, respectively, P less than 0.01) versus insulin alone, whereas 10-pmol.kg-1.min-1 amylin infusion (plasma concn 5 x 10(-11) M) failed to affect insulin-mediated glucose disposal. After amylin infusion, the contribution of whole-body glycolysis to overall glucose disposal increased from 43-48 to 62-79%, whereas muscle glycogen synthesis decreased significantly at all peptide concentrations greater than 3 x 10(-10) M, completely accounting for the decrease in glucose uptake. Skeletal muscle glucose-6-phosphate concentration rose from 0.219 +/- 0.038 mumol/g (insulin alone) to 0.350 +/- 0.018, 0.440 +/- 0.020, and 0.505 +/- 0.035 mumol/g (insulin plus amylin at 50, 200, or 500 pmol.kg-1.min-1, P less than 0.01). Suppression of hepatic glucose production by insulin was unaffected by a 50-pmol.kg-1.min-1 amylin infusion (18.5 +/- 4.3 vs. 21.7 +/- 2.9 mumol.kg-1.min-1), whereas it was slightly but significantly impaired by amylin infusion at 200 pmol.kg-1.min-1 (17.8 +/- 3.9 vs. 24.7 +/- 4.5 mumol.kg-1.min-1, P less than 0.05).(ABSTRACT TRUNCATED AT 250 WORDS)     

Influence of dietary fat composition on development of insulin resistance in rats. Relationship to muscle triglyceride and omega-3 fatty acids in muscle phospholipid

High levels of some but not all dietary fats lead to insulin resistance in rats. The aim of this study was to investigate the important determinants underlying this observation. Insulin action was assessed with the euglycemic clamp. Diets high in saturated, monounsaturated (omega-9), or polyunsaturated (omega-6) fatty acids led to severe insulin resistance; glucose infusion rates [GIR] to maintain euglycemia at approximately 1000 pM insulin were 6.2 +/- 0.9, 8.9 +/- 0.9, and 9.7 +/- 0.4 mg.kg-1. min-1, respectively, versus 16.1 +/- 1.0 mg.kg-1.min-1 in chow-fed controls. Substituting 11% of fatty acids in the polyunsaturated fat diet with long-chain omega-3 fatty acids from fish oils normalized insulin action (GIR 15.0 +/- 1.3 mg.kg-1.min-1). Similar replacement with short-chain omega-3 (alpha-linolenic acid, 18:3 omega 3) was ineffective in the polyunsaturated diet (GIR 9.9 +/- 0.5 mg.kg-1.min-1) but completely prevented the insulin resistance induced by a saturated-fat diet (GIR 16.0 +/- 1.5 mg.kg-1.min-1) and did so in both the liver and peripheral tissues. Insulin sensitivity in skeletal muscle was inversely correlated with mean muscle triglyceride accumulation (r = 0.95 and 0.86 for soleus and red quadriceps, respectively; both P less than 0.01). Furthermore, percentage of long-chain omega-3 fatty acid in phospholipid measured in red quadriceps correlated highly with insulin action in that muscle (r = 0.97). We conclude that 1) the particular fatty acids and the lipid environment in which they are presented in high-fat diets determine insulin sensitivity in rats; 2) impaired insulin action in skeletal muscle relates to triglyceride accumulation, suggesting intracellular glucose-fatty acid cycle involvement; and 3) long-chain omega-3 fatty acids in phospholipid of skeletal muscle may be important for efficient insulin action.     

Acute Pain Induces Insulin Resistance in Humans

Background: Painful trauma results in a disturbed metabolic state with impaired insulin sensitivity, which is related to the magnitude of the trauma. The authors explored whether pain per se influences hepatic and extrahepatic actions of insulin.

Methods: Ten healthy male volunteers underwent two randomly sequenced hyperinsulinemic-euglycemic (insulin infusion rate, 0.6 mU [middle dot] kg-1 [middle dot] min-1 for 180 min) clamp studies 4 weeks apart. Self-controlled painful electrical stimulation was applied to the abdominal skin for 30 min, to a pain intensity of 8 on a visual analog scale of 0-10, just before the clamp procedure (study P). In the other study, no pain was inflicted (study C).

