Carmine G. Fanelli

Plasma Leptin and Insulin Relationships in Obese and Nonobese Humans

Hyperinsulinemia is associated with an overexpression of mRNA for the ob protein leptin in rodent models of genetic obesity, and insulin has been reported to directly stimulate leptin mRNA in rat adipocytes. Human obesity is also associated with increased leptin mRNA as well as plasma levels, but there have been no reports of the effect of insulin on leptin secretion. We, therefore, tested the hypothesis that insulin stimulates leptin secretion in humans. Using a newly developed leptin assay, immunoreactive leptin was measured in fasting and postprandial plasma samples from 27 healthy adults and in samples before and during euglycemic-hyperinsulinemic then stepped hypoglycemic (hourly steps at 85, 75, 65, 55, and 45 mg/dl) clamps from 10 healthy subjects and 11 patients with IDDM. Plasma leptin was correlated (r = 0.84, P = 0.0005) with BMI in obese but not nonobese subjects and with fasting (r = 0.75, P = 0.008) but not postprandial plasma insulin levels. (Leptin levels did not change postprandially.) Euglycemic hyperinsulinemia did not alter leptin levels, nor did hyperinsulinemic hypoglycemia. Thus, because circulating leptin levels are not increased during postprandial hyperinsulinemia or during euglycemic (or hypoglycemic) hyperinsulinemia, we conclude that, at least in the short term, insulin does not increase leptin secretion in humans and that hyperleptinemia in obese individuals is not likely the result of hyperinsulinemia.

Meticulous Prevention of Hypoglycemia Normalizes the Glycemic Thresholds and Magnitude of Most of Neuroendocrine Responses to, Symptoms of, and Cognitive Function During Hypoglycemia in Intensively Treated Patients With Short-Term IDDM

To test the hypothesis that hypoglycemia unawareness is largely secondary to recurrent therapeutic hypoglycemia in IDDM, we assessed neuroendocrine and symptom responses and cognitive function in 8 patients with short-term IDDM (7 yr) and hypoglycemia unawareness. Patients were assessed during a stepped hypoglycemic clamp, before and after 2 wk and 3 mo of meticulous prevention of hypoglycemia, which resulted in a decreased frequency of hypoglycemia (0.49 ± 0.05 to 0.045 ± 0.03 episodes/patient-day) and an increase in HbA1c (5.8 ± 0.3 to 6.9 ± 0.2%) (P < 0.05). We also studied 12 nondiabetic volunteer subjects. At baseline, lower than normal symptom and neuroendocrine responses occurred at lower than normal plasma glucose, and cognitive function deteriorated only marginally during hypoglycemia. After 2 wk of hypoglycemia prevention, the magnitude of symptom and neuroendocrine responses (with the exception of glucagon and norepinephrine) nearly normalized, and cognitive function deteriorated at the same glycemic threshold and to the same extent as in nondiabetic volunteer subjects. At 3 mo, the glycemic thresholds of symptom and neuroendocrine responses normalized, and surprisingly, some of the responses of glucagon recovered. We concluded that hypoglycemia unawareness in IDDM is largely reversible and that intensive insulin therapy and a program of intensive education may substantially prevent hypoglycemia and at the same time maintain the glycemic targets of intensive insulin therapy, at least in patients with IDDM of short duration.

PHYSIOLOGY OF GLUCOSE COUNTERREGULATION TO HYPOGLYCEMIA

Prevention of hypoglycemia is essential for the preservation of brain metabolism and survival of the whole body. Normally, glucose is the only substrate used by the brain to meet its metabolic requirements. Therefore, a continuous supply of circulatory glucose is a necessary prerequisite for normal cerebral metabolism. When plasma glucose concentration decreases (e.g., during prolonged fasting or after administration of glucose-lowering drugs) several physiologic responses are activated to prevent further decreases in blood glucose. The first response is known as counterregulation, a system that prevents and corrects hypoglycemia through the release of counterregulatory hormones.

