Donald C. Simonson

Impaired insulin action in puberty: a contributing factor to poor glycemic control in adolescents with diabetes

Patients with insulin-dependent diabetes mellitus often have poor metabolic control during puberty. To determine whether puberty is associated with decreased insulin-stimulated glucose metabolism, we compared the results of euglycemic insulin-clamp studies in adults and prepubertal and pubertal children with and without insulin-dependent diabetes. In nondiabetic pubertal children, insulin-stimulated glucose metabolism (201 +/- 12 mg per square meter of body surface area per minute) was sharply reduced, as compared with that of prepubertal children and adults (316 +/- 34 and 290 +/- 21 mg per square meter, respectively; P less than 0.01), despite comparable hyperinsulinemia (insulin levels of 80 to 90 microU per milliliter). Similarly, the response to insulin was 25 to 30 percent lower in the diabetic pubertal children than in the diabetic prepubertal children (P less than 0.05) and adults (P = 0.07). At each stage of development, the stimulating effect of insulin on glucose metabolism was decreased by 33 to 42 percent in the children with diabetes (P less than 0.01). In all the groups of children studied, the response to insulin was inversely correlated with mean 24-hour levels of growth hormone (r = -0.52, P = 0.01). Among the diabetic children, the glycosylated hemoglobin levels were substantially higher in the pubertal children than in the prepubertal children (P less than 0.02), although the daily insulin doses tended to be higher. These data suggest that insulin resistance occurs during puberty in both normal children and children with diabetes. The combined adverse effects of puberty and diabetes on insulin action may help explain why control of glycemia is so difficult to achieve in adolescent patients.

Acute Hyperglycemia Attenuates Endothelium-Dependent Vasodilation in Humans In Vivo

BACKGROUND Endothelial function is impaired in patients with diabetes mellitus. However, the factors contributing to this defect are currently unknown. Hyperglycemia attenuates endothelium-dependent relaxation in normal rabbit arteries in vitro and rat arterioles in vivo. Accordingly, this study examined the effect of acute hyperglycemia on endothelium-dependent vasodilation in nondiabetic humans in vivo. METHODS AND RESULTS Endothelium-dependent vasodilation was assessed through brachial artery infusion of methacholine chloride both before and during 6 hours of local hyperglycemia (300 mg/dL) achieved by intra-arterial infusion of 50% dextrose. Forearm blood flow was determined by plethysmography. In a group of 10 subjects, there was a trend toward attenuated methacholine-mediated vasodilation during hyperglycemia compared with euglycemia (P=.07 by ANOVA; maximal response, 13.3+/-2.8 versus 14.7+/-1.5 mL x min(-1) x 100 mL(-1), respectively). In these subjects, the systemic serum insulin levels increased significantly during the dextrose infusion (P<.001). To eliminate the confounding vasoactive effects of insulin, the protocol was repeated during systemic infusion of octreotide (30 ng x kg(-1) x min(-1)) to inhibit pancreatic secretion of insulin. In these subjects (n=10), hyperglycemia significantly attenuated the forearm blood flow response to methacholine (P<.01 by ANOVA; maximal response, 16.9+/-2.5 before versus 12.7+/-1.8 mL x min(-1) x 100 mL(-1) during hyperglycemia). Methacholine-mediated vasodilation was not attenuated by an equimolar infusion of mannitol (P>.40), nor did hyperglycemia reduce endothelium-independent vasodilation to verapamil (P>.50). CONCLUSIONS Acute hyperglycemia impairs endothelium-dependent vasodilation in healthy humans in vivo. This finding suggests that elevated glucose may contribute to the endothelial dysfunction observed in patients with diabetes mellitus.

Hepatic and peripheral insulin resistance: A common feature of Type 2 (non-insulin-dependent) and Type 1 (insulin-dependent) diabetes mellitus

