Direct Laser Acceleration in Underdense Plasmas with Multi-PW Lasers: A Path to High-Charge, GeV-Class Electron Bunches

The direct laser acceleration (DLA) of electrons in underdense plasmas can provide hundreds of nC of electrons accelerated to near-GeV energies using currently available lasers. Here we demonstrate the key role of electron transverse displacement in the acceleration and use it to analytically predict the expected maximum electron energies. The energy scaling is shown to be in agreement with full-scale quasi-3D particle-in-cell simulations of a laser pulse propagating through a preformed guiding channel and can be directly used for optimizing DLA in near-future laser facilities. The strategy towards optimizing DLA through matched laser focusing is presented for a wide range of plasma densities paired with current and near-future laser technology. Electron energies in excess of 10 GeV are accessible for lasers at I∼10^{21}  W/cm^{2}.

Does conventional early life academic excellence predict later life scientific discovery? An assessment of the lives of great medical innovators

BACKGROUND

Perhaps, as never before, we need innovators. With our growing population numbers, and with increasing pressures on our education systems, are we in danger of becoming more rigid and formulaic and increasingly inhibiting innovation? When young can we predict who will become the great innovators? For example, in medicine, who will change clinical practice?

AIMS

We therefore determined to assess whether the current academic excellence approach to medical school entrance would have captured previous great innovators in medicine, assuming that they should all have well fulfilled current entrance requirements.

METHODS

The authors assembled a list of 100 great medical innovators which was then approved, rejected or added to by a jury of 12 MD fellows of the Royal Society of Canada. Two reviewers, who had taken both the past and present Medical College Admission Test as part of North American medical school entrance requirements, independently assessed each innovator's early life educational history in order to predict the innovator's likely success at medical school entry, assuming excellence in all entrance requirements.

RESULTS

Thirty-one percent of the great medical innovators possessed no medical degree and 24% would likely be denied entry to medical school by today's standards (e.g. had a history of poor performance, failure, dropout or expulsion) with only 24% being guaranteed entry. Even if excellence in only one topic was required, the figure would only rise to 41% certain of medical school entry.

CONCLUSION

These data show that today's medical school entry standards would have barred many great innovators and raise questions about whether we are losing medical innovators as a consequence. Our findings have important implications for promoting flexibility and innovation for medical education, and for promoting an environment for innovation in general.

Collisionless shock acceleration in the corona of an inertial confinement fusion pellet with possible application to ion fast ignition

Two-dimensional particle-in-cell simulations are used to explore collisionless shock acceleration in the corona plasma surrounding the compressed core of an inertial confinement fusion pellet. We show that an intense laser pulse interacting with the long scale-length plasma corona is able to launch a collisionless shock around the critical density. The nonlinear wave travels up-ramp through the plasma reflecting and accelerating the background ions. Our results suggest that protons with characteristics suitable for ion fast ignition may be achieved in this way. This article is part of a discussion meeting issue 'Prospects for high gain inertial fusion energy (part 2)'.

Abstract P3-12-02: Intraoperative radiotherapy outcomes in early-stage breast cancer: A study in elderly Canadian women

Objective. Breast-conserving therapy with external beam radiotherapy (EBRT) is currently the standard of care for women with early breast cancer. Our aim was to determine if early-stage breast cancers treated with lumpectomy and primary intraoperative radiotherapy (IORT) have comparable local recurrence rates. This is the first study examining the Canadian experience with IORT.

Methods. Patients who underwent breast-conserving therapy with pre-pathology IORT between 2007- 2017 were retrospectively identified. The primary outcome measure was ipsilateral breast tumor recurrence (IBRT). A time to event analysis was performed; Kaplan-Meier estimates report the fraction of patients living free of recurrence. Secondary outcomes included acute and chronic wound complications.

Results. 106 patients with a median age of 70 (IQR 65-75) were included. Median follow-up was 33 months. The majority of patients had screen-detected (94.3%), estrogen-receptor positive (96.2%), HER2neu negative (93.4%), invasive ductal carcinomas (92.5%). Only 50 (47.6%) were prescribed adjuvant endocrine blockade. IBTR occurred in 5 (4.7%) patients. Five and ten-year local recurrence-free rates were 0.95 and 0.81, respectively. The superficial skin infection rate was 9.4%. Acute symptomatic seromas occurred in 23 (21.7%), while only 10 (9.4%) persisted chronically.

Conclusion. In this cohort of Canadian post-menopausal women treated with breast-conserving surgery and IORT, the IBTR approached 5%. Despite selection of low-risk patients, the local recurrence rate is higher than what is reported in the literature with EBRT. The low rates of prescribed adjuvant systemic therapy may have contributed to this outcome.

Citation Format: Elmi M, Tigert A, Escallon J, Zagorski B, Leong W, Vranic M, Fyles A, Vitkin A, Cil T, McCready D. Intraoperative radiotherapy outcomes in early-stage breast cancer: A study in elderly Canadian women [abstract]. In: Proceedings of the 2018 San Antonio Breast Cancer Symposium; 2018 Dec 4-8; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2019;79(4 Suppl):Abstract nr P3-12-02.

Collimated protons accelerated from an overdense gas jet irradiated by a 1 µm wavelength high-intensity short-pulse laser

We have investigated proton acceleration in the forward direction from a near-critical density hydrogen gas jet target irradiated by a high intensity (10¹⁸ W/cm²), short-pulse (5 ps) laser with wavelength of 1.054 μm. We observed the signature of the Collisionless Shock Acceleration mechanism, namely quasi-monoenergetic proton beams with small divergence in addition to the more commonly observed electron-sheath driven proton acceleration. The proton energies we obtained were modest (~MeV), but prospects for improvement are offered through further tailoring the gas jet density profile. Also, we observed that this mechanism is very robust in producing those beams and thus can be considered as a future candidate in laser-driven ion sources driven by the upcoming next generation of multi-PW near-infrared lasers.

