Thiemo Veneman

Role of Reduced Suppression of Glucose Production and Diminished Early Insulin Release in Impaired Glucose Tolerance

BACKGROUND Insulin resistance and impaired insulin secretion both occur in non-insulin-dependent diabetes (NIDDM), but their relative importance is unclear. Hyperglycemia itself has adverse effects on tissue insulin sensitivity and insulin secretion that make it difficult to distinguish between primary and secondary abnormalities. To avoid this problem we studied subjects with postprandial glucose intolerance but not sustained hyperglycemia. METHODS We compared the rate of systemic appearance and disappearance of glucose, the output of endogenous hepatic glucose, splanchnic and muscle uptake of glucose, and plasma insulin and glucagon responses after the ingestion of 1 g of glucose per kilogram of body weight in 15 subjects with impaired glucose tolerance (8 of them nonobese and 7 obese) and in 16 normal subjects (9 nonobese and 7 obese) who were matched for age and weight. RESULTS After glucose ingestion the mean (+/- SE) rate of total systemic appearance of glucose was significantly higher in both the nonobese subjects (455 +/- 12 mmol per five hours) and the obese subjects (486 +/- 17 mmol per five hours) with impaired glucose tolerance than in the respective normal subjects (411 +/- 11 and 436 +/- 7 mmol per five hours). This difference was fully accounted for by the reduced suppression of endogenous hepatic glucose in the subjects with impaired glucose tolerance (a reduction of about 28 percent, vs. 48 percent in the normal subjects; P less than 0.01). Despite late hyperinsulinemia, at 30 minutes the subjects with impaired glucose tolerance had smaller increases in plasma insulin and smaller reductions in plasma glucagon (both P less than 0.01). Molar ratios of plasma insulin to plasma glucagon levels correlated inversely (r = -0.62, P less than 0.001) with the rates of systemic glucose appearance; the latter correlated positively (r = 0.72, P less than 0.0001) with peak plasma glucose concentrations. CONCLUSIONS Impaired glucose tolerance, the precursor of NIDDM, results primarily from reduced suppression of hepatic glucose output due to abnormal pancreatic islet-cell function. The late hyperinsulinemia may be the consequence of an inadequate early beta-cell response rather than of insulin resistance.

Contribution of abnormal muscle and liver glucose metabolism to postprandial hyperglycemia in NIDDM.

To assess the role of muscle and liver in the pathogenesis of postprandial hyperglycemia in non-insulin-dependent diabetes mellitus (NIDDM), we administered an oral glucose load enriched with [14C]glucose to 10 NIDDM subjects and 10 age- and weight-matched nondiabetic volunteers and compared muscle glucose disposal by measuring forearm balance of glucose, lactate, alanine, O2, and CO2 (with forearm calorimetry). In addition, we used the dual-lable isotope method to compare overall rates of glucose appearance (Ra) and disappearance (Rd), suppression of endogenous glucose output, and splanchnic glucose sequestration. During the initial 1-1.5 h after glucose ingestion, plasma glucose increased by approximately 8 mM in NIDDM vs. approximately 3 mM in nondiabetic subjects (P less than 0.01); overall glucose Ra was nearly 11 g greater in NIDDM than nondiabetic subjects (45.1 +/- 2.3 vs. 34.4 +/- 1.5 g, P less than 0.01), but glucose Rd was not significantly different in NIDDM (35.1 +/- 2.4 g) and nondiabetic (33.3 +/- 2.7 g) subjects. The greater overall glucose Ra of NIDDM subjects was due to 6.8 g greater endogenous glucose output (13.7 +/- 1.1 vs. 6.8 +/- 1.0 g, P less than 0.01) and 3.8 g less oral glucose splanchnic sequestration of the oral load (31.4 +/- 1.5 vs. 27.5 +/- 0.9 g, P less than 0.05). Although glucose taken up by muscle was not significantly different in NIDDM and nondiabetic subjects (39.3 +/- 3.5 vs. 41.0 +/- 2.5 g/5 h), a greater amount of the glucose taken up by muscle in NIDDM was released as lactate and alanine (11.7 +/- 1.0 vs. 5.2 +/- 0.3 g in nondiabetic subjects, P less than 0.01), and less was stored (11.7 +/- 1.3 vs. 16.9 +/- 1.5 g, P less than 0.05). We conclude that increased systemic glucose delivery, due primarily to reduced suppression of endogenous hepatic glucose output and, to a lesser extent, reduced splanchnic glucose sequestration, is the predominant factor responsible for postprandial hyperglycemia in NIDDM.

