Susan D. Boulware

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.

Localized 13C NMR Spectroscopy in the Human Brain of Amino Acid Labeling from d‐[1‐13C]Glucose

Abstract: Cerebral metabolism of d[1‐13C]glucose was studied with localized 13C NMR spectroscopy during intravenous infusion of enriched [1‐13C]glucose in four healthy subjects. The use of three‐dimensional localization resulted in the complete elimination of triacylglycerol resonance that originated in scalp and subcutaneous fat. The sensitivity and resolution were sufficient to allow 4 min of time‐resolved observation of label incorporation into the C3 and C4 resonances of glutamate and C4 of glutamine, as well as C3 of aspartate with lower time resolution. [4‐13C]Glutamate labeled rapidly reaching close to maximum labeling at 60 min. The label flow into [3‐13C]glutamate clearly lagged behind that of [4‐13C]glutamate and peaked at t = 110–140 min. Multiplets due to homonuclear 13C‐13C coupling between the C3 and C4 peaks of the glutamate molecule were observed in vivo. Isotopomer analysis of spectra acquired between 120 and 180 min yielded a 13C isotopic fraction at C4 glutamate of 27 ± 2% (n = 4), which was slightly less than one‐half the enrichment of the C1 position of plasma glucose (63 ± 1%), p < 0.05. By comparison with an external standard the total amount of [4‐13C]glutamate was directly quantified to be 2.4 ± 0.1 µmol/ml‐brain. Together with the isotopomer data this gave a calculated brain glutamate concentration of 9.1 ± 0.7 µmol/ml, which agrees with previous estimates of total brain glutamate concentrations. The agreement suggests that essentially all of the brain glutamate is derived from glucose in healthy human brain.

1H NMR studies of glucose transport in the human brain

The difference between 1H nuclear magnetic resonance (NMR) spectra obtained from the human brain during euglycemia and during hyperglycemia is depicted as well-resolved glucose peaks. The time course of these brain glucose changes during a rapid increase in plasma glucose was measured in four healthy subjects, aged 18–22 years, in five studies. Results demonstrated a significant lag in the rise of glucose with respect to plasma glucose. The fit of the integrated symmetric Michaelis–Menten model to the time course of relative glucose signals yielded an estimated plasma glucose concentration for half maximal transport, Kt, of 4.8 ± 2.4 mM (mean ± SD), a maximal transport rate, Tmax, of 0.80 ± 0.45 μmol g–1 min–1, and a cerebral metabolic glucose consumption rate (CMR)glc of 0.32 ± 0.16 μmol g“1 min”1. Assuming cerebral glucose concentration to be 1.0 μmol/g at euglycemia as measured by 13C NMR, the fit of the same model to the time course of brain glucose concentrations resulted in Kt = 3.9 ± 0.82 mM, Tmax = 1.16 ± 0.29 μmol g–1 min–1, and CMRglc = 0.35 ± 0.10 μmol g–1 min–1. In both cases, the resulting time course equaled that predicted from the determination of the steady-state glucose concentration by 13C NMR spectroscopy within the experimental scatter. The agreement between the two methods of determining transport kinetics suggests that glucose is distributed throughout the entire aqueous phase of the human brain, implying substantial intracellular concentration.

Mild hypoglycemia and impairment of brain stem and cortical evoked potentials in healthy subjects

To evaluate the impact of mild hypoglycemia on CNS function in healthy adults, we measured brain stem auditory evoked potentials and P300 potentials (elicited by cognitive processing of auditory stimuli) during hypoglycemic or euglycemic insulin clamps (80 mU · m−2 · min−1). In the hypoglycemic clamp study (n = 8), plasma glucose was allowed to fall from 4.6 to 3 mM in hourly ∼0.5-mM steps and subsequently returned to euglycemic baseline levels. In the euglycemic clamp study (n = 8), plasma glucose was maintained at baseline levels throughout. Neither brain stem nor P300 responses changed during the euglycemic control study; symptoms and counterregulatory hormones were also unaffected. During the hypoglycemia study, epinephrine and growth hormone rose once plasma glucose reached 3.4 ± 0.1 mM. Brain stem and P300 potentials remained unchanged until the 3-mM glucose step, when neurophysiological changes suddenly developed in conjunction with reported symptoms. At this glucose level, the wave V component of the brain stem potential was selectively altered in 7 of 8 subjects. Furthermore, P300 latency significantly increased, and amplitude diminished. Changes in both brain stem and cortical (P300) responses reversed when euglycemia was restored. We conclude that modest reductions in plasma glucose (to 3 mM) produce marked alterations in both brain stem and cortical responses to auditory stimuli. These changes in neural function appear at the same time as symptoms and follow rather than precede the rise in counterregulatory hormones during hypoglycemia. Our data suggest that the adverse effects of mild hypoglycemia on brain function are not limited to higher centers but also involve the brain stem.