Results: Pain reduced whole-body insulin-stimulated glucose uptake from 6.37 +/- 1.87 mg [middle dot] kg-1 [middle dot] min-1 (mean +/- SD) in study C to 4.97 +/- 1.38 mg [middle dot] kg-1 [middle dot] min-1 in study P (P < 0.01) because of a decrease in nonoxidative glucose disposal, as determined by indirect calorimetry (2.47 +/- 0.88 mg [middle dot] kg-1 [middle dot] min-1 in study P vs. 3.41 +/- 1.03 mg [middle dot] kg-1 [middle dot] min-1 in study C;P < 0.05). Differences in glucose oxidation rates were not statistically significant. The suppression of isotopically determined endogenous glucose output during hyperinsulinemia tended to be decreased after pain (1.67 +/- 0.48 mg [middle dot] kg-1 [middle dot] min-1 in study P vs. 2.04 +/- 0.45 mg [middle dot] kg-1 [middle dot] min-1 in study C;P = 0.06). Pain elicited a twofold to threefold increase in serum cortisol (P < 0.01), plasma epinephrine (P < 0.05), and serum free fatty acids (P < 0.05). Similarly, circulating concentrations of glucagon and growth hormone tended to increase during pain.     

Metabolic effects of hyperglycemia and hyperinsulinemia on fate of intracellular glucose in NIDDM

Hyperglycemia in non-insulin-dependent diabetes mellitus (NIDDM) stimulates peripheral glucose uptake, which tends to compensate for impaired insulin-mediated glucose uptake. The metabolic fate of glucose and suppression of fat oxidation may differ, however, when glucose uptake is stimulated primarily by insulin or hyperglycemia. To address this issue, three hyperinsulinemic glucose-clamp studies were performed in combination with indirect calorimetry in seven nonobese subjects with NIDDM. In the first two experiments, when glucose uptake was matched at approximately 8 mg.kg-1 fat-free mass (FFM).min-1 with primarily hyperinsulinemia (1350 +/- 445 pM) or hyperglycemia (20.8 +/- 1.8 mM), identical rates of glucose oxidation (3.21 +/- 0.29 and 3.10 +/- 0.23 mg.kg-1 FFM.min-1, NS) and nonoxidative glucose metabolism (5.19 +/- 0.75 and 5.46 +/- 0.61 mg.kg-1 FFM.min-1, NS) were achieved. When glucose uptake was increased further to 11.11 +/- 0.36 mg.kg-1 FFM.min-1 with less insulin (625 +/- 70 pM) and hyperglycemia, glucose oxidation (3.85 +/- 0.26 mg.kg-1 FFM.min-1) and nonoxidative glucose metabolism (7.26 +/- 0.51 mg.kg-1 FFM.min-1) rose significantly (both P less than 0.05 from matched studies at lower rates of glucose uptake). During all glucose-clamp studies, free fatty acids were comparably suppressed by 40-46% (all P less than 0.005 vs. basal values), whereas fat oxidation was suppressed by 70-80% (all P less than 0.005 vs. basal values). A strong negative correlation was observed between rates of glucose and fat oxidation (r = -0.88, P less than 0.001) when all studies were combined.(ABSTRACT TRUNCATED AT 250 WORDS)     

Metabolic effects of IGF-I in diabetic rats

Insulinlike growth factor I (IGF-I) stimulates glucose utilization (GU) in nondiabetic rats. We compared the effects of IGF-I and insulin on glucose metabolism in control (fed plasma glucose 7.7 +/- 0.1 mM, n = 30) and partially (90%) pancreatectomized diabetic (plasma glucose 18.4 +/- 0.8 mM, n = 30) awake unstressed rats. IGF-I was infused at 0.65 or 1.96 nmol.kg-1.min-1 and insulin at 22 or 29 pmol.kg-1.min-1 in combination with [3-3H]glucose while euglycemia was maintained by a variable glucose infusion. In controls, GU during the 0.65- and 1.96-nmol.kg-1.min-1 IGF-I infusions (127 +/- 7 and 168 +/- 4 mumol.kg-1.min-1, respectively) was similar to rates observed during the 22- and 29-pmol.kg-1.min-1 insulin infusions (121 +/- 2 and 156 +/- 5 mumol.kg-1.min-1). Whole-body glycolytic rate (3H2O generation) and muscle glycogen synthetic rate were identical during insulin and IGF-I infusions. In diabetic rats, GU was reduced by 30% versus control rats (P less than 0.01) during both the low-dose (88 +/- 7 vs. 121 +/- 7 mumol.kg-1.min-1) and higher-dose (109 +/- 4 vs. 156 +/- 5 mumol.kg-1.min-1) insulin clamps. The defect in insulin action involved both muscle glycogen synthesis and glycolysis. In diabetic rats, IGF-I elicited rates of GU similar to controls (115 +/- 10 and 164 +/- 12 mumol.kg-1.min-1 during the 0.65- and 1.96-nmol.kg-1.min-1 infusions, respectively) and corrected the intracellular defects in glycogen synthesis and glycolysis.(ABSTRACT TRUNCATED AT 250 WORDS)     

Insulin is required for the liver to respond to intraportal glucose delivery in the conscious dog.

