M. M. Ventura

Acute Antihyperglycemic Mechanisms of Metformin in NIDDM: Evidence for Suppression of Lipid Oxidation and Hepatic Glucose Production

To establish the antihyperglycemic mechanisms of metformin in non-insulin-dependent diabetes mellitus (NIDDM) independently of the long-term, aspecific effects of removal of glucotoxicity, 21 NIDDM subjects (14 obese, 7 nonobese) were studied on two separate occasions, with an isoglycemic (plasma glucose ∼9 mM) hyperinsulinemic (two-step insulin infusion, 2 h each, at the rate of 4 and 40 mU · m−2 · min−1) clamp combined with [3−3H]glucose infusion and indirect calorimetry, after administration of either metformin (500 mg per os, at –5 and –1 h before the clamp) or placebo. Compared with placebo, hepatic glucose production (HGP) decreased ∼30% more after metformin (from 469 ± 50 to 330 ± 54 μmol/min), but glucose uptake did not increase. Metformin suppressed free fatty acids (FFAs) by ∼17% (from 0.42 ± 0.04 to 0.35 ± 0.04 mM) and lipid oxidation by ∼25% (from 4.5 ± 0.4 to 3.4 ± 0.4 μmol · kg−1 · min−1) and increased glucose oxidation by ∼ 16% (from 16.2 ± 1.4 to 19.3 ± 1.3 μmol.kg−1 · min−1) compared with placebo (P < 0.05), but did not affect nonoxidative glucose metabolism, protein oxidation, or total energy expenditure. Suppression of FFA and lipid oxidation after metformin correlated with suppression of HGP (r = 0.70 and r = 0.51, P < 0.001). The effects of metformin in obese and nonobese subjects were no different. We conclude that the specific, antihyperglycemic effects of metformin in the clinical condition of hyperglycemia in NIDDM are primarily due to suppression of HGP, not stimulation of glucose uptake, and are mediated, at least in part, by suppression of FFA and lipid oxidation.

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.

Comparison of Glucose Counterregulation During Short-Term and Prolonged Hypoglycemia in Normal Humans

To compare glucose counter regulatory mechanisms during short-term hypoglycemia and prolonged hypoglycemia, insulin was infused either intravenously (160 mil · M−2 · min) for 10 min or subcutaneously (15 mU · M−2 · min) for 12 h in normal volunteers. With each type of insulin infusion, hypoglycemia (∼50 mg/dl) was either allowed to develop or was prevented (control experiments) by the glucose-clamp technique. During prolonged hypoglycemia, both increased glucose production (1.55 ± 0.05 versus 0.33 ± 0.14 mg · kg−1 · min in control experiments at 12 h, P < 0.01) and suppressed glucose utilization (1.55 ± 0.06 versus 3.17 ± 0.15 mg · kg−1 · min in control studies at 12 h, P < 0.01) were involved in counterregulation. During short-term hypoglycemia, only increased glucose production (3.23 ± 0.33 versus 0.06 ± 0.03 mg · kg−1 · min in control experiments at 60 min) was involved, since glucose clearance actually increased (3.99 ± 0.20 versus 2.88 ± 0.02 ml · kg−1 · min in control experiments at 60 min, P < 0.01). Estimated portal venous insulin concentrations decreased 40% (basal 24 ± 3 versus 14 ± 1 mU/ml at 60 min, P < 0.01) in the short-term hypoglycemia experiments but remained at basal levels (basal 25 ± 1 versus ∼26 μU/min between 1 and 12 h) during prolonged hypoglycemia. Despite the fact that hypoglycemia was more gradually induced in the prolonged hypoglycemia model, peak counterregulatory hormone responses were at least as great as those during short-term hypoglycemia. Plasma free fatty acids and ketone bodies increased 150–200% above basal (both P < 0.01) with counterregulation during prolonged hypoglycemia but did not increase above basal levels with counterregulation during short-term hypoglycemia. Finally, plasma alanine remained unchanged during short-term hypoglycemia but decreased nearly 40% during prolonged hypoglycemia (basal 327 ± 20 versus 208 ± 21 μM at 12 h, P < 0.01). We conclude that glucose counterregulatory mechanisms differ during short-term and prolonged hypoglycemia. In the former, only an increase in glucose production mediated by suppression of insulin secretion and increased glucagon secretion is involved, whereas during prolonged hypoglycemia, both a suppression of glucose utilization, and an increase in glucose production are important. These latter changes in glucose production and utilization may be influenced by changes in circulating nonglucose substrates (e.g., alanine, free fatty acids, and ketone bodies) as well as by hormonal factors acting on both hepatic and extrahepatic tissues.

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.

Studies on overnight insulin requirements and metabolic clearance rate of insulin in normal and diabetic man: relevance to the pathogenesis of the dawn phenomenon

SummaryIn order to assess whether the metabolic clearance of insulin changes overnight, 11 patients with Type 1 (insulin-dependent) diabetes and low insulin antibody titre, and 6 nondiabetic subjects were studied. In these studies insulin was always infused by a Harvard pump. Initially, the nocturnal insulin requirements were assessed in the diabetic patients by an overnight feedback insulin infusion to maintain euglycaemia. The insulin requirements decreased continuously after midnight to a nadir of 0.115 ± 0.014 mU · kg−1 · min−1 at 04.30 hours, but after 05.00 hours the insulin requirements increased nearly 40 percent to a maximum of 0.16 ±0.012 mU · kg−1 · min−1 at 07.00 hours. To assess whether plasma insulin clearance changes overnight, the diabetic patients were studied on two different occasions, from 22.00–02.30 hours and from 04.00–08.30 hours. During each of these two studies insulin was infused in sequential steps of 90 min each at the rate of 0.13, 0.40 and 0.20 mU · kg−1 · min−1. Despite changes in plasma free insulin concentration, the metabolic clearance of insulin in the interval 22.00–02.30 hours (12.6±0.17 ml · kg−1 · min−1) was no different from that of the interval 04.00–08.30 hours (12.5 ± 0.19 ml · kg−1 · min−1). The nondiabetic subjects were studied on two different occasions to assess whether the metabolic clearance of insulin changes overnight. Somatostatin (0.25 mg/h) and insulin (0.3 mU kg−1 · min−1) were infused from 22.00–02.30 hours on one occasion, and from 04.00–08.30 hours on the other. The metabolic clearance of plasma free insulin in the interval 22.00–02.30 hours was no different from that of the interval 04.00–08.30 hours (12.6 ± 0.20 vs 12.9 ± 0.25 ml · kg−1 · min−1 nor was it different from that of the diabetic patients.It is concluded that, first, the metabolic clearance rate of insulin does not change overnight either in diabetic or in nondiabetic subjects; second, that it is independent of plasma insulin concentration; and third, that its value is comparable in nondiabetic subjects and in diabetic patients with a low titre of insulin antibodies. Thus, changes in insulin sensitivity rather than changes in insulin clearance are implicated in the pathogenesis of the dawn phenomenon.