Seeded QED cascades in counterpropagating laser pulses

The growth rates of seeded QED cascades in counterpropagating lasers are calculated with first-principles two- and three-dimensional QED-PIC (particle-in-cell) simulations. The dependence of the growth rate on the laser polarization and intensity is compared with analytical models that support the findings of the simulations. The models provide insight regarding the qualitative trend of the cascade growth when the intensity of the laser field is varied. A discussion about the cascade's threshold is included, based on the analytical and numerical results. These results show that relativistic pair plasmas and efficient conversion from laser photons to γ rays can be observed with the typical intensities planned to operate on future ultraintense laser facilities such as ELI or Vulcan.

All-optical radiation reaction at 10²¹ W/cm²

Using full-scale 3D particle-in-cell simulations we show that the radiation reaction dominated regime can be reached in an all-optical configuration through the collision of a ~1 GeV laser wakefield accelerated electron bunch with a counterpropagating laser pulse. In this configuration the radiation reaction significantly reduces the energy of the particle bunch, thus providing clear experimental signatures for the process with currently available lasers. We also show that the transition between the classical and quantum radiation reaction could be investigated in the same configuration with laser intensities of 10²³ W/cm².

Impairment of hippocampal neurogenesis in streptozotocin-treated diabetic rats

OBJECTIVES

Adult neurogenesis in dentate gyrus (DG) is an evolutionarily preserved trait in most mammals examined thus far. Neuronal proliferation and subsequent integration of new neurons into the hippocampal circuit are regulated processes that can have profound effects on an animal's behaviour. A streptozotocin model of type I diabetes, characterized by low insulin and high plasma glucose levels, affects not only body's overall metabolism but also brain activity.

MATERIALS AND METHODS

Neurogenesis was measured within the DG of the hippocampus using immunohistochemical markers Ki67, Doublecortin, Calbindin (CaBP) and bromodeoxyuridine (BrdU).

RESULTS

Cell proliferation, measured with the endogenous marker Ki67, was reduced by 45%, and cell survival, measured with BrdU, was reduced by 64% of the control. Combined effects on proliferation and survival produced dramatically lower neuronal production. Among the surviving cells only 33% matured normally as judged by the co-labelling of BrdU and CaBP.

CONCLUSION

Such a reduction lowered the number of surviving cells with neuronal phenotype by over 80% of the control values and this is expected to cause a significant functional impairment of learning and memory in diabetic animals. These results may shed light on causes of diabetic neuropathology and provide an explanation for the memory deficiencies seen in some diabetic patients.

Diabetes and the hypothalamo-pituitary-adrenal (HPA) axis

Patients and animals with poorly controlled or uncontrolled diabetes present with diurnal hypersecretion of glucocorticoids and altered regulation of the hypothalamo-pituitary-adrenocortical (HPA) axis. Although some of these changes are reversed with insulin replacement therapy, neuroendocrine function is not always restored to normal, even with rigorous glycemic control. In addition, stress responsiveness is also impaired in diabetes and this has important implications in the way patients with diabetes cope with many stress challenges, including the metabolic challenge of insulin-induced hypoglycemia. HPA dysregulation in diabetes appears to involve complex interactions between impaired glucocorticoid negative feedback sensitivity and factors such as hypoinsulinemia, hyperglycemia and/or hypoleptinemia, that may increase central drive of the axis. This review examines some of the evidence indicating hyperactivation of the HPA axis in patients with diabetes. Using the streptozotocin-diabetic rat as a model of type-1 diabetes, we will focus on elucidating some of the mechanisms underlying HPA dysregulation in diabetes. Hyperactivation of the HPA axis in diabetes is associated with increased expression of hypothalamic corticotrophin-releasing hormone (CRH) mRNA and hippocampal mineralocorticoid receptor (MR) mRNA. Although insulin replacement restores ACTH and corticosterone levels to normal, likely through glucocorticoid-mediated suppression of ACTH secretion, CRH and MR mRNA expression remain elevated. A better understanding of these mechanisms may be important in developing new treatment modalities for patients with diabetes mellitus.

Mechanisms of impaired hypothalamic-pituitary-adrenal (HPA) function in diabetes: reduced counterregulatory responsiveness to hypoglycaemia

In summary, our data suggest that in uncontrolled diabetes, increased HPA activity is caused by increased central drive at or above the level of the PVN. Insulin treatment only restores HPA activity at and below the pituitary level, presumably by GC-mediated suppression of ACTH secretion. We hypothesize that the defective HPA response to hypoglycaemia is at least in part due to a lack of a decrease in MR mRNA in response to hypoglycaemia, and diminished sensitivity of the pituitary and adrenal gland to stimulation. Interestingly, insulin treatment restores the HPA response, but not the defective epinephrine response. Therefore, defective epinephrine responses are not linked to defective HPA responses. Similarly, antecedent hypoglycaemia specifically impairs epinephrine responses, but not HPA responses to hypoglycaemia. These studies have revealed some of the mechanisms of impaired HPA function in diabetes and its impaired responsiveness to hypoglycaemia. Further investigations are essential for understanding poor counterregulation in insulin-treated diabetes and may lead to new strategies for preventing hypoglycaemia.

Molecular regulation of the hypothalamo-pituitary-adrenal axis in streptozotocin-induced diabetes: effects of insulin treatment

Increased hypothalamo-pituitary-adrenocortical (HPA) activity in diabetes is likely important in the development of some pathologies associated with the disorder. We hypothesized that central regulation of HPA activity differs among normal, streptozotocin (STZ)-diabetic, and insulin-treated diabetic rats. Blood glucose, ACTH, and corticosterone were elevated, 8 d after inducing diabetes. Insulin treatment normalized these parameters. Plasma norepinephrine was similar in all groups, but epinephrine was lower in STZ-diabetic and higher in insulin-treated rats vs. normals. Increased ACTH with diabetes corresponded with increased hypothalamic CRH mRNA, but no change in pituitary POMC mRNA. With insulin-treatment, CRH mRNA remained elevated, and POMC mRNA was unaltered. Hippocampal MR mRNA expression was dramatically increased with diabetes and, moreover, was not normalized by insulin. No differences in GR mRNA were detected between normal and STZ-diabetic rats. However, insulin treatment increased GR mRNA levels in the paraventricular nucleus and pituitary. We postulate that, in STZ-diabetes: 1) increased HPA activity is caused by increased central drive at and/or above the level of the paraventricular nucleus and is associated with decreased epinephrine; and 2) normalized pituitary-adrenal activity with insulin may be caused by the compensatory increase in GR mRNA allowing glucocorticoid-mediated suppression of ACTH secretion despite the residual increase in central HPA activity. Thus, insulin apparently restored HPA activity at and below the pituitary but, surprisingly, not above it.