Induction of Hypoglycemia Unawareness by Asymptomatic Nocturnal Hypoglycemia

Hypoglycemia has been incriminated as a possible factor responsible for development of the hypoglycemia unawareness phenomenon in patients with type I diabetes. Many patients with this condition, however, do not have a history of recent hypoglycemia. Because asymptomatic nocturnal hypoglycemia commonly occurs in type I diabetes, we tested the hypothesis that such episodes might be capable of inducing this phenomenon. Accordingly, autonomic and neuroglycopenic symptoms, counterregulatory hormone responses, and cognitive function were assessed during standardized insulin-induced hypoglycemia in 10 normal volunteer subjects on two occasions—once after induction of asymptomatic nocturnal hypoglycemia and once after control studies in which saline rather than insulin was infused overnight. Compared with control experiments, asymptomatic nocturnal hypoglycemia increased the threshold (required greater hypoglycemia for initiation) and reduced the magnitude of autonomic and neuroglycopenic symptoms, counterregulatory hormone responses, and cognitive dysfunction during subsequent hypoglycemia (all, P < 0.05). These results indicate that asymptomatic hypoglycemia may induce hypoglycemia unawareness and, thus, may explain why not every patient with this condition has a history of prior hypoglycemia. Our results therefore support the concept that in type I diabetes this phenomenon may be largely attributable to antecedent hypoglycemia.

Effect of Hyperketonemia and Hyperlacticacidemia on Symptoms, Cognitive Dysfunction, and Counterregulatory Hormone Responses During Hypoglycemia in Normal Humans

The brain usually depends almost exclusively on glucose for its energy requirements. During hypoglycemia associated with prolonged fasting or strenuous exercise, circulating ketone-body and lactate levels increase severalfold; in both situations, certain signs and symptoms of hypoglycemia are diminished. Therefore, to test the hypothesis that hyperketonemia or hyperlacticacidemia of the magnitude seen during certain clinical situations can substitute for glucose as an energy source for the brain and alter physiological responses to hypoglycemia, we assessed autonomic and neuroglycopenic symptoms, counterregulatory hormone responses, and cognitive function during standardized insulin-induced hypoglycemia in normal volunteers with and without infusion of (β-hydroxybutyrate (BOHB) or lactate designed to reproduce circulating levels of these substrates seen during prolonged fasting and strenuous exercise. Compared with paired control experiments, infusion of BOHB and lactate increased the glycemic threshold (required greater hypoglycemia for initiation) and reduced the magnitude of autonomic and neuroglycopenic symptoms, counterregulatory hormone responses, and cognitive dysfunction (all P < 0.05). The hypoglycemic threshold for autonomic symptoms increased from 3.8 ± 0.1 to 3.1 ± 0.2 mmoM during BOHB infusion and from 3.7 ± 0.1 to 2.8 ± 0 . 1 mmol<1 during lactate infusion, and the threshold for neuroglycopenic symptoms increased from 2.8 ± 0.1 to 2.4 ± 0.1 and 2.3 ± 0.1 mmol/1, respectively. The magnitude for autonomic symptoms decreased from 12 ± 2 and 11 ± 1 to 6 ± 2 and 4 ± 1 during BOHB and lactate infusion, respectively. Neuroglycopenic symptoms decreased from 11 ± 2 to 3 ± 1 during both series of experiments. Infusion of BOHB and lactate-reduced responses for all counterregulatory hormones, the reduction being the greatest for epinephrine (∼57 and 73%, during BOHB and lactate infusion, respectively) and least for cortisol (7sim;28 and 29%, respectively). These results indicate that under certain clinical conditions, BOHB and lactate may substitute for glucose as a fuel for the brain and alter physiological responses to hypoglycemia.

Impaired postprandial glucose utilization in non-insulin-dependent diabetes mellitus.

