P. De Feo

Long-term recovery from unawareness, deficient counterregulation and lack of cognitive dysfunction during hypoglycaemia, following institution of rational, intensive insulin therapy in IDDM.

SummaryHypoglycaemia unawareness, is a major risk factor for severe hypoglycaemia and a contraindication to the therapeutic goal of near-normoglycaemia in IDDM. We tested two hypotheses, first, that hypoglycaemia unawareness is reversible as long as hypoglycaemia is meticulously prevented by careful intensive insulin therapy in patients with short and long IDDM duration, and that such a result can be maintained long-term. Second, that intensive insulin therapy which strictly prevents hypoglycaemia, can maintain long-term near-normoglycaemia. We studied 21 IDDM patients with hypoglycaemia unawareness and frequent mild/severe hypoglycaemia episodes while on “conventional” insulin therapy, and 20 nondiabetic control subjects. Neuroendocrine and symptom responses, and deterioration in cognitive function were assessed in a stepped hypoglycaemia clamp before, and again after 2 weeks, 3 months and 1 year of either intensive insulin therapy which meticulously prevented hypoglycaemia (based on physiologic insulin replacement and continuous education, experimental group, EXP, n=16), or maintenance of the original “conventional” therapy (control group, CON, n=5). At entry to the study, all 21 IDDM-patients had subnormal neuroendocrine and symptom responses, and less deterioration of cognitive function during hypoglycaemia. After intensive insulin therapy in EXP, the frequency of hypoglycaemia decreased from 0.5±0.05 to 0.045±0.02 episodes/patient-day; HbA1C increased from 5.83±0.18 to 6.94±0.13% (range in non-diabetic subjects 3.8–5.5%) over a 1-year period; all counterregulatory hormone and symptom responses to hypoglycaemia improved between 2 weeks and 3 months, with the exception of glucagon which improved at 1 year; and cognitive function deteriorated further as early as 2 weeks (p<0.05). The improvement in responses was maintained at 1 year. The improvement in plasma adrenaline and symptom responses inversely correlated with IDDM duration. In contrast, in CON, neither frequency of hypoglycaemia, nor neuroendocrine responses to hypoglycaemia improved. Thus, meticulous prevention of hypoglycaemia by intensive insulin therapy reverses hypoglycaemia unawareness even in patients with long-term IDDM, and is compatible with long-term near-normoglycaemia. Because carefully conducted intensive insulin therapy reduces, not increases the frequency of moderate/severe hypoglycaemia, intensive insulin therapy should be extended to the majority of IDDM patients in whom it is desirable to prevent/delay the onset/progression of microvascular complications.

Demonstration of a Dawn Phenomenon in Normal Human Volunteers

To ascertain whether the dawn phenomenon occurs in nondiabetic individuals and, if so, whether it is due to an increase in glucose production or a decrease in glucose utilization, we determined plasma concentrations of glucose, insulin, C-peptide, and counter regulatory hormones, as well as rates of glucose production, glucose utilization, and insulin secretion at one-halfhourly intervals between 1:00 and 9:00 a.m. in eight normal volunteers. After 5:30 a.m., plasma glucose, insulin, and C-peptide concentrations all increased significantly; rates of glucose production, glucose utilization, and insulin secretion also increased (all P < 0.05). Plasma cortisol, epinephrine, and norepinephrine increased significantly from nocturnal nadirs between 4:00 and 6:30 a.m. Plasma growth hormone, which had increased episodically between 1:00 and 4:30 a.m., decreased thereafter nearly 50% (P < 0.05). Plasma glucagon did not change significantly throughout the period of observation. These results indicate that a dawn-like phenomenon, initiated by an increase in glucose production, occurs in nondiabetic individuals. Thus, early morning increases in plasma glucose concentrations and insulin requirements observed in IDDM and NIDDM may be an exaggeration of a physiologic circadian variation in hepatic insulin sensitivity induced by antecedent changes in catecholamine and/or growth hormone secretion.