Effects of antecedent hypoglycemia, hyperinsulinemia, and excess corticosterone on hypoglycemic counterregulation

This study aimed to differentiate the effects of repeated antecedent hypoglycemia, antecedent marked hyperinsulinemia, and antecedent increases in corticosterone on counterregulation to subsequent hypoglycemia in normal rats. Specifically, we examined whether exposure to hyperinsulinemia or elevated corticosterone per se could impair subsequent counterregulation. Four groups of male Sprague-Dawley rats were used: 1) normal controls (N) had 4 days of sham antecedent treatment; 2) an antecedent hypoglycemia group (AH) had 7 episodes of hyperinsulinemic hypoglycemia over 4 days; 3) an antecedent hyperinsulinemia group (AE) had 7 episodes of hyperinsulinemic euglycemia; and 4) an antecedent corticosterone group (AC) had 7 episodes of intravenous corticosterone to simulate the hypoglycemic corticosterone levels in AH rats. On day 5, hyperinsulinemic euglycemic-hypoglycemic clamps were performed. Epinephrine responses to hypoglycemia were impaired (P < 0.05 vs. N) after antecedent hypoglycemia and hyperinsulinemia. This correlated with diminished (P < 0.05 vs. N) absolute glucose production responses in AH rats and diminished incremental glucose production responses in AE rats. Paradoxically, norepinephrine responses were increased (P < 0.05 vs. N) after antecedent hypoglycemia. Glucagon and corticosterone responses were unaffected by antecedent hypoglycemia and hyperinsulinemia. In AC rats, incremental but not absolute glucose production responses were decreased (P < 0.05 vs. N). However, neuroendocrine counterregulation was unaltered. We conclude that both antecedent hypoglycemia and hyperinsulinemia impair epinephrine and glucose production responses to subsequent hypoglycemia, suggesting that severe recurrent hyperinsulinemia may contribute to the development of hypoglycemia-associated autonomic failure.

Impact of endovascular repair for abdominal aortic aneurysms in octogenarians

A total of 50 consecutive patients (86% male; median age, 82 years) underwent endovascular repair of abdominal aortic aneurysms (AAAs) ranging from 4.0 to 9.0 cm (median, 5.2 cm). Efficacy of aneurysm exclusion was assessed by angiography, duplex scan, and/or contrast-enhanced computed tomography (CT). Acute technical success was 82%. Access failed in one patient, and immediate conversion to open operation was required in two patients. Improper deployment of the endoluminal graft (ELG) across the renal arteries occurred in one patient. The median operation time, estimated blood loss, packed red blood cells received, contrast volume, and length of intensive care and hospital stay were 128 min, 200 mL, 0.1 unit, 297 mL, 0.9 days, and 3 days, respectively. ELG limb thrombosis was seen in one patient. There were 4 (8%) early endoleaks, and 2 endoleaks were discovered in other patients at 3 and 6 months. Local/vascular and remote/systemic postoperative complications were seen in 13 (26%) and 9 (18%) patients, respectively. At a median follow-up of 11 months (range 2 to 36 months), clinical success was 78%. The aneurysm sac diameter (n = 49) decreased from a preoperative median of 5.2 to 4.7 cm (p = 0.0001). Technical success was high, and results at 11 months were satisfactory. Long-term outcomes require further study.

The effect of systemic versus portal insulin delivery in pancreas transplantation on insulin action and VLDL metabolism

Combined kidney-pancreas transplantation (KPT) with anastomosis of the pancreatic vein to the systemic circulation (KPT-S) or to the portal circulation (KPT-P) provides a human model in which the chronic effects of portal versus systemic insulin delivery on glucose and VLDL metabolism can be examined. Despite similar plasma glucose and C-peptide levels, KPT-S (n = 9) had an approximate twofold elevation of fasting and intravenous glucose-stimulated plasma insulin levels compared with both KPT-P (n = 7) and healthy control subjects (n = 15). The plasma free fatty acid (FFA) levels were elevated in both transplant groups versus control subjects, but the plasma insulin elevation necessary to lower plasma FFA by 50% was approximately two times higher in KPT-S versus KPT-P and control subjects. Endogenous glucose production was similar in KPT-S and KPT-P, despite approximately 35% higher hepatic insulin levels in the latter, and was suppressed to a greater extent during a euglycemic-hyperinsulinemic clamp in KPT-S versus KPT-P. Total-body glucose utilization during the euglycemic-hyperinsulinemic clamp was approximately 40% lower in KPT-S versus KPT-P, indicating peripheral tissue but not hepatic insulin resistance in KPT-S versus KPT-P. Both transplant groups had an approximate twofold elevation of triglyceride (TG)-rich lipoprotein apolipoprotein B (apoB) and lipids versus control subjects. Elevation of VLDL-apoB and VLDL-TG in both transplant groups was entirely explained by an approximately 50% reduction in clearance of VLDL compared with healthy control subjects. In the presence of increased FFA load but in the absence of hepatic overinsulinization and marked hepatic insulin resistance, there was no elevation of VLDL secretion in KPT-S versus KPT-P and control subjects. These findings suggest that chronic systemic hyperinsulinemia and peripheral tissue insulin resistance with the consequent elevation of plasma FFA flux are insufficient per se to cause VLDL overproduction and that additional factors, such as hepatic hyperinsulinemia and/or gross insulin resistance, may be an essential prerequisite in the pathogenesis of VLDL overproduction in the common form of the insulin resistance syndrome.