Independent Effects of Youth and Poor Diabetes Control on Responses to Hypoglycemia in Children

To evaluate the effects of childhood and poorly controlled insulin-dependent diabetes mellitus (IDDM) on counterregulatory hormone and symptomatic responses to hypoglycemia, we studied 16 nondiabetic children (13 ± 2 yr), 19 nondiabetic adults (26 ± 3 yr), and 13 children with IDDM (14 ± 2 yr, HbA1, 15.1 ± 3.3%) during a gradual reduction in plasma glucose with the glucose-clamp technique. Plasma glucose was reduced from ∼5 to ∼2.8 mM over 240 min with serial assessment of counterregulatory hormone levels and symptom awareness. The plasma glucose level that triggered a sustained rise in plasma epinephrine was consistently higher in nondiabetic children than in adults (3.9 ± 0.06 vs. 3.2 ± 0.06 mM, P < 0.001). Poorly controlled IDDM further elevated the glucose threshold for epinephrine release to normoglycemic levels (4.9 ± 0.2 mM, P < 0.001 vs. both control groups). Age and IDDM also produced an upward shift in the glucose level at which growth hormone release and symptom awareness were initiated. In contrast to the effect on glucose thresholds, maximal epinephrine responses and symptom scores were increased only by age and not IDDM (2-fold higher in children). We conclude that childhood and poor diabetes control independently contribute to an upward shift in glucose thresholds for counterregulatory hormone release and symptom awareness during mild hypoglycemia. Normoglycemic counterregulation may interfere with efforts to control diabetes in young patients.

Effect of growth hormone treatment on hyperinsulinemia associated with turner syndrome

To determine whether the insulin resistance in patients with Turner syndrome, which may be exaggerated by treatment with human growth hormone, leads to excessive insulin secretion, we applied the hyperglycemic glucose-clamp technique to produce a standard hyperglycemic stimulus (6.9 mmol/L, or 125 mg/dl, greater than fasting plasma glucose level for 120 minutes) in seven patients with Turner syndrome and in seven healthy children. These studies were repeated in the patients after 6 to 12 months of therapy with growth hormone. Fasting plasma levels of insulin were comparable in control subjects and patients before therapy but increased significantly in the patients after 6 to 12 months of treatment with growth hormone. Despite identical glucose increments in the two groups during the glucose-clamp procedure, both first- and second-phase insulin responses were significantly greater in the patients than in the control subjects. Moreover, the hyperinsulinemic responses to glucose were markedly exaggerated in the patients after their treatment with growth hormone, reaching values (first phase 474 +/- 100 pmol and second phase 826 +/- 100 pmol; p less than 0.02 vs pretreatment values) that were almost threefold greater than those in control subjects (p less than 0.001). Nevertheless, the rate of insulin-stimulated glucose metabolism during the last 60 minutes of the clamp procedure was similar in all three groups of studies. Glycosylated hemoglobin, total cholesterol level, and blood pressure remained normal in patients after therapy with growth hormone. We conclude that glucose-stimulated insulin response is increased in patients with Turner syndrome and that these alterations are further exaggerated by treatment with growth hormone. These hyperinsulinemic responses appear to compensate for reductions in insulin sensitivity.

Enhanced adrenomedullary response and increased susceptibility to neuroglycopenia: Mechanisms underlying the adverse effects of sugar ingestion in healthy children

OBJECTIVE Eating simple sugars has been suggested as having adverse behavioral and cognitive effects in children, but a physiologic mechanism has not been established. This study was performed to address this issue. DESIGN Metabolic, hormonal, and symptomatic responses to a standard oral glucose load (1.75 gm/kg; maximum, 120 gm) were compared in 25 healthy children and 23 young adults, and the hypoglycemic clamp, together with measurements of P300 auditory evoked potentials, was used to assess whether children are more vulnerable than adults to neuroglycopenia. SETTING Childrens Clinical Research Center, Yale University School of Medicine. RESULTS Baseline and oral glucose-stimulated plasma glucose and insulin levels were similar in both groups, including the nadir glucose level 3 to 5 hours after oral administration of glucose (3.4 +/- 0.1 mmol/L (61 +/- 1.8 mg/dl) in children and 3.5 +/- 0.1 mmol/L (63 +/- 1.8 mg/dl) in adults). The late glucose decrease stimulated a rise in plasma epinephrine levels that was twofold higher in children than in adults (2260 +/- 289 vs 1031 +/- 147 pmol/L (407 +/- 52 vs 186 +/- 26 pg/ml), p < 0.01) and a significant increase in hypoglycemic symptom scores in children (p < 0.01), but not in adults. During control experiments, in which six of the healthy children ingested a sugar-free drink, there were no significant changes in plasma glucose levels, hormone concentrations, or hypoglycemic symptom scores. During the hypoglycemic clamp, P300 potentials did not change in any of eight adult subjects until the plasma glucose concentration was lowered to 3.0 mmol/L (54 mg/dl), whereas similar changes in P300 potentials were observed in six of seven children at glucose levels 3.6 to 4.2 mmol/L (65 to 75 mg/dl). CONCLUSION Enhanced adrenomedullary responses to modest reductions in plasma glucose concentration and increased susceptibility to neuroglycopenia may be important contributing factors to adverse behavioral and cognitive effects after sugar ingestion in healthy children.