To determine whether insulin is essential for the augmented hepatic glucose uptake observed in the presence of intraportal glucose delivery, SRIF was used to induce acute insulin deficiency in 5 conscious dogs, and glucose was infused into the portal vein or a peripheral vein in two sequential, randomized periods. Insulin and C-peptide levels were below the limits of detection after SRIF infusion, and the load of glucose presented to the liver was approximately doubled and equivalent during the portal and peripheral periods. Net hepatic glucose output was 2.9 +/- 0.9 and 2.1 +/- 1.1 mumol.kg-1.min-1 during portal and peripheral glucose delivery, respectively. In an additional set of protocols, pancreatectomized dogs were used to investigate the effects of prolonged insulin deficiency (n = 5) and acute insulin replacement (n = 4) on the hepatic response to intraportal glucose delivery. In the prolonged insulin deficiency protocol, SRIF was used to lower glucagon and thereby reduce circulating glucose levels, and glucose was infused into the portal or peripheral circulations in two sequential, randomized periods. As with acute insulin deficiency, net hepatic glucose output was still evident and similar (3.6 +/- 1.1 and 4.2 +/- 1.3 mumol.kg-1.min-1) during portal and peripheral glucose delivery, respectively. When the pancreatectomized dogs were restudied using a similar protocol, but one in which insulin was replaced (4X-basal), and the glucose load to the liver was matched to that which occurred in the prolonged insulin deficiency protocol, net hepatic glucose uptake was 23.6 +/- 6.1 mumol.kg-1.min-1 during portal glucose delivery but only 10.3 +/- 3.5 mumol.kg-1.min-1 during peripheral glucose delivery. These results suggest that the induction of net hepatic glucose uptake and the augmented hepatic response to intraportal glucose delivery require the presence of insulin.     

Efficacy of pulsatile versus continuous insulin administration on hepatic glucose production and glucose utilization in type I diabetic humans

To evaluate the role of pulsatile insulin administration, hepatic glucose production (HGP) and utilization were studied in type I diabetic patients in the fasting state and during a euglycemic insulin (1 mU X kg-1 X min-1 i.v.) clamp with continuous and pulsatile insulin administration. In the latter study, insulin was infused at twice the continuous rate with 3-min-on/7-min-off intervals, thereby reducing total insulin delivery by 40%. The restraining effect of pulsatile insulin on basal HGP (1.91 +/- 0.35 mg X kg-1 X min-1) was equipotent to continuous insulin exposure (1.80 +/- 0.17 mg X kg-1 X min-1). During the insulin-clamp studies, HGP was equally suppressed by pulsed (0.62 +/- 0.12 mg X kg-1 X min-1) as by continuous insulin infusion (0.63 +/- 0.12 mg X kg-1 X min-1). Insulin-stimulated glucose utilization was not significantly altered in either study (2.55 +/- 0.27 vs. 2.92 +/- 0.23 mg X kg-1 X min-1). When in further studies the total insulin dose given during the pulsatile study was infused continuously (0.6 mU X kg-1 X min-1), HGP in the basal state and residual HGP during the insulin-clamp study were 25-30% higher than in the pulsatile experiments, whereas glucose utilization was not significantly different. In conclusion, by reducing total hormone delivery by up to 40%, but given in a pulsatile fashion, insulin is equally potent in controlling HGP as continuous insulin administration. This greater efficacy of pulsatile exposure in suppressing HGP is accompanied by an equipotent effect on glucose utilization.(ABSTRACT TRUNCATED AT 250 WORDS)     

Reduced in vivo biological activity of in vitro glycosylated insulin

We evaluated the in vivo biological activity of in vitro extensively glycosylated insulin (GI) with the euglycemic-hyperinsulinemic glucose-clamp technique in postabsorptive nondiabetic subjects. Insulin-mediated glucose disposal was approximately 30% lower (P less than .03) with GI (9.2 +/- 1.2 mg.kg-1.min-1, mean +/- SE) than with the nonglycosylated hormone (12.6 +/- 0.7 mg.kg-1.min-1) at comparable plasma insulin concentrations (approximately 90 microU/ml). Binding of GI to a specific receptor on circulating cells (erythrocytes and monocytes) was normal. We conclude that in vitro extensive glycosylation of insulin reduces its biological activity in vivo, as reflected by insulin-mediated glucose disposal, probably at a postreceptor level.     