Norepinephrine infusion during moderate-intensity exercise increases glucose production and uptake

A role for the increase in circulating norepinephrine (NE) during intense exercise [IE; > or = 80% maximum O(2) uptake (VO(2max))] in the marked increment in glucose rate of production (Ra) during IE is hypothesized. Seven fit male subjects (27 +/- 2 yr old; body mass index, 23 +/- 1 kg/m(2); VO(2max), 63 +/- 5 mL/kg.min) underwent 40 min of postabsorptive moderate-intensity (53% VO(2max)) cycle ergometer exercise (126 +/- 14 W), once without [control (CON)] and once with NE infusion (0.1 microg/kg.min) from 30-40 min (NE). With infusion, plasma NE reached 15.9 +/- 1.0 nM (8-fold rest, 2-fold CON). Ra doubled to 4.40 +/- 0.44 in CON, but rose to 7.55 +/- 0.68 mg/kg.min with NE infusion (P = 0.003). Ra correlated strongly (r(2) = 0.92, P < 0.02) with plasma NE during and immediately after infusion. With NE infusion, peak glucose uptake [rate of disappearance (Rd), 6.57 +/- 0.59 vs. 4.53 +/- 0.55 mg/kg.min, P < 0.02] and glucose metabolic clearance rate (P < 0.05) were higher than in CON. Glycemia rose minimally during the NE infusion but did not differ between groups at any time during exercise. Glucagon-to-insulin ratio increased minimally, and epinephrine increased approximately 2.5- to 3-fold at peak but did not differ between groups. Thus, NE infusion during moderate exercise led to increments in Ra and Rd in fit individuals, supporting a possible contributory role for the increase of plasma NE in IE. NE effects on Rd and metabolic clearance rate during exercise may differ from its effects at rest.

Low-dose IGF-I has no selective advantage over insulin in regulating glucose metabolism in hyperglycemic depancreatized dogs

At supraphysiological levels, IGF-I bypasses some forms of insulin resistance and has been proposed as a therapeutic agent in the treatment of diabetes. Unfortunately, side effects of high-dose IGF-I (100-250 microg/kg) have precluded its clinical use. Low-dose IGF-I (40-80 microg/kg), however, shows minimal side effects but has not been systematically evaluated. In our previous study under conditions of declining glucose, low-dose IGF-I infusion was more effective in stimulating glucose utilization, but less effective in suppressing glucose production and lipolysis than low-dose insulin. However, under conditions of hyperglycemia, we could not observe any differential effects between high-dose infusions of IGF-I and insulin. To determine whether the differential effects of IGF-I and insulin are dose-related or related to the prevailing glucose level, 3 h glucose clamps were performed in the same animal model as in the previous studies, i.e. the moderately hyperglycemic (175 mg/dl) insulin-infused depancreatized dog, with additional infusions of low-dose IGF-I (67.8 microg/kg, i.e. 29.1 microg/kg bolus plus 0.215 microg/kg( )per min infusion; n=5) or insulin 49.5 mU/kg (9 mU/kg bolus plus 0.45 mU/kg per min; n=7). As in the previous study under conditions of declining glucose, low-dose IGF-I had significant metabolic effects in vivo, in our model of complete absence of endogenous insulin secretion. Glucose production was similarly suppressed with both IGF-I and insulin, by 54+/-3 and 56+/-2% s.e. (P=NS) respectively. Glucose utilization was stimulated to the same extent (IGF-I 5.2+/-0.2, insulin 5.5+/-0.3 mg/kg per min, P=NS). Glucagon, free fatty acid, glycerol, alanine and beta-hydroxybutyrate, were suppressed, while lactate and pyruvate levels were raised, similarly with IGF-I and insulin. We conclude that: (i) differential effects of IGF-I and insulin may be masked under hyperglycemic conditions, independent of the hormone dose; (ii) low-dose IGF-I has no selective advantage over additional insulin in suppressing glucose production and lipolysis, nor in stimulating glucose utilization during hyperglycemia and subbasal insulin infusion when insulin secretion is absent, as in type 1 diabetes mellitus.

Effect of JTT-501 on net hepatic glucose balance and peripheral glucose uptake in alloxan-induced diabetic dogs

JTT-501, a new insulin sensitizer, improves peripheral glucose uptake in insulin-resistant animals such as KK-Ay mice and Zucker fatty rats. However, the effect of JTT-501 on hepatic glucose metabolism has not been addressed. To investigate this effect, experiments were performed on 6 alloxan-diabetic dogs. Three experiments were conducted for each dog: the treatment experiment, which followed a 10-day oral treatment with JTT-501 30 mg x kg(-1) x d(-1), and 2 control experiments 2 weeks before and 2 weeks after the treatment experiment. A hyperinsulinemic-hyperglycemic clamp was performed with the tracer dilution method (intraportal insulin infusion rate, 18 pmol x kg(-1) x min(-1)). Arterial hyperglycemia (approximately 10 mmol/L) was maintained by adjusting the peripheral glucose infusion rate. After a 45-minute basal period (period I), portal glucose infusion (22.2 micromol x kg(-1)min(-1)) was administered for 120 minutes (period II). This was followed by a 90-minutes recovery period (period III). JTT-501 increased insulin-stimulated glucose utilization (P < .05) and enhanced insulin-mediated suppression of glucose production (P < .05) in periods I and III. Net hepatic glucose balance (NHGB) determined by the arterial-venous (A-V) difference method was increased by JTT-501 in period II (P < .01). We conclude that JTT-501 enhances both hepatic and peripheral insulin sensitivity and therefore may have important therapeutic effects in type 2 diabetes.