The importance of impaired glucose utilization in the pathogenesis of postprandial hyperglycemia in non-insulin-dependent diabetes mellitus (NIDDM) is controversial. Three methods were used to assess glucose utilization following ingestion of a mixed meal in 18 NIDDM and 12 nondiabetic subjects. Dual glucose isotopes were used to determine first-pass splanchnic glucose uptake, suppression of endogenous glucose production, and systemic glucose utilization. Leg balance was used to evaluate skeletal muscle glucose metabolism, and systemic and limb indirect calorimetry were used to assess glucose and lipid oxidation. NIDDM subjects had marked postprandial hyperglycemia as compared with nondiabetics (15.35 +/- 0.72 v 5.83 +/- 0.28 mmol, P < .001), accompanied by lower postprandial insulin (179 +/- 25 v 253 +/- 46 pmol, P < .01) and elevated plasma free fatty acids ([FFA] 569 +/- 34 v 314 +/- 20 mumol/L, P < .001). Cumulative postprandial glucose appearance was nearly twofold greater in NIDDM (82.2 +/- 4.7 v 48.7 +/- 4.9 g.5h, P < .001) due to increased endogenous glucose production (56.4 +/- 4.8 v 24.5 +/- 1.9 g, P < .001), whereas first-pass splanchnic uptake of ingested glucose was normal in NIDDM. Cumulative postprandial glucose utilization in NIDDM, after correction for urinary glucose, was unchanged from postabsorptive rates, a pattern also found for postprandial glucose oxidation. Cumulative leg glucose uptake was somewhat less in NIDDM subjects (123 +/- 18 v 173 +/- 14 mumol/100 mL leg tissue.5 h, P = .06), whereas lactate and alanine net release across the leg were nevertheless twofold greater in NIDDM (P = .04) and accounted for nearly half of the leg glucose metabolism in NIDDM.(ABSTRACT TRUNCATED AT 250 WORDS)

An improved method to calculate adipose tissue interstitial substrate recovery for microdialysis studies.

We simultaneously compared the conventional, time-consuming point of no net flux method for calculation of interstitial substrate recovery necessary for in vivo microdialysis studies with a simple isotopic method using rat epididymal fat pads. The recovery (%) calculated with the conventional method and the isotopic method for glucose (7.4 +/- 1.1 vs. 6.6 +/- 0.6), glycerol (23 +/- 4 vs. 26 +/- 5) and lactate (40 +/- 8 vs. 38 +/- 5), respectively, were not significantly different. Moreover, the overall correlation coefficient (N = 25) between the methods was 0.87, p < 0.001. We therefore conclude that the methods yield comparable results, and the more convenient isotopic method should become the method of choice for determining adipose tissue interstitial recovery for glucose, lactate and glycerol.

Contribution of Impaired Muscle Glucose Clearance to Reduced Postabsorptive Systemic Glucose Clearance in NIDDM

The reduced postabsorptive rates of systemic glucose clearance in non-insulin-dependent diabetes mellitus (NIDDM) are thought to be the consequence of insulin resistance in peripheral tissues. Although the peripheral tissues involved have not been identified, it is generally assumed to be primarily muscle, the major site of insulin-mediated glucose disposal. To test this hypothesis, we measured postabsorptive systemic and forearm glucose utilization and clearance in 15 volunteers with NIDDM and 15 age- and weightmatched nondiabetic volunteers. Although systemic glucose utilization was increased in NIDDM subjects (14.5 ± 0.5 vs. 11.2 ± 0.2 μmol · kg−1 · min−1, P < 0.001), systemic glucose clearance was reduced 1.40 ± 0.06 vs. 2.13 ± 0.05 ml · kg−1 · min−1, P < 0.01). Although forearm glucose utilization was increased in NIDDM subjects (0.663 ± 0.058 vs. 0.411 ± 0.019 μmol · dl−1 · min−1, P < 0.001), forearm glucose dl−1 clearance was reduced (0.628 ± 0.044 vs. 0.774 ± 0.037 ml · L−1 · min−1, P < 0.01). However, extrapolation of forearm data to total-body muscle indicated that impaired clearance reduced muscle glucose disposal by only 61 ± 21 μmol<min, whereas impaired systemic clearance reduced systemic glucose disposal by 662 ± 82 μmol<min. Thus, impaired muscle glucose clearance accounted for <10% of the reduced systemic glucose clearance in NIDDM subjects. Therefore, we conclude that muscle insulin resistance plays only a minor role in the reduced systemic glucose clearance found in NIDDM in the postabsorptive state and propose that reduced brain glucose clearance is largely responsible.

Diabetes and driving: Desired data, research methods and their pitfalls, current knowledge, and future research.