SummaryHepatic glucose production (3H-glucose technique) and insulin-mediated glucose uptake (insulin clamp technique) were measured in 38 Type 2 (non-insulin-dependent) and 11 Type 1 (insulin-dependent) diabetic patients. Fasting plasma glucose concentration was 8.3 ± 0.5 mmol/l in the former, and 9.6 ± 1.3 mmol/1 in the latter group; the respective fasting plasma insulin levels were 19 ± 2 mU/l (p < 0.005 versus 13 ± 1 mU/l in 33 age-matched control subjects), and 9 ± 1 mU/l (p < 0.01 versus 14 ± 1 mU/l in 36 younger control subjects). In the fasting state, hepatic glucose production was slightly increased (15%, 0.1 > p > 0.05) in the Type 2 diabetic patients and markedly elevated (65%, p < 0.001) in the Type 1 patients compared with their respective control groups. In both groups of diabetic subjects, the rates of hepatic glucose production were inappropriately high for the prevailing plasma glucose and insulin levels, indicating the presence of hepatic resistance to insulin. Basal plasma glucose clearance was also significantly reduced in both the Type 2 (34%) and the Type 1 (14%) diabetic subjects. The fasting plasma glucose concentration correlated directly with hepatic glucose production, and inversely with plasma glucose clearance. During the insulin clamp, plasma insulin was maintained at approximately 100 mU/l in all groups, while plasma glucose was maintained constant at the respective fasting levels. Total glucose uptake was reduced in both the Type 2 (4.57 ± 0.31 versus 6.39 ± 0.25 mg · min–1 · kg–1 in the control subjects, p < 0.01) and the Type 1 (4.77 ± 0.48 versus 7.03 ± 0.22 mg · min–1 · kg–1, p < 0.01) diabetic patients. Insulin-stimulated glucose clearance was reduced to a similar extent in Type 2 (54%) and Type 1 (61%) diabetic subjects, and correlated directly with fasting glucose clearance. These results show that insulin resistance is a common feature of both types of diabetes and can be demonstrated in the basal as well as the insulin-stimlated state. Both hepatic and peripheral resistance to the action of insulin contribute to diabetic hyperglycaemia.

Fasting hyperglycemia in non-insulin-dependent diabetes mellitus: contributions of excessive hepatic glucose production and impaired tissue glucose uptake

The factors responsible for fasting hyperglycemia were investigated in 77 normal weight non-insulin-dependent diabetic (NIDD) and 72 age-, sex-, and weight-matched control individuals. In diabetic subjects with mild fasting hyperglycemia (less than 140 mg/dL) hepatic glucose production (1.85 +/- 0.03 mg/kg.min) was similar to controls (1.84 +/- 0.02); the major factor responsible for the elevated basal glucose level in the diabetic group was a decreased efficiency in the tissue uptake of glucose, as reflected by a 30% decline in the rate of glucose clearance (1.56 +/- 0.03 v 2.00 +/- 0.03 mL/kg.min, P less than .001). In contrast, in diabetic subjects with fasting plasma glucose concentrations above 140 mg/dL, basal hepatic glucose production was significantly elevated (2.42 +/- 0.08 mg/kg.min, P less than .001) and correlated closely with the increase in fasting plasma glucose concentration (r = .796, P less than .001). The basal rate of whole body glucose clearance reached a plateau value at fasting glucose levels of 160 to 180 mg/dL and did not contribute to the further rise in fasting plasma glucose concentrations above 160 to 180 mg/dL. Decreased efficiency of tissue glucose uptake is responsible the development of fasting hyperglycemia in patients with mild NIDDM (fasting plasma glucose less than 140 mg/dL). As the diabetic state worsens, an increase in basal hepatic glucose production is the major factor responsible for the progressive rise in fasting glucose levels.

Insulin Resistance is a Prominent Feature of Insulin-dependent Diabetes

Tissue sensitivity to insulin was examined in 36 control subjects and 19 insulin-dependent diabetics with diabetes of long-standing duration (mean = 10 ± 3 yr) employing the insulin clamp technique (Δ plasma insulin concentration ∼100 βU/ml). Eleven of the diabetics (group I) were studied at their fasting hyperglycemic level (173 mg/dl); the remaining 8 diabetics (group II) were studied after lowering their plasma glucose concentration to euglycemic levels (90 mg/dl). Despite plasma glucose levels that were almost twice as great in the diabetics (group 1,173 versus 91 mg/dl, P < 0.001), insulin-mediated glucose metabolism, 4.77 ± 0.18 mg/kg · min, was reduced by 32% versus controls, 7.03 ± 0.22 mg/kg · min (P < 0.01). When the control subjects were restudied at plasma glucose levels (166 ± 2 mg/dl) that were comparable to those of the diabetics, insulin-mediated glucose metabolism was 12.14 ± 0.96 mg/kg · min (P < 0.01). In diabetics studied at euglycemic levels (group II) insulin-mediated glucose metabolism, 3.39 ± 0.30 mg/kg · min, was reduced even further. The metabolic clearance rate in the 19 diabetics, 3.31 ± 0.23 mg/kg · min, was reduced by 58% compared with controls, 7.83 / 0.25 (P < 0.001). These results emphasize the severe degree of insulin resistance that exists in the insulin-dependent diabetics. Basal hepatic glucose production in the diabetic group, 2.96 ± 0.24 mg/kg · min, was 26% greater than in the controls, 2.35 ± 0.04 (P < 0.001). The fasting plasma glucose concentration displayed a strong positive correlation (r = 0.857, P < 0.001) with basal hepatic glucose production and was weakly and inversely correlated (r = −0.413, P = 0.07) with the basal glucose clearance. Following hyperinsulinemia, however, suppression of hepatic glucose production was ∼ 9 5% in both diabetics and controls, suggesting that peripheral tissues are primarily responsible for the observed impairment in insulin-mediated glucose uptake. The present results indicate that impaired insulin action is a common feature of insulin-dependent diabetics, despite daily insulin requirements (35 ± 2 U/day) that would not clinically characterize them as being insulin resistant.