Glucoregulatory responses to intense exercise performed in the postprandial state

A seven- to eightfold increment in hepatic glucose production (endogenous R(a)) occurs in postabsorptive (PA) intense exercise (IE). A similar response is likely present in the postprandial (PP) state, when most such exercise is performed, because 1) little evidence for increased intestinal absorption of glucose during exercise exists, and 2) intravenous glucose does not prevent it. We investigated IE in 10 PA and 8 PP fit, lean, young males who had exercised for 15 min at >84% maximum O(2) uptake, starting 3 h after a 412-kcal mixed meal. The meal induced a small rise in glycemia with sustained insulin and glucagon increases. Preexercise glucose total R(a) and utilization (R(d)) were equal and approximately 130% of the PA level. Exercise hyperglycemia in PP was delayed and diminished and, in early recovery, was of shorter duration and lesser magnitude (P = 0.042). Peak catecholamine (12- to 16-fold increase) and R(a) (PP: 11.5 +/- 1.4, PA: 13.8 +/- 1.4 mg. kg(-1). min(-1)) responses did not differ, and their responses during exercise were significantly correlated. Exercise glucagon, insulin, and glucagon-to-insulin responses were small or not significant. R(d) reached the same peak (PP: 8.0 +/- 0.6, PA: 9.3 +/- 0.8 mg. kg(-1). min(-1)) but was greater at 20-120 min of recovery in PP (P = 0.001). Therefore, the total R(a) response to IE is preserved despite the possibility of prior PP suppression of endogenous R(a) and is consistent with catecholamine mediation. Post-IE hyperglycemia is reduced in the postprandial state.

Epinephrine infusion during moderate intensity exercise increases glucose production and uptake

The glucoregulatory response to intense exercise [IE, >80% maximum O(2) uptake (VO(2 max))] comprises a marked increment in glucose production (R(a)) and a lesser increment in glucose uptake (R(d)), resulting in hyperglycemia. The R(a) correlates with plasma catecholamines but not with the glucagon-to-insulin (IRG/IRI) ratio. If epinephrine (Epi) infusion during moderate exercise were able to markedly stimulate R(a), this would support an important role for the catecholamines' response in IE. Seven fit male subjects (26 +/- 2 yr, body mass index 23 +/- 0.5 kg/m(2), VO(2 max) 65 +/- 5 ml x kg(-1) x min(-1)) underwent 40 min of postabsorptive cycle ergometer exercise (145 +/- 14 W) once without [control (CON)] and once with Epi infusion [EPI (0.1 microg x kg(-1) x min(-1))] from 30 to 40 min. Epi levels reached 9.4 +/- 0.8 nM (20x rest, 10x CON). R(a) increased approximately 70% to 3.75 +/- 0.53 in CON but to 8.57 +/- 0.58 mg x kg(-1) x min(-1) in EPI (P < 0.001). Increments in R(a) and Epi correlated (r(2) = 0.923, P </= 0.01). In EPI, peak R(d) (5.55 +/- 0.54 vs. 3.38 +/- 0.46 mg x kg(-1) x min(-1), P = 0.006) and glucose metabolic clearance rate (MCR, P = 0.018) were higher. The R(a)-to-R(d) imbalance in EPI caused hyperglycemia (7.12 +/- 0.22 vs. 5.59 +/- 0.22 mM, P = 0.001) until minute 60 of recovery. A small and late IRG/IRI increase (P = 0.015 vs. CON) could not account for the R(a) increase. Norepinephrine (approximately 4x increase at peak) did not differ between EPI and CON. Thus Epi infusion during moderate exercise led to increments in R(a) and R(d) and caused rises of plasma glucose, lactate, and respiratory exchange ratio in fit individuals, supporting a regulatory role for Epi in IE. Epi's effects on R(d) and MCR during exercise may differ from its effects at rest.

Glucoregulation during and after intense exercise: effects of alpha-adrenergic blockade

In intense exercise (>80% maximal oxygen consumption [VO2 max]), the 7- to 8-fold increase in glucose production (Ra) is tightly correlated with the greater than 14-fold increase in plasma norepinephrine (NE) and epinephrine (EPI). To distinguish the relative roles of alpha- and beta-adrenergic receptors, the responses of 12 control (C) lean, healthy, fit young male subjects to 87% VO2 max cycle ergometer exercise were compared with those of 7 subjects (at 83% VO2max) receiving intravenous phentolamine (Ph). The Ph group received a 70-microg/kg bolus and then 7 microg/kg/min from -30 minutes, during exercise and for 60 minutes of recovery. The data were analyzed by comparing exercise responses to exhaustion in Ph subjects (11.4 +/- 0.6 min) with those at both 12 minutes and at exhaustion in C subjects (14.6 +/- 0.3 min) and during recovery. There were no significant differences between groups in the plasma glucose response during exercise, but values were higher in C versus Ph subjects during the first 40 minutes of postexercise "recovery." The Ra response during the first 12 minutes of exercise was not different by repeated-measures ANOVA, reaching 10.6 +/- 1.3 mg/kg/min in C and 9.6 +/- 1.5 in Ph subjects at 12 minutes. However, in C subjects, Ra increased significantly to 14.1 +/- 1.2 mg/kg/min by exhaustion, and remained higher versus Ph subjects until 15 minutes of recovery. The Rd during recovery was not different between groups; thus, the higher Ra in C subjects in early recovery was responsible for the greater hyperglycemia observed in C subjects. Ph subjects showed a more rapid, marked increment (P = .002) in both plasma NE (to 64 v38 nmol/L) and EPI at exhaustion, and catecholamine concentrations remained higher in Ph versus C subjects during recovery. Whereas plasma insulin (IRI) declined in the C group, it increased 3-fold (P = .001) in the Ph group during exercise and until 15 minutes of recovery. Ph had no effect on glucagon (IRG). Thus, the glucagon to insulin ratio decreased in Ph subjects from baseline levels during exercise and early recovery, but increased in C subjects. The increase in Ra among Ph subjects despite the decrease in the glucagon to insulin ratio supports our earlier evidence that these hormones are not principal regulators of the Ra in intense exercise. The shorter time to exhaustion and markedly higher catecholamine levels in Ph subjects limited our ability to isolate the effects of alpha-adrenergic receptors on the Ra.alpha-Adrenergic receptors appear to have little influence on the Rd.