Differential effects of insulin deficiency on albumin and fibrinogen synthesis in humans.

Insulin deficiency decreases tissue protein synthesis, albumin mRNA concentration, and albumin synthesis in rats. In contrast, insulin deficiency does not change, or, paradoxically, increases estimates of whole body protein synthesis in humans. To determine if such estimates of whole body protein synthesis could obscure potential differential effects of insulin on the synthetic rates of individual proteins, we determined whole body protein synthesis and albumin and fibrinogen fractional synthetic rates using 5-h simultaneous infusions of [14C]leucine and [13C]bicarbonate, in six type 1 diabetics during a continuous i.v. insulin infusion (to maintain euglycemia) and after short-term insulin withdrawal (12 +/- 2 h). Insulin withdrawal increased (P less than 0.03) whole body proteolysis by approximately 35% and leucine oxidation by approximately 100%, but did not change 13CO2 recovery from NaH13CO3 or estimates of whole body protein synthesis (P = 0.21). Insulin deficiency was associated with a 29% decrease (P less than 0.03) in the albumin fractional synthetic rate but a 50% increase (P less than 0.03) in that of fibrinogen. These data provide strong evidence that albumin synthesis in humans is an insulin-sensitive process, a conclusion consistent with observations in rats. The increase in fibrinogen synthesis during insulin deficiency most likely reflects an acute phase protein response due to metabolic stress. These data suggest that the absence of changes in whole body protein synthesis after insulin withdrawal is the result of the summation of differential effects of insulin deficiency on the synthesis of specific body proteins.

Demonstration of a critical role for free fatty acids in mediating counterregulatory stimulation of gluconeogenesis and suppression of glucose utilization in humans.

In vitro studies indicate that FFA compete with glucose as an oxidative fuel in muscle and, in addition, stimulate gluconeogenesis in liver. During counterregulation of hypoglycemia, plasma FFA increase and this is associated with an increase in glucose production and a suppression of glucose utilization. To test the hypothesis that FFA mediate changes in glucose metabolism that occur during counterregulation, we examined the effects of acipimox, an inhibitor of lipolysis, on glucose production and utilization ([3-3H]glucose), and incorporation of [U-14C]-alanine into glucose during insulin-induced hypoglycemia. Eight normal volunteers were infused with insulin for 8 h to produce modest hypoglycemia (approximately 3 mM) on two occasions, first without acipimox (control) and then with acipimox administration (250 mg per os at 60 and 240 min). Despite identical plasma insulin concentrations, glucose had to be infused in the acipimox experiments (glucose-clamp technique) to maintain plasma glucose concentrations identical to those in control experiments. Acipimox completely prevented counterregulatory increases in lipolysis so that during the last 4 h plasma FFA were below baseline values and averaged 67 +/- 13 vs. 725 +/- 65 microM in control experiments, P < 0.001. Concomitantly, overall glucose production was reduced by 40% (5.5 +/- 11 vs. 9.3 +/- 0.7 mumol/kg per min, P < 0.001), and gluconeogenesis from alanine was reduced by nearly 70% (0.32 +/- 0.09 vs. 1.00 +/- 0.18 mumol/kg per min, P < 0.001), while glucose utilization increased by 15% (10.8 +/- 1.4 vs. 9.3 +/- 0.7 mumol/kg per min). We conclude that FFA play a critical role in mediating changes in glucose metabolism during counterregulation, and that under these conditions, FFA exert a much more profound effect on hepatic glucose production than on glucose utilization.

Effects of whole-body vibration exercise on the endocrine system of healthy men.