Quantitative aspects of glucose production and metabolism in healthy elderly subjects

The metabolic basis for the reduced glucose tolerance that occurs during aging in humans has been explored with the aid of a primed constant intravenous infusion method of labeled glucose (6-3H; 6,6,2H- and U-13C-glucose). Healthy young adult men and women (24 +/- 3 yr) and elderly men and women (75 +/- 4 yr) participated in a series of studies designed to quantify rates of plasma glucose appearance, oxidation, and recycling while subjects were in the postabsorptive (basal) state and to determine rates of hepatic glucose production and glucose disappearance in response to intravenous glucose at approximately 1 and 2 mg x kg-1min-1 and also 4 mg x kg-1min-1 without or with a simultaneous infusion of insulin to maintain normoglycemia. Basal rates of glucose production were 2.41 +/- 0.06 and 2.18 /+/- 0.05 mg x kg-1min-1 in the young adults and elderly, respectively (P less than 0.05). Recycling of glucose carbon and glucose oxidation rates did not differ significantly between the two age groups. Infusion of unlabeled glucose reduced hepatic glucose production to the same extent in the two groups, indicating that the mechanisms responsible for altered hepatic glucose production with intravenous glucose administration remain intact during human aging. Plasma insulin changes were similar in young adult and elderly subjects receiving 4 mg x kg-1min-1 unlabeled glucose except that the higher plasma glucose levels in the elderly were associated with higher insulin levels. For elderly subjects, the amount of exogenous insulin required to maintain normoglycemia at the 4 mg x kg-1min-1 glucose infusion rate was about twice that necessary in young adults.     

Dietary substitution of medium-chain triglycerides improves insulin-mediated glucose metabolism in NIDDM subjects.

Dietary medium-chain triglycerides (MCT) may improve insulin-mediated glucose metabolism. To examine this possibility, 10 non-insulin-dependent diabetes mellitus (NIDDM) patients, 4 hypertriglyceridemic, and 6 normotriglyceridemic nondiabetic control subjects were examined with a 5-day cross-over design, in which the short-term metabolic effects of a 40% fat diet containing 77.5% of fat calories as MCT were compared with an isocaloric long-chain triglyceride-containing diet. In diabetic patients, MCT failed to alter fasting serum glucose concentrations but reduced preprandial glycemic excursions by 45% (F = 7.9, P less than 0.01). On MCT, the amount of glucose needed to maintain euglycemia during an intravenous insulin infusion was increased in diabetic subjects by 30%, in hypertriglyceridemic subjects by 30%, and in normotriglyceridemic control subjects by 17%. MCT increased mean +/- SE insulin-mediated glucose disposal (4.52 +/- 0.56 vs. 2.89 +/- 0.21 mg.kg-1.min-1; n = 3, P less than 0.05) but failed to alter basal glucose metabolism or insulin-mediated suppression of hepatic glucose output. Metabolic responses to MCT were observed independent of sulfonylurea therapy or severity of fasting hyperglycemia. No change in fasting serum insulin or triglyceride concentrations were seen with MCT administration. Although MCT increased mean fasting serum beta-hydroxybutyrate levels from 0.10 +/- 0.03 to 0.26 +/- 0.06 mM (P less than 0.05) in normotriglyceridemic nondiabetic subjects, no change was seen in diabetic patients. Thus, MCT-containing diets increased insulin-mediated glucose metabolism in both diabetic patients and nondiabetic subjects. In diabetic subjects, this effect appears to be mediated by increases in insulin-mediated glucose disposal.(ABSTRACT TRUNCATED AT 250 WORDS)     

Increased basal glucose production and utilization in children with recent obesity versus adults with long-term obesity