Beta-blockade, but not normoglycemia or hyperinsulinemia, markedly diminishes stress-induced hyperglycemia in diabetic dogs

Stress-induced hyperglycemia can lead to significant deterioration in glycemic control in individuals with diabetes. Previously, we have shown in normal dogs that, after intracerebroventricular (ICV) administration of carbachol (a model of moderate stress), increases in both the metabolic clearance rate (MCR) of glucose and endogenous glucose production (GP) occur. However, in hyperglycemic diabetic dogs subjected to the same stress, the MCR of glucose does not increase and glycemia therefore markedly deteriorates because of stimulation of GP. Our aims were to determine the following: 1) whether insulin-induced acute normalization of glycemia, with or without beta-blockade, would correct glucose clearance and prevent the hyperglycemic effect of stress, and 2) whether hyperinsulinemia per se could correct these abnormalities. Stress was induced by ICV carbachol in 27 experiments in five alloxan-administered diabetic dogs subjected to the following protocols in random order: 1) basal insulin infusion (BI) to restore normoglycemia; 2) basal insulin infusion with beta-blockade (BI+block); 3) normoglycemic-hyperinsulinemic clamp with threefold elevation of insulin above basal (3x BI); and 4) normoglycemic-hyperinsulinemic clamp with fivefold elevation of insulin above basal (5 x BI). The BI+block protocol fully prevented stress-induced hyperglycemia, both by increasing MCR (deltaMCR at peak: 0.72 +/- 0.25 ml x kg(-1) x min(-1) vs. no change in BI, P < 0.05) and by diminishing the stress-induced increment in GP observed in BI (deltaGP at peak: 3.72 +/- 0.09 micromol x kg(-1) x min(-1) for BI+block vs. 14.10 +/- 0.31 micromol x kg(-1) x min(-1) for BI, P < 0.0001). In contrast, 3x BI and 5x BI treatments with normoglycemic-hyperinsulinemic clamps proportionately increased basal MCR at baseline, but paradoxically were not associated with an increase in MCR in response to stress, which induced a twofold increase in GP. Thus, in alloxan-administered diabetic dogs, stress increased GP but not MCR, despite normalization of glycemia with basal or high insulin. In contrast, beta-adrenergic blockade almost completely restored the metabolic response to stress to normal and prevented marked hyperglycemia, both by limiting the rise in GP and by increasing glucose MCR. We conclude that acute normalization of glycemia with basal insulin or hyperinsulinemia does not prevent hyperglycemic effects of stress unless accompanied by beta-blockade, and we speculate that short-term beta-blockade may be a useful treatment modality under some stress conditions in patients with diabetes.

Immediate endovascular repair for descending thoracic aortic transection secondary to blunt trauma

PURPOSE

To report the immediate endovascular treatment of a thoracic aortic tear secondary to blunt trauma.

METHODS AND RESULTS

A 39-year-old man was injured in a motor vehicle collision. In addition to significant trauma to the head, chest, and abdomen, there were signs of a deceleration injury to the thoracic aorta. After urgent celiotomy to repair a lacerated spleen, the thoracic aortic transection was treated intraluminally using an endograft made of Gianturco Z-stents covered with polytetrafluoroethylene. The patient recovered from his injuries, and the thoracic endograft shows no evidence of endoleak 7 months after treatment.

CONCLUSIONS

Endoluminal techniques can be used successfully in the immediate repair of thoracic aortic injuries.

Gender differences in glucoregulatory responses to intense exercise

We compared glucoregulatory responses to intense exercise (14 min at 88% maximum O(2) uptake) between genders (16 men, 12 women). Analysis of covariance of maximum O(2) uptake showed no gender effect, with 82% of variance due to fat-free mass (FFM). Glycemia rose comparably during exercise but was higher in women during recovery (P = 0.02). Glucose production [rate of appearance (R(a)); in mg/min] increased markedly in both; stepwise multiple regression and analysis of covariance of R(a) (peak and incremental area under the curve) showed no effect of gender, body weight, or FFM. Glucose uptake [rate of disappearance (R(d))] increased less than R(a) and slower in women. R(d) area under the curve related to FFM (P = 0.01) but not gender or body weight. Norepinephrine and epinephrine responses (13-18x baseline) were the same and correlated significantly with R(a). Exercise insulin and glucagon changes were slight, but postexercise hyperinsulinemia was greater in women (P = 0.018), along with higher R(d). Therefore, intense exercise glucoregulation is qualitatively similar between genders, with a "feed-forward" regulation of R(a) (consistent with catecholamine mediation). However, women have a lesser R(d) response, related to FFM. This combination leads to greater recovery-period hyperglycemia and hyperinsulinemia.

Intraoperative subclavian artery stenting to salvage a LIMA graft

The internal mammary artery is the preferred conduit for coronary bypass grafting; however, suboptimal flow through the internal mammary artery is sometimes found during the operation, and the conduit is abandoned. Subclavian artery stenosis, a well-recognized cause of reduced internal mammary artery flow, is easily and effectively treated with endovascular techniques. We describe a case of intraoperative primary stent deployment in a high-grade subclavian artery stenosis compromising internal mammary artery flow.