Whole-body vibration is reported to increase muscle performance, bone mineral density and stimulate the secretion of lipolytic and protein anabolic hormones, such as GH and testosterone, that might be used for the treatment of obesity. To date, as no controlled trial has examined the effects of vibration exercise on the human endocrine system, we performed a randomized controlled study, to establish whether the circulating concentrations of glucose and hormones (insulin, glucagon, cortisol, epinephrine, norepinephrine, GH, IGF-1, free and total testosterone) are affected by vibration in 10 healthy men [age 39±3, body mass index (BMI) of 23.5±0.5 kg/m2, mean±SEM]. Volunteers were studied on two occasions before and after standing for 25 min on a ground plate in the absence (control) or in the presence (vibration) of 30 Hz whole body vibration. Vibration slightly reduced plasma glucose (30 min: vibration 4.59±0.21, control 4.74±0.22 mM, p=0.049) and increased plasma norepinephrine concentrations (60 min: vibration 1.29±0.18, control 1.01±0.07 nM, p=0.038), but did not change the circulating concentrations of other hormones. These results demonstrate that vibration exercise transiently reduces plasma glucose, possibly by increasing glucose utilization by contracting muscles. Since hormonal responses, with the exception of norepinephrine, are not affected by acute vibration exposure, this type of exercise is not expected to reduce fat mass in obese subjects.

Nocturnal spikes of growth hormone secretion cause the dawn phenomenon in Type 1 (insulin-dependent) diabetes mellitus by decreasing hepatic (and extrahepatic) sensitivity to insulin in the absence of insulin waning

SummaryThe aim of the present studies was to test the hypothesis that the dawn phenomenon in Type 1 (insulin-de-pendent) diabetes mellitus is due to a decrease in insulin sensitivity caused by nocturnal spikes of growth hormone. Twelve subjects with Type 1 diabetes were studied on two different occasions, from 24.00 to 02.00 hours, and from 06.00 to 08.00 hours with the euglycaemic clamp technique at two plasma free insulin levels (≈25 mU/l,n=7; ≈80 mU/l,n=5). To eliminate the confounding factor of insulin waning of previous Biostator studies, prior to clamp experiments the diabetic subjects were infused with i.v. insulin by means of a syringe pump according to their minute-to-minute insulin requirements. Insulin sensitivity decreased at dawn as compared to the early night hours (≈30% increase in the rate of hepatic glucose production, ≈25% decrease in the rate of peripheral glucose utilisation). Plasma insulin clearance did not change overnight. In seven Type 1 diabetic subjects, suppression of nocturnal spikes of growth hormone secretion by somatostatin during basal glucagon and growth hormone replacement resulted in complete abolition of the increased rate of hepatic glucose production at dawn. Replacement of nocturnal spikes of growth hormone faithfully reproduced the increase in hepatic glucose production at dawn of the control study. It is concluded that the dawn phenomenon in Type 1 diabetes mellitus examined during optimal insulin replacement, first, is due solely to a decrease in insulin sensitivity and not to an increase in insulin clearance; second, that the decrease in insulin sensitivity at dawn takes place both in the liver and peripheral tissues; third, that the decrease in hepatic (and most likely extrahepatic) insulin sensitivity at dawn is caused by nocturnal spikes of growth hormone secretion.

Adrenergic mechanisms contribute to the late phase of hypoglycemic glucose counterregulation in humans by stimulating lipolysis.