Effect of Intensive Insulin Therapy on Glycemic Thresholds for Counterregulatory Hormone Release

To evaluate the effect of strict glycemie control of insulin-dependent diabetes mellitus (IDDM) on the plasma glucose threshold initiating counterregulatory hormone responses to hypoglycemia, we used the glucose clamp technique to produce a standardized gradual glucose decline from 90 to 40 mg/dl in seven young IDDM patients before and after 2–6 mo of intensified insulin therapy. Before intensive therapy [hemoglobin A1 (HbA1) 9.6 ± 1.1%], epinephrine responses were triggered at a higher plasma glucose level (67 ± 4 mg/dl) than in normal control subjects (56 ± 1 mg/dl, P < .05), and clinical symptoms of hypoglycemia appeared at glucose levels of 50–60 mg/dl. After intensive therapy (HbA, 7.1 ± 0.7%), the glucose threshold for epinephrine release consistently declined to values (46 ± 2 mg/dl) below normal (P < .01). Furthermore, epinephrine concentrations were markedly reduced at each hypoglycemie level, and a greater hypoglycemie stimulus was required to elicit symptoms. The glucose threshold stimulating release of growth hormone also significantly declined after intensive therapy. We conclude that strict glycemie control of IDDM lowers the plasma glucose level required to generate epinephrine release during hypoglycemia. This may diminish patient recognition of moderate hypoglycemia and increase the risk of severe hypoglycemia in intensively treated IDDM.

Sleep Restriction for 1 Week Reduces Insulin Sensitivity in Healthy Men

OBJECTIVE Short sleep duration is associated with impaired glucose tolerance and an increased risk of diabetes. The effects of sleep restriction on insulin sensitivity have not been established. This study tests the hypothesis that decreasing nighttime sleep duration reduces insulin sensitivity and assesses the effects of a drug, modafinil, that increases alertness during wakefulness. RESEARCH DESIGN AND METHODS This 12-day inpatient General Clinical Research Center study included 20 healthy men (age 20–35 years and BMI 20–30 kg/m2). Subjects spent 10 h/night in bed for ≥8 nights including three inpatient nights (sleep-replete condition), followed by 5 h/night in bed for 7 nights (sleep-restricted condition). Subjects received 300 mg/day modafinil or placebo during sleep restriction. Diet and activity were controlled. On the last 2 days of each condition, we assessed glucose metabolism by intravenous glucose tolerance test (IVGTT) and euglycemic-hyperinsulinemic clamp. Salivary cortisol, 24-h urinary catecholamines, and neurobehavioral performance were measured. RESULTS IVGTT-derived insulin sensitivity was reduced by (means ± SD) 20 ± 24% after sleep restriction (P = 0.001), without significant alterations in the insulin secretory response. Similarly, insulin sensitivity assessed by clamp was reduced by 11 ± 5.5% (P < 0.04) after sleep restriction. Glucose tolerance and the disposition index were reduced by sleep restriction. These outcomes were not affected by modafinil treatment. Changes in insulin sensitivity did not correlate with changes in salivary cortisol (increase of 51 ± 8% with sleep restriction, P < 0.02), urinary catecholamines, or slow wave sleep. CONCLUSIONS Sleep restriction (5 h/night) for 1 week significantly reduces insulin sensitivity, raising concerns about effects of chronic insufficient sleep on disease processes associated with insulin resistance.

Effect of insulin and plasma amino acid concentrations on leucine metabolism in man. Role of substrate availability on estimates of whole body protein synthesis.