Glucoregulation during and after intense exercise: effects of beta-adrenergic blockade in subjects with type 1 diabetes mellitus

In intense exercise (>80% maximum oxygen uptake) a huge, up to 8-fold increase in glucose production (Ra) is tightly correlated to marked increases in plasma norepinephrine (NE) and epinephrine. Both Ra and glucose uptake (Rd) are enhanced, not reduced, during beta-adrenergic blockade in normal subjects. Beta-blockade also caused a greater fall in immunoreactive insulin (IRI) during exercise, which could, in turn, have increased Ra directly or via an increased glucagon/insulin ratio. To control for adrenergic effects on endogenous insulin secretion, we tested type 1 diabetic subjects (DM) made euglycemic by overnight i.v. insulin that was kept constant in rate during and after exercise. Their responses to postabsorptive cycle ergometer exercise at 85-87% maximum oxygen uptake for approximately 14 min were compared to those of similar male control (CP) subjects. Six DM and seven CP subjects received i.v. 150 microg/kg propranolol over 20 min, then 80 microg/kg x min from -30 min, during exercise and for 60 min during recovery. Plasma glucose increased from similar resting values to peaks of 6.8 mmol/L in DM and 6.5 mmol/L in CP, then returned to resting values in CP within 20 min, but in DM, remained higher than in CP from 8-60 min (P = 0.049). Ra rose rapidly until exhaustion, to 13.3 mg/kg x min in CP and 11.6 in DM (P = NS). Ra declined rapidly in recovery, although somewhat more slowly in DM (P = 0.013 from 2-15 min). The Rd increased to 10.6 in CP and 9.2 mg/kg x min in DM (P = NS), then declined similarly in early recovery, but remained higher in CP from 50-100 min (P = 0.05). The rises in plasma glucose during exercise in both groups were thus due to the increments in Rd less than those in Ra. The higher recovery glucose in DM was due to the slower decline in Ra and the lower Rd in later recovery. IRI was higher in DM than in CP before exercise (P = 0.011), and whereas it decreased in CP (P < 0.05), it increased approximately 2-fold in DM, thus being higher throughout exercise (P = 0.003). The glucagon/insulin ratio was unchanged in DM, but increased in CP during exercise (P = 0.002). NE showed a rapid, marked increment during exercise to peak values of 23.7 nmol/L in CP and 25.7 nmol/L in DM (P = NS), and epinephrine showed parallel responses. Both correlated significantly with the Ra responses. In summary, the Ra responses of both DM and CP during exercise were greater than those of control unblocked subjects (previously reported) despite higher IRI (all exogenous) in DM. This suggests an important contribution of direct alpha-adrenergic stimulation to this Ra effect.

Glucose uptake during centrally induced stress is insulin independent and enhanced by adrenergic blockade

Glucose utilization increases markedly in the normal dog during stress induced by the intracerebroventricular (ICV) injection of carbachol. To determine the extent to which insulin, glucagon, and selective (alpha/beta)-adrenergic activation mediate the increment in glucose metabolic clearance rate (MCR) and glucose production (R(a)), we used five groups of normal mongrel dogs: 1) pancreatic clamp (PC; n = 7) with peripheral somatostatin (0.8 microg x kg(-1) x min(-1)) and intraportal replacement of insulin (1,482 +/- 84 pmol x kg(-1) x min(-1)) and glucagon (0.65 ng x kg(-1) x min(-1)) infusions; 2) PC plus combined alpha (phentolamine)- and beta (propranolol)-blockade (7 and 5 microg x kg(-1) x min(-1), respectively; alpha+beta; n = 5); 3) PC plus alpha-blockade (alpha; n = 6); 4) PC plus beta-blockade (beta; n = 5); and 5) a carbachol control group without PC (Con; n = 10). During ICV carbachol stress (0-120 min), catecholamines, ACTH, and cortisol increased in all groups. Baseline insulin and glucagon levels were maintained in all groups except Con, where glucagon rose 33%, and alpha, where insulin increased slightly but significantly. Stress increased (P < 0.05) plasma glucose in Con, PC, and alpha but decreased it in beta and alpha+beta. The MCR increment was greater (P < 0.05) in beta and alpha+beta than in Con, PC, and alpha. R(a) increased (P < 0.05) in all groups but was attenuated in alpha+beta. Stress-induced lipolysis was abolished in beta (P < 0.05). The marked rise in lactate in Con, PC, and alpha was abolished in alpha+beta and beta. We conclude that the stress-induced increase in MCR is largely independent of changes in insulin, markedly augmented by beta-blockade, and related, at least in part, to inhibition of lipolysis and glycogenolysis, and that R(a) is augmented by glucagon and alpha- and beta-catecholamine effects.

Opposite effects of acute hypoglycemia and acute hyperglycemia on glucose transport and glucose transporters in perfused rat skeletal muscle

This study was undertaken to characterize the effects of glycemia per se (glucose effectiveness) on muscle glucose transport. Isolated rat hindlimbs were perfused in situ for 2 h with perfusate containing either low (2 mmol/l, n = 7), normal (6.5 mmol/l, n = 6), or high (20 mmol/l, n = 6) concentrations of glucose, without insulin, to simulate hypo-, eu-, and hyperglycemic conditions. The effect of varying glucose concentrations on muscle glucose transport was assessed by an ensuing 30-min perfusion with 5.5 mmol/l glucose perfusate without insulin. The 2-h of low glucose perfusion induced significant increases in both muscle glucose clearance (approximately 2.3-fold, P < 0.01) and plasma membrane GLUT4 content (approximately 20%, P < 0.05) relative to normal. In contrast, high glucose perfusion decreased glucose clearance (approximately 1.7-fold, P < 0.01) and plasma membrane GLUT4 content (approximately 20%, P < 0.05). Glucose extraction during the following 30-min perfusion was 2.5-fold greater (P < 0.0001) in the low group and threefold less (P < 0.0001) in the high group, relative to normal. 2-[3H]deoxyglucose-6-phosphate content in both red (soleus) and white (extensor digitorum longus) muscles increased approximately twofold after 2 h of low glucose perfusion (P < 0.0001) and decreased > or =2-fold after high glucose perfusion (P < 0.0001), relative to normal. It is concluded that glycemia regulates glucose transport in skeletal muscle independently of insulin, achieved at least partially via changes in plasma membrane GLUT4. We propose that high glucose levels can acutely downregulate GLUT4 and glucose clearance, thus limiting excessive glucose uptake in muscle. Conversely, low glucose-induced upregulation of muscle glucose clearance and GLUT4 can compensate for reduced glucose availability in the circulation.