Three studies were performed on nine normal volunteers to assess whether catecholamine-mediated lipolysis contributes to counterregulation to hypoglycemia. In these three studies, insulin was intravenously infused for 8 h (0.30 mU.kg-1.min-1 from 0 to 180 min, and 0.40 mU.kg-1.min-1 until 480 min). In study I (control study), only insulin was infused; in study II (direct + indirect effects of catecholamines), propranolol and phentolamine were superimposed to insulin and exogenous glucose was infused to reproduce the same plasma glucose (PG) concentration of study I. Study III (indirect effect of catecholamines) was the same as study II, except heparin (0.2 U.kg-1.min-1 after 80 min), 10% Intralipid (1 ml.min-1 after 160 min) and variable glucose to match PG of study II, were also infused. Glucose production (HGO), glucose utilization (Rd) [3-3H]glucose, and glucose oxidation and lipid oxidation (LO) (indirect calorimetry) were determined. In all three studies, PG decreased from approximately 4.8 to approximately 2.9 mmol/liter (P = NS between studies), and plasma glycerol and FFA decreased to a nadir at 120 min. Afterwards, in study I plasma glycerol and FFA increased by approximately 75% at 480 min, but in study II they remained approximately 40% lower than in study I, whereas in study III they rebounded as in study I (P = NS). In study II, LO was lower than in study I (1.69 +/- 0.13 vs. 3.53 +/- 0.19 mumol.kg-1.min-1, P less than 0.05); HGO was also lower between 60 and 480 min (7.48 +/- 0.57 vs. 11.6 +/- 0.35 mumol.kg-1.min-1, P less than 0.05), whereas Rd was greater between 210 and 480 min (19 +/- 0.38 vs. 11.4 +/- 0.34 mumol.kg-1.min-1, respectively, P less than 0.05). In study III, LO increased to the values of study I; between 4 and 8 h, HGO increased by approximately 2.5 mumol.kg-1.min-1, and Rd decreased by approximately 7 mumol.kg-1.min-1 vs. study II. We conclude that, in a late phase of hypoglycemia, the indirect effects of catecholamines (lipolysis mediated) account for at least approximately 50% of the adrenergic contribution to increased HGO, and approximately 85% of suppressed Rd.

Role of hepatic autoregulation in defense against hypoglycemia in humans.

To assess the role of hepatic autoregulation in defense against hypoglycemia, we compared the effects of complete blockade of glucose counterregulation with those of blockade of only neurohumoral counterregulation during moderate (approximately 50 mg/dl) and severe (approximately 30 mg/dl) hypoglycemia induced by physiologic hyperinsulinemia during subcutaneous infusion of insulin in normal volunteers. Compared with observations in control experiments, neurohumoral counterregulatory blockade (somatostatin, propranolol, phentolamine, and metyrapone), during which identical moderate hypoglycemia was achieved using the glucose clamp technique, resulted in suppressed glucose production (0.62 +/- 0.08 vs. 1.56 +/- 0.07 mg/kg per min at 12 h, P less than 0.01) and augmented glucose utilization (2.17 +/- 0.18 vs. 1.57 +/- 0.07 mg/kg per min at 12 h, P less than 0.01). Complete blockade of counterregulation (neurohumoral blockade plus prevention of hypoglycemia) did not further enhance the suppressive effects of insulin on glucose production. However, when severe hypoglycemia was induced during neurohumoral counterregulatory blockade, glucose production was nearly two times greater (1.05 +/- 0.05 mg/kg per min at 9 h) than that observed during complete counterregulatory blockade (0.58 +/- 0.08 mg/kg per min at 9 h, P less than 0.01) and that observed during mere neurohumoral blockade with moderate hypoglycemia (0.59 +/- 0.06 mg/kg per min at 9 h, P less than 0.01). These results demonstrate that glucose counterregulation involves both neurohumoral and hepatic autoregulatory components: neurohumoral factors, which require only moderate hypoglycemia for their activation, augment glucose production and reduce glucose utilization; hepatic autoregulation requires severe hypoglycemia for its activation and may thus serve as an emergency system to protect the brain when other counterregulatory factors fail to prevent threatening hypoglycemia.

The dawn phenomenon in type 1 (insulin-dependent) diabetes mellitus: magnitude, frequency, variability, and dependency on glucose counterregulation and insulin sensitivity.