Impact of Recent Antecedent Hypoglycemia on Hypoglycemic Cognitive Dysfunction in Nondiabetic Humans

To test the hypothesis that glycemic thresholds for hypoglycemic cognitive dysfunction, like those for neuroendocrine responses to and symptoms of hypoglycemia, shift to lower plasma glucose concentrations after recent antecedent hypoglycemia, 16 healthy young adult subjects (7 women and 9 men) were studied on two separate occasions in random sequence, once with hyperinsulinemic hypoglycemia (2.6 ± 0.1 mmol/l, 47 ± 1 mg/dl) and once with otherwise identical hyperinsulinemic euglycemia (4.8 ± 0.1 mmol/l, 86 ± 5 mg/dl) between 1430 and 1630. Neuroendocrine, symptomatic, and cognitive responses to hyperinsulinemic stepped hypoglycemic (4.7, 4.2, 3.6, 3.0, 2.8, 2.5, and 2.2 mmol/l; 85, 75, 65, 55, 50, 45, and 40 mg/dl) clamps were quantitated the following morning on both occasions. Cognitive function tests included measures of information processing (Serial Addition), attention (Stroop Arrow Word), pattern recognition and memory (Delayed Non-Match to Sample), and declarative memory (Paragraph Recall). As expected, plasma glucagon (P = 0.0094), epinephrine (P = 0.0063), and pancreatic polypeptide (P = 0.0046) responses to stepped hypoglycemia were reduced significantly, and symptomatic responses tended to be reduced after afternoon hypoglycemia. Performance on the cognitive function tests deteriorated (P < 0.0001) during stepped hypoglycemic clamps, but there were no significant overall effects of antecedent hypoglycemia on hypoglycemic cognitive dysfunction. Although deterioration was reduced (P < 0.05) from the 2.8 mmol/l (50 mg/dl) to the 2.5 mmol/l (45 mg/dl) steps on the Serial Addition and Delayed Non-Match to Sample tasks after afternoon hypoglycemia, comparable differences were not found on the Stroop Arrow Word or Paragraph Recall tasks. Thus, glycemic thresholds for hypoglycemic cognitive dysfunction, unlike those for neuroendocrine responses to and symptoms of hypoglycemia, do not seem to shift to substantially lower plasma glucose concentrations after recent antecedent hypoglycemia in nondiabetic humans.

Pharmacokinetics and Pharmacodynamics of Therapeutic Doses of Basal Insulins NPH, Glargine, and Detemir After 1 Week of Daily Administration at Bedtime in Type 2 Diabetic Subjects: A randomized cross-over study

OBJECTIVE To compare the pharmacokinetics and pharmacodynamics of NPH, glargine, and detemir insulins in type 2 diabetic subjects. RESEARCH DESIGN AND METHODS This study used a single-blind, three-way, cross-over design. A total of 18 type 2 diabetic subjects underwent a euglycemic clamp for 32 h after a subcutaneous injection of 0.4 units/kg at 2200 h of either NPH, glargine, or detemir after 1 week of bedtime treatment with each insulin. RESULTS The glucose infusion rate area under the curve0–32 h was greater for glargine than for detemir and NPH (1,538 ± 688; 1,081 ± 785; and 1,170 ± 703 mg/kg, respectively; P < 0.05). Glargine suppressed endogenous glucose production more than detemir (P < 0.05) and similarly to NPH (P = 0.16). Glucagon, C-peptide, free fatty acids, and β-hydroxy-butyrate were more suppressed with glargine than detemir. All 18 subjects completed the glargine study, but two subjects on NPH and three on detemir interrupted the study because of plasma glucose >150 mg/dL. CONCLUSIONS Compared with NPH and detemir, glargine provided greater metabolic activity and superior glucose control for up to 32 h.

Prevention of Hypoglycemia While Achieving Good Glycemic Control in Type 1 Diabetes: The role of insulin analogs

Insulin therapy in diabetes, both at onset and after several years’ duration, is primarily directed to maintain near-normoglycemia to prevent the onset and/or delay progression of long-term complications (1,2). However, it is important that regimens of insulin therapy are designed not only to aim at near-normalizing blood glucose, but also to minimize the risk of hypoglycemia. Subjects with type 1 diabetes continuously drift between hyperglycemia and hypoglycemia. If the former prevails, long-term complications are frequently expected (1). On the other hand, hypoglycemia is not only dangerous and unpleasant, but may over time lead to the syndrome of hypoglycemia unawareness (3). This is relevant in type 1 diabetes but also in type 2 diabetes, since over time, many type 2 diabetic subjects develop progressive pancreatic β-cell dysfunction requiring insulin therapy. Because in subjects with advanced type 2 diabetes the neuroendocrine responses to hypoglycemia are as abnormal as in type 1 diabetic patients (4), insulin therapy may become responsible for frequent and/or severe hypoglycemia in type 2 diabetic patients as well. The goal of minimizing the risk of hypoglycemia while achieving good glycemic control is feasible as long as 1 ) a rational plan of insulin therapy is adopted, 2 ) blood glucose is properly monitored, 3 ) blood glucose targets are individualized, and 4 ) education programs are widely implemented. In the present article, the importance of the use of insulin analogs as a key tool to achieve good glycemic control and prevent hypoglycemia is emphasized. Normal nondiabetic subjects maintain plasma glucose <100 mg/dl in the fasting and <135 mg/dl in the postprandial period. In the fasting state, this is due to the continuous release of insulin from the pancreas, which results in steady plasma insulin, thus restraining hepatic glucose production and thereby preventing fasting hyperglycemia. At mealtime, the normal pancreas releases …

Mechanisms of insulin resistance after insulin-induced hypoglycemia in humans: the role of lipolysis.