We examined the effect of insulin and plasma amino acid concentrations on leucine kinetics in 15 healthy volunteers (age 22 +/- 2 yr) using the euglycemic insulin clamp technique and an infusion of [1-14C]leucine. Four different experimental conditions were examined: (a) study one, high insulin with reduced plasma amino acid concentrations; (b) study two, high insulin with maintenance of basal plasma amino acid concentrations; (c) study three, high insulin with elevated plasma amino acid concentrations; and (d) study four, basal insulin with elevated plasma amino acid concentrations. Data were analyzed using both the plasma leucine and alpha-ketoisocaproate (the alpha-ketoacid of leucine) specific activities. In study one total leucine flux, leucine oxidation, and nonoxidative leucine disposal (an index of whole body protein synthesis) all decreased (P less than 0.01) regardless of the isotope model utilized. In study two leucine flux did not change, while leucine oxidation increased (P less than 0.01) and nonoxidative leucine disposal was maintained at the basal rate; endogenous leucine flux (an index of whole body protein degradation) decreased (P less than 0.01). In study three total leucine flux, leucine oxidation, and nonoxidative leucine disposal all increased significantly (P less than 0.01). In study four total leucine flux, leucine oxidation, and nonoxidative leucine disposal all increased (P less than 0.001), while endogenous leucine flux decreased (P less than 0.001). We conclude that: (a) hyperinsulinemia alone decreases plasma leucine concentration and inhibits endogenous leucine flux (protein breakdown), leucine oxidation, and nonoxidative leucine disposal (protein synthesis); (b) hyperaminoacidemia, whether in combination with hyperinsulinemia or with maintained basal insulin levels decreases endogenous leucine flux and stimulates both leucine oxidation and nonoxidative leucine disposal.

Increased insulin secretion in puberty: A compensatory response to reductions in insulin sensitivity

Recent studies have suggested that insulin action is reduced during puberty in normal children. To determine whether such resistance leads to excessive insulin secretion, we used the hyperglycemic clamp technique to produce a standard hyperglycemic stimulus (125 mg/dl above fasting levels for 120 minutes) in 9 preadolescent and 14 adolescent healthy children and in 14 normal adults. Fasting plasma insulin and C-peptide concentrations were higher in adolescents than in preadolescents and adults (p less than or equal to 0.02). Despite identical glucose increments during the glucose clamp procedure, both first- and second-phase plasma insulin and C-peptide responses were also markedly greater in adolescents than in preadolescents or adults (p less than 0.01 vs. other groups). Despite sharply increased insulin responses in adolescents, the amount of exogenous glucose required to maintain hyperglycemia was similar in all three groups. Insulin responses in the children were directly correlated with fasting plasma levels of insulin-like growth factor I (r = 0.60 to 0.70, p less than 0.01). We conclude that glucose-stimulated insulin secretion is normally increased during puberty, a response that may compensate for puberty-induced defects in insulin sensitivity.

Defective Glucose Counterregulation after Strict Glycemic Control of Insulin-Dependent Diabetes Mellitus

We infused small doses of insulin (0.3 mU per kilogram of body weight per minute; range, 0.9 to 1.7 U per hour) for three hours into 8 subjects who did not have diabetes, 11 patients with well-controlled diabetes (hemoglobin A1, 7.6 +/- 0.7 percent), and 10 patients with poorly controlled diabetes (hemoglobin A1, 11.5 +/- 1.7 percent) to simulate the mild peripheral hyperinsulinemia observed during insulin treatment. Normoglycemia was established in the patients during the night before study. During the insulin infusion, the plasma glucose level stabilized at 60 to 70 mg per deciliter (3.3 to 3.9 mmol per liter) in the subjects without diabetes and the patients with poorly controlled diabetes, because of a rebound increase in hepatic glucose production. In contrast, hypoglycemia developed in the patients with well-controlled diabetes (42 +/- 2 mg of glucose per deciliter, or 2.3 +/- 0.1 mmol per liter, P less than 0.01) as glucose production remained suppressed. The hypoglycemia in the patients with well-controlled diabetes was associated with a lowering of the plasma threshold of glucose that triggered a release of epinephrine (less than 45 mg of glucose per deciliter, or 2.5 mmol per liter, vs. greater than 55 mg per deciliter, or 3.1 mmol per liter, in the other groups, P less than 0.01) as well as an enhanced sensitivity to the suppressive effects of insulin on hepatic glucose production. Nearly identical disturbances in glucose counterregulation and decreased perception of hypoglycemia developed when four of the subjects with poorly controlled diabetes were restudied after intensive treatment. We conclude that strict control of diabetes induces physiologic alterations (delayed release of epinephrine and persistent suppression of glucose production) that impair glucose counterregulation to doses of insulin in the therapeutic range. These defects may contribute to the increased incidence of severe hypoglycemia reported during intensive insulin therapy.