The issue of traffic safety of patients with diabetes is rising on political as well as scientific levels. Driving by diabetic patients may be impaired by three factors: hyperglycemia, hypoglycemia, and diabetes complications. Management of diabetic patients is progressively aiming at near normoglycemia (1,2). Consequently, the rate of hypoglycemia and hypoglycemia unawareness has markedly increased (3–8). These factors could pose an increased threat to diabetic patients’ fitness to drive. Current legal restrictions regarding diabetes and driving privileges vary throughout the world, but laws are generally prompted by the impending danger of hypoglycemia during driving. To research the various aspects of diabetes and driving, several study methods have been applied. In consideration of dissenting opinions and laws, rules, and regulations, we will discuss in this article the currently available data on diabetes and driving, potential pitfalls in research, and give recommendations for future research. In modern traffic, the increasing age of drivers (9) and their medical conditions can be risk factors for traffic incidents and accidents. The amounting prevalence of diabetes also leads to an increased number of diabetic drivers. Driving by diabetic patients may be impaired by three factors: hyperglycemia, hypoglycemia, and diabetes complications. In recent years, it has become apparent that acute hyperglycemia, and possibly also chronic hyperglycemia, may be associated with cognitive function loss (10–15). However, the cognitive dysfunction occurring during hypoglycemia is most striking (16–18). After the Diabetes Control and Complications Trial (1) and the U.K. Prospective Diabetes Study (2), which showed that diabetes complications are reduced with tight glucose control, management of diabetic patients is progressively aiming at near normoglycemia. Consequently, the rate of hypoglycemia has increased. We now know that even a single episode of hypoglycemia leads to impaired hypoglycemia awareness (19). Hypoglycemia unawareness currently affects ∼25% …

Dose-response characteristics for glucose-stimulated insulin release in man and assessment of influence of glucose on arginine-stimulated insulin release

Glucose potentiates arginine-induced insulin release. We investigated the dose-response characteristics for both phases of glucose-induced insulin release in normal man, and studied the influence of hyperglycemia on arginine-induced insulin secretion. Dose-response curves of plasma C-peptide increments achieved during 60-minute hyperglycemia clamps (7, 11, 17, 24, and 32 mmol/L) with and without a primed continuous infusion of arginine (infusion rate, 15 mg/kg/min) were analyzed with a modified Michaelis-Menten equation. The ED50 (half-maximally stimulating blood glucose concentration) of first-phase insulin release (determined from plasma C-peptide increments at 5 minutes) was significantly lower than the ED50 for the second phase (60 minutes; 8.4 +/- 0.8 v 14.3 +/- 1.3 mmol/L, respectively, P less than .002). Combined glucose-arginine stimulation significantly increased insulin release. Vmax of both phases of glucose-arginine-stimulated insulin release were positively correlated (r = .75, P less than .05). The ED50 of the influence of glucose on first-phase arginine-induced insulin release was significantly lower than the ED50 for the second phase (9.0 +/- 1.1 v 12.7 +/- 1.0 mmol/L, respectively, P less than .02). For each insulin secretion phase separately, the ED50 for the influence of hyperglycemia on arginine-induced insulin release were not significantly different from the ED50 for glucose-induced insulin secretion (without arginine). When dose-response curves of plasma insulin increments were analyzed with the same equation, the ED50 of second-phase glucose-induced plasma insulin increments was significantly higher than the ED50 assessed from the plasma C-peptide increments (21.6 +/- 2.8 v 14.3 +/- 1.3 mmol/L, respectively, P less than .05).(ABSTRACT TRUNCATED AT 250 WORDS)

Evidence against the hypothesis that hyperinsulinemia increases sympathetic nervous system activity in man

To test the hypothesis that physiologic hyperinsulinemia activates the sympathetic nervous system in humans, we measured changes in plasma norepinephrine as well as epinephrine concentrations during euglycemic hyperinsulinemic clamp experiments in which normal volunteers were infused with insulin for up to 12 hours, at rates chosen to simulate the basal and postprandial hyperinsulinemia seen in insulin-resistant states. Infusions of insulin increased plasma insulin threefold (to approximately 200 pmol/L) and 15-fold (to approximately 1,000 pmol/L) in simulations of fasting and postprandial hyperinsulinemia. In neither experiment did plasma norepinephrine or epinephrine change significantly. In control experiments in which saline was infused for 12 hours, plasma epinephrine increased twofold (P less than .05), but plasma norepinephrine did not change. Therefore, we conclude that hyperinsulinemia of the magnitude seen in the insulin-resistant humans does not increase sympathetic nervous system activity.