OBJECTIVE Changes in glucose metabolism occurring during counterregulation are, in part, mediated by increased plasma free fatty acids (FFAs), as a result of hypoglycemia-activated lipolysis. However, it is not known whether FFA plays a role in the development of posthypoglycemic insulin resistance as well. RESEARCH DESIGN AND METHODS We conducted a series of studies in eight healthy volunteers using acipimox, an inhibitor of lipolysis. Insulin action was measured during a 2-h hyperinsulinemic-euglycemic clamp (plasma glucose [PG] 5.1 mmo/l) from 5:00 p.m. to 7:00 p.m. or after a 3-h morning hyperinsulinemic-glucose clamp (from 10 a.m. to 1:00 p.m.), either euglycemic (study 1) or hypoglycemic (PG 3.2 mmol/l, studies 2–4), during which FFA levels were allowed to increase (study 2), were suppressed by acipimox (study 3), or were replaced by infusing lipids (study 4). [6,6-2H2]-Glucose was infused to measure glucose fluxes. RESULTS Plasma adrenaline, norepinephrine, growth hormone, and cortisol levels were unchanged (P > 0.2). Glucose infusion rates (GIRs) during the euglycemic clamp were reduced by morning hypoglycemia in study 2 versus study 1 (16.8 ± 2.3 vs. 34.1 ± 2.2 μmol/kg/min, respectively, P < 0.001). The effect was largely removed by blockade of lipolysis during hypoglycemia in study 3 (28.9 ± 2.6 μmol/kg/min, P > 0.2 vs. study 1) and largely reproduced by replacement of FFA in study 4 (22.3 ± 2.8 μmol/kg/min, P < 0.03 vs. study 1). Compared with study 2, blockade of lipolysis in study 3 decreased endogenous glucose production (2 ± 0.3 vs. 0.85 ± 0.1 μmol/kg/min, P < 0.05) and increased glucose utilization (16.9 ± 1.85 vs. 28.5 ± 2.7 μmol/kg/min, P < 0.05). In study 4, GIR fell by ∼23% (22.3 ± 2.8 μmol/kg/min, vs. study 3, P = 0.058), indicating a role of acipimox per se on insulin action. CONCLUSION Lipolysis induced by hypoglycemia counterregulation largely mediates posthypoglycemic insulin resistance in healthy subjects, with an estimated overall contribution of ∼39%.

Forearm norepinephrine spillover during standing, hyperinsulinemia, and hypoglycemia