SummaryIn 114 subjects with Type 1 (insulin-dependent) diabetes mellitus the nocturnal insulin requirements to maintain euglycaemia were assessed by means of i. v. insulin infusion by a Harvard pump. The insulin requirements decreased after midnight to a nadir of 0.102±0.03 mU·kg−1·min−1 at 02.40 hours. Thereafter, the insulin requirements increased to a peak of 0.135±0.06mU·kg−1·min−1 at 06.40 hours (p<0.05). The dawn phenomenon (increase in insulin requirements by more than 20% after 02.40 hours lasting for at least 90 min) was present in 101 out of the 114 diabetic subjects, and its magnitude (% increase in insulin requirements between 05.00–07.00 hours vs that between 01.00–03.00 hours) was 19.4±0.54% and correlated inversely with the duration of diabetes (r = −0.72, p<0.001), but not with age. The nocturnal insulin requirements and the dawn phenomenon were highly reproducible on three separate nights. In addition, glycaemic control, state of counterregulation to hypoglycaemia and insulin sensitivity all influenced the magnitude of the dawn phenomenon as follows. In a subgroup of 84 subjects with Type 1 diabetes, the multiple correlation analysis showed that not only duration of diabetes (t = −9.76, p<0.0001), but also % HbA1 significantly influenced the magnitude of the dawn phenomenon (t = 2.03, p<0.05). After 5–9 months of intensive therapy, the magnitude of the dawn phenomenon decreased from 24+-2% to 18±2% (p<0.05) in seven Type 1 diabetic subjects with initially poor glycaemic control, whereas it increased from 17±2% to 24±3% (p<0.05) in five Type 1 diabetic subjects in whom glycaemic control had deteriorated for 2 weeks. In 18 Type 1 diabetic subjects the magnitude of the dawn phenomenon correlated with the indices of adequate glucose counterregulation, namely plasma glucose concentration at the hypoglycaemic nadir (r = −0.79) and the rate of plasma glucose recovery from hypoglycaemia (r = −0.74) (both p<0.01). Finally, in 10 diabetic subjects in whom insulin sensitivity was examined by the euglycaemic glucose clamp technique, there was a correlation between the residual rate of hepatic glucose production (r = 0.78, p<0.005) as well as between the rate of peripheral glucose utilization and the magnitude of the dawn phenomenon (r = −0.70, p<0.025). In conclusion, the dawn phenomenon is a very frequent event in Type 1 diabetes; its magnitude (∼20%) is much lower than that indicated by previous Biostator studies; it is highly reproducible from day to day; it is influenced by the duration of diabetes, glycaemic control, state of the counterregulation system to hypoglycaemia and insulin sensitivity.

Metabolic response to exercise

At the beginning, the survival of humans was strictly related to their physical capacity. There was the need to resist predators and to provide food and water for life. Achieving these goals required a prompt and efficient energy system capable of sustaining either high intensity or maintaining prolonged physical activity. Energy for skeletal muscle contraction is supplied by anaerobic and aerobic metabolic pathways. The former can allow short bursts of intense physical activity (60-90 sec) and utilizes as energetic source the phosphocreatine shuttle and anaerobic glycolysis. The aerobic system is the most efficient ATP source for skeletal muscle. The oxidative phosporylation of carbohydrates, fats and, to a minor extent, proteins, can sustain physical activity for many hours. Carbohydrates are the most efficient fuel for working muscle and their contribution to total fuel oxidation is positively related to the intensity of exercise. The first metabolic pathways of carbohydrate metabolism to be involved are skeletal muscle glycogenolysis and glycolysis. Later circulating glucose, formed through activated gluconeogenesis, becomes an important energetic source. Among glucose metabolites, lactate plays a primary role as either direct or indirect (gluconeogenesis) energy source for contracting skeletal muscle. Fat oxidation plays a primary role during either low-moderate intensity exercise or protracted physical activity (over 90-120 min). Severe muscle glycogen depletion results in increased rates of muscle proteolysis and branched chain amino acid oxidation. Endurance training ameliorates physical performance by improving cardiopulmonary efficiency and optimizing skeletal muscle supply and oxidation of substrates. 2003, Editrice Kurtis