To characterize the abnormalities of glucose homeostasis and insulin action early in the course of human obesity, we studied in vivo glucose kinetics in seven children who were recently massively overweight. At time of study they were gaining weight at a rate of 13.5 +/- 1.4 kg/yr. They were compared with six age-matched control subjects. Six adults with long-term obesity and five normal adults were studied in parallel. The obese children and adults were normoglycemic and hyperinsulinemic. We found that glucose production and utilization were remarkably higher in obese children (295 +/- 18 mg/min; 7.6 mg.kg-1 lean body mass.min-1) than in control children (129 +/- 13 mg/min; 4.4 mg.kg-1 lean body mass.min-1, P less than .01) and obese adults (151 +/- 8 mg/min; 3.1 +/- 0.3 mg.kg-1 lean body mass.min-1, P less than .01). Obese adults had normal rates of glucose production and utilization. Insulin- and non-insulin-mediated glucose uptake, estimated with somatostatin-induced suppression of endogenous insulin secretion, contributed almost equally to the excess glucose utilization observed in the obese children. When studied with the euglycemic-hyperinsulinemic clamp, obese children could not increase glucose disposal to the same extent as normal children and were not able to adequately suppress their endogenous glucose production. Recently obese children are therefore characterized by an increased basal glucose turnover rate and an already established insulin resistance of the liver and probably the skeletal muscles.     

In vivo regulation of non-insulin-mediated and insulin-mediated glucose uptake by cortisol

In vivo glucose uptake (Rd) occurs via two mechanisms: insulin-mediated glucose uptake (IMGU), which occurs in insulin-sensitive tissues, and non-insulin-mediated glucose uptake (NIMGU), which occurs in both insulin-sensitive and non-insulin-sensitive tissues. To determine whether these two pathways for in vivo glucose disposal are regulated independently, we studied the effect of stress levels of cortisol on IMGU and NIMGU in seven normal subjects after an overnight fast. To study NIMGU, somatostatin (SRIF, 600 micrograms/h) was infused to suppress endogenous insulin secretion and create severe insulinopenia, and glucose turnover was measured isotopically while serum glucose was clamped at approximately 200 mg/dl for 240 min. Separate studies were performed during the overnight infusion of saline or hydrocortisone (HCT; 2.0 micrograms.kg-1.min-1). The final 120 min of each study were used for data analysis. Under these conditions, insulin action is absent, and Rd = NIMGU. NIMGU was 204 +/- 11 mg/min and 208 +/- 8 mg/dl during saline and HCT, respectively (P NS). Therefore, HCT did not modulate NIMGU. To measure the effect of cortisol on Rd, hyperglycemic (200 mg/dl)-hyperinsulinemic clamp studies (30 mU.m-2. min-1) were performed during the infusion of saline or HCT. The results demonstrate that during saline infusion, steady-state rates of Rd (10.4 +/- 0.8 mg.kg-1.min-1) were achieved by 160 min; in contrast, during HCT infusion, Rd never reached steady state but increased from 4.5 +/- 0.2 in the 2nd h to 7.6 +/- 0.4 mg.kg-1.min-1 in the 4th h, P less than .01.(ABSTRACT TRUNCATED AT 250 WORDS)     

Effects of amino acids on glucose disposal

Free fatty acids are known to inhibit carbohydrate disposal and oxidation. This action may play an important role in the pathophysiology of insulin resistance and non-insulin-dependent diabetes mellitus. To investigate whether amino acids (AAs) have similar actions, we determined the effects of an intravenously infused mixture of 15 AAs on carbohydrate disposal during euglycemic-hyperinsulinemic clamps associated with either basal or high glucagon concentrations in healthy male volunteers. Plasma glucose concentration was clamped at approximately 4.7 mM (coefficient of variation 4.7%). Insulin infusion (7.18 pmol.kg-1.min-1) raised serum insulin concentrations from 36-50 pM to between 300 and 600 pM. AA infusions (0.5 g.kg-1.h-1.4 h) raised plasma alpha-amino N2 concentrations about five- to six-fold. Infusion of AAs, somatostatin (somatotropin release inhibitory factor, SRIF), and high-glucagon replacement (3.0 ng.kg-1.min-1) reduced the rate of exogenous glucose infusion needed to maintain euglycemia from 51.1 +/- 7.2 mumol.kg-1.min-1 (saline + SRIF + high glucagon) to 28.3 +/- 11.1 mumol.kg-1.min-1 and stimulated endogenous glucose production (from 0 to approximately 17 mumol.kg-1.min-1). Thus, glucose disposal (exogenous infusion plus endogenous production of glucose) remained essentially unchanged. During infusion of AAs + SRIF + basal glucagon replacement (0.25 ng.kg-1.min-1), endogenous glucose production remained completely suppressed, and the rates of exogenous glucose infusion did not change (compared with saline + SRIF + basal glucagon replacement). The data showed that 1) hyperaminoacidemia associated with hyperglucagonemia stimulated endogenous glucose production despite hyperinsulinemia, and 2) intravenous infusion of a mixture of 15 AAs had no inhibitory effect on insulin-stimulated total-body glucose disposal.