Plasma norepinephrine (NE) concentrations are a fallible index of sympathetic neural activity because circulating NE can be derived from sympathetic nerves, the adrenal medullas, or both and because of regional differences in sympathetic neural activity. We used isotope dilution measurements of systemic and forearm NE spillover rates (SNESO and FNESO, respectively) to study the sympathochromaffin system during prolonged standing, hyperinsulinemic euglycemia, and hyperinsulinemic hypoglycemia in healthy humans. Prolonged standing led to decrements in blood pressure without increments in heart rate, the pattern of incipient vasodepressor syncope. FNESO was not increased (0.58 +/- 0.20 to 0. 50 +/- 0.21 pmol. min-1. 100 ml tissue-1), suggesting that the approximately twofold increments in plasma NE and SNESO were derived from sympathetic nerves other than those in the forearm (with a possible contribution from the adrenal medullas). Hyperinsulinemia per se (euglycemia maintained) stimulated sympathetic neural activity, as evidenced by increments in FNESO (0.57 +/- 0.11 to 1.25 +/- 0.25 pmol. min-1. 100 ml tissue-1, P < 0.05), but not adrenomedullary activity. Hypoglycemia per se stimulated adrenomedullary activity (plasma epinephrine from 190 +/- 70 to 1720 +/- 320, pmol/l, P < 0.01). Although SNESO (P < 0.05) and perhaps plasma NE (P < 0.06) were raised to a greater extent during hyperinsulinemic hypoglycemia than during hyperinsulinemic euglycemia, FNESO was not. Thus these data do not provide direct support for the concept that hypoglycemia per se also stimulates sympathetic neural activity.Plasma norepinephrine (NE) concentrations are a fallible index of sympathetic neural activity because circulating NE can be derived from sympathetic nerves, the adrenal medullas, or both and because of regional differences in sympathetic neural activity. We used isotope dilution measurements of systemic and forearm NE spillover rates (SNESO and FNESO, respectively) to study the sympathochromaffin system during prolonged standing, hyperinsulinemic euglycemia, and hyperinsulinemic hypoglycemia in healthy humans. Prolonged standing led to decrements in blood pressure without increments in heart rate, the pattern of incipient vasodepressor syncope. FNESO was not increased (0.58 ± 0.20 to 0.50 ± 0.21 pmol ⋅ min-1 ⋅ 100 ml tissue-1), suggesting that the approximately twofold increments in plasma NE and SNESO were derived from sympathetic nerves other than those in the forearm (with a possible contribution from the adrenal medullas). Hyperinsulinemia per se (euglycemia maintained) stimulated sympathetic neural activity, as evidenced by increments in FNESO (0.57 ± 0.11 to 1.25 ± 0.25 pmol ⋅ min-1 ⋅ 100 ml tissue-1, P < 0.05), but not adrenomedullary activity. Hypoglycemia per se stimulated adrenomedullary activity (plasma epinephrine from 190 ± 70 to 1720 ± 320, pmol/l, P < 0.01). Although SNESO ( P < 0.05) and perhaps plasma NE ( P < 0.06) were raised to a greater extent during hyperinsulinemic hypoglycemia than during hyperinsulinemic euglycemia, FNESO was not. Thus these data do not provide direct support for the concept that hypoglycemia per se also stimulates sympathetic neural activity.

Differential Effects of Adiposity on Pharmacodynamics of Basal Insulins NPH, Glargine, and Detemir in Type 2 Diabetes Mellitus

OBJECTIVE To assess the role of adiposity on the pharmacodynamics of basal insulins NPH, detemir, and glargine in type 2 diabetes mellitus (T2DM), as estimated by glucose infusion rate (GIR) and endogenous glucose production (EGP) rate in the euglycemic clamp. RESEARCH DESIGN AND METHODS We examined the variables that best predicted GIR and EGP in 32-h clamp studies after treatment with subcutaneous injection of 0.4 units/kg NPH, detemir, and glargine in 18 T2DM subjects (crossover). RESULTS A multiple regression analysis revealed that BMI best predicted GIR variation during the clamp. BMI was inversely correlated with GIR in all three insulin treatments, but was statistically significant in detemir treatment only. BMI correlated positively with residual suppression of EGP in detemir, but not with glargine and NPH treatments. CONCLUSIONS Adiposity blunts the pharmacodynamics of all basal insulins in T2DM. However, as adiposity increases, the effect of detemir is lower versus NPH and glargine.

Pivotal Role of Timely Basal Insulin Replacement After Metformin Failure in Sustaining Long-Term Blood Glucose Control at a Target in Type 2 Diabetes

Type 2 diabetes over time associates with the development of vascular complications (1). The causative role of long-term elevation of blood glucose is well established, at least for microvascular complications, since intervention strategies directed at reducing hyperglycemia lower onset and/or progression of microangiopathy (1,2). The role of hyperglycemia and its treatment in the development of macrovascular complications is less well established. In fact, it takes a longer time to observe a positive effect of better blood glucose control (in addition to reducing the multiple risk factors often associated with type 2 diabetes such as hypertension, visceral obesity, and hyperdyslipidemia) on macroangiopathy compared with microangiopathy (1,3,4). Today’s understanding of the complex relationship between hyperglycemia and complications in type 2 diabetes predicates that only an early and aggressive blood glucose–lowering intervention (in addition to reduction of the above mentioned risk factors), successfully sustained over time, will translate into benefits on macrovascular complications several years later (likely 10–15 years) (1,3–5). Thus, the present recommendation is to intensively treat people with type 2 diabetes from the clinical onset of the disease, particularly subjects with short diabetes duration who likely have not yet developed vascular complications and who presumably have a long life-expectancy (6). At present, the question is not whether to intensively treat people with type 2 diabetes at onset of the disease to prevent long-term complications. The question rather is how to intensively treat type 2 diabetes over the many years and decades of the progression of the disease to consistently keep A1C levels <7.0% over the entire cycle of type 2 diabetes. At present, this question is difficult to answer, primarily because of the lack of evidence of long-term effects of one specific intervention, compared with several other possible intervention strategies …