Metabolic Effects of Insulin-Like Growth Factor I in Normal Humans

Since the development of recombinant DNA technology, there has been a rapid expansion of interest in the use of human insulin-like growth factor I (IGF-I) synthesized by recombinant DNA technology for the treatment of clinical disorders. This article reviews recent studies of the metabolic effects of recombinant human IGF-I in normal humans. These studies demonstrated that under euglycemic conditions, IGF-I had potent effects on glucose (hepatic and peripheral), lipid and amino acid metabolism that closely resemble those of insulin, despite a concomitant inhibitory effect on insulin secretion. Hypoglycemia produced by IGF-I infusions (free-fall study and glucose clamps) had a different effect on counterregulatory responses compared with insulin. The glucagon response was absent, growth hormone (GH) release was attenuated, while norepinephrine levels were increased. Suppression of glucagon release during hypoglycemia impaired glucose recovery. Paradoxically, awareness of hypoglycemia was enhanced with IGF-I, partly due to stimulation of sympathetic activity. Studies performed under hyperglycemic conditions showed that IGF-I inhibited glucose-stimulated insulin secretion, but that this inhibitory effect was partially overcome by increasing the hyperglycemic stimulus. Moreover, despite the decrease in insulin secretion, glucose disposal was accelerated by IGF-I. These observations imply that IGF-I might be effective in human diabetes. In particular, normalization of the decreased basal IGF-I levels, which are characteristic of poorly controlled patients with insulin-dependent diabetes mellitus (IDDM), in pubertal patients might lower glucagon and GH levels and improve cellular metabolism in muscle.(ABSTRACT TRUNCATED AT 250 WORDS)

Non-invasive measurements of the cerebral steady-state glucose concentration and transport in humans by 13C nuclear magnetic resonance

Glucose transport across the blood-brain barrier (BBB) has been shown to be a carrier-mediated, saturable process of the facilitated diffusion type (4). The cerebral steady-state glucose concentration depends on the kinetic properties of transport and is lower than in blood. Direct measurement of brain glucose is not possible in humans by non-invasive techniques using radioactive tracers because the signal of labeled glucose cannot be distinguished from that of its metabolic products.

Does Insulin Species Modify Counterregulatory Hormone Response to Hypoglycemia

Objective To examine whether pork and human insulin induce different counterregulatory responses to hypoglycemia. Research Design and Methods The responses to a mild hypoglycemic stimulus were determined in 35 healthy young adults with the glucose-clamp technique to ensure standardization of glucose and insulin levels. Either pork (n = 15) or human (n = 20) regular insulin was infused (0.8 mU·kg−1·min−1) to lower plasma glucose from 4.7 ± 0.07 to 3.3 ± 0.04 mM (both groups) over ∼ 40 min. Plasma glucose was maintained at that level (with variable rate glucose infusion) for an additional 60 min. Results Steady-state insulin levels were similar in both groups (316 ± 50 vs. 280 ± 29 pM, pork vs. human). Before insulin administration, basal counterregulatory hormone levels were indistinguishable. Most importantly, after plasma glucose was lowered, hormonal responses were nearly identical. No significant differences in peak values of epinephrine (1769 ± 404 vs. 1775 ± 311 pM, pork vs. human), norepinephrine (1.64 ± 0.23 vs. 1.87 ± 0.20 nM, pork vs. human), glucagon (163 ± 29 vs. 175 ± 20 ng/L, pork vs. human), growth hormone (14 ± 3 vs. 17 ± 3 μ/L, pork vs. human), or cortisol (543 ± 83 vs. 458 ± 28 nM, pork vs. human) occurred. Conclusions Our data suggest that pork and human insulin produce a comparable and robust hormonal response in healthy adults under conditions of controlled hypoglycemia.