Glucagon-like peptide 1 increases insulin sensitivity in depancreatized dogs

To determine whether glucagon-like peptide (GLP)-1 increases insulin sensitivity in addition to stimulating insulin secretion, we studied totally depancreatized dogs to eliminate GLP-1's incretin effect. Somatostatin was infused (0.8 microg x kg(-1) x min(-1)) to inhibit extrapancreatic glucagon in dogs, and basal glucagon was restored by intraportal infusion (0.65 ng x kg(-1) x min(-1)). To simulate the residual intraportal insulin secretion in type 2 diabetes, basal intraportal insulin infusion was given to obtain plasma glucose concentrations of approximately 10 mmol/l. Glucose was clamped at this level for the remainder of the experiment, which included peripheral insulin infusion (high dose, 5.4 pmol x kg(-1) x min(-1), or low dose, 0.75 pmol x kg(-1) x min(-1)) with or without GLP-1(7-36) amide (1.5 pmol x kg(-1) x min(-1)). Glucose production and utilization were measured with 3-[3H]glucose, using radiolabeled glucose infusates. In 12 paired experiments with six dogs at the high insulin dose, GLP-1 infusion resulted in higher glucose requirements than saline (60.9+/-11.0 vs. 43.6+/-8.3 micromol x kg(-1) x min(-1), P< 0.001), because of greater glucose utilization (72.6+/-11.0 vs. 56.8+/-9.7 micromol x kg(-1) x min(-1), P<0.001), whereas the suppression of glucose production was not affected by GLP-1. Free fatty acids (FFAs) were significantly lower with GLP-1 than saline (375.3+/-103.0 vs. 524.4+/-101.1 micromol/l, P<0.01), as was glycerol (77.9+/-17.5 vs. 125.6+/-51.8 micromol/l, P<0.05). GLP-1 receptor gene expression was found using reverse transcriptase-polymerase chain reaction of poly(A)-selected RNA in muscle and adipose tissue, but not in liver. Low levels of GLP-1 receptor gene expression were also found in adipose tissue using Northern blotting. In 10 paired experiments with five dogs at the low insulin dose, GLP-1 infusion did not affect glucose utilization or FFA and glycerol suppression when compared with saline, suggesting that GLP-1's effect on insulin action was dependent on the insulin dose. In conclusion, in depancreatized dogs, GLP-1 potentiates insulin-stimulated glucose utilization, an effect that might be contributed in part by GLP-1 potentiation of insulin's antilipolytic action.

Resistance to insulin's acute direct hepatic effect in suppressing steady-state glucose production in individuals with type 2 diabetes

We and others have shown that insulin acutely suppresses glucose production in fasting nondiabetic humans and dogs, by both a direct hepatic effect and an indirect (extrahepatic) effect, and in diabetic dogs by an indirect effect alone. In type 2 diabetes, there is resistance to insulin's ability to suppress hepatic glucose production, but it has not previously been determined whether the resistance is primarily at the level of the hepatocyte or the peripheral tissues. To determine whether the diabetic state reduces the direct effect of insulin in humans, we studied nine patients with untreated type 2 diabetes who underwent three studies each, 4-6 weeks apart. 1) Portal study (POR): intravenous tolbutamide was infused for 3 h with calculation of pancreatic insulin secretion from peripheral plasma C-peptide. 2) Peripheral study (PER): equidose insulin was infused by peripheral vein. 3) Half-dose peripheral insulin study (1/2 PER): matched peripheral insulin levels with study 1. In all studies, glucose was clamped at euglycemia, glucose turnover was measured with the constant specific activity method, and 3-[3H]glucose was purified by high-performance liquid chromatography. Peripheral insulin was lower in POR versus PER but slightly higher in POR versus 1/2 PER, although most of the difference could be accounted for by higher proinsulin levels in POR (stimulated by tolbutamide). Calculated portal insulin was approximately 1.3-fold higher in POR versus PER and approximately 2.2-fold higher in POR versus 1/2 PER. In the final 30 min of the clamp, glucose production reached a lower steady-state level in PER than in POR (4.0 +/- 0.4 vs. 5.3 +/- 0.5 pmol(-1) x kg(-1) x min(-1), P < 0.05), despite the higher hepatic insulin level in POR. In contrast with our studies in nondiabetic individuals, glucose production was not more suppressed at steady state in POR versus 1/2 PER (5.3 +/- 0.4 micromol x kg(-1) x min(-1)), despite much higher hepatic insulin levels in POR. In conclusion, this is the first study in patients with type 2 diabetes to characterize insulin resistance to the acute direct suppressive effect of insulin on hepatic glucose production.

Counterregulatory response to hypoglycemia differs according to the insulin delivery route, but does not affect glucose production in normal humans

The magnitude of the counterregulatory response to insulin-induced hypoglycemia is primarily determined by the degree of hypoglycemia. We examined whether the route of acute insulin delivery (portal or peripheral venous) is also important in determining the magnitude of the counterregulatory response to hypoglycemia in nine healthy nondiabetic men. Pancreatic insulin secretion, stimulated by an i.v. tolbutamide infusion (portal insulin study), was matched with an exogenous insulin infusion into the peripheral vein 4-6 weeks later (peripheral insulin study). Each study consisted of a 150-min baseline tracer equilibration period, a 180-min euglycemic hyperinsulinemic (portal or peripheral insulin delivery) period, a 60-min hypoglycemic period in which insulin secretion diminished during tolbutamide or was reduced during exogenous insulin, and a 30-min recovery period. Peripheral venous glucose concentrations were well matched in the portal and peripheral studies during euglycemia and hypoglycemia (glucose nadir, 2.9 +/- 0.1 mmol/L in the portal and 2.7 +/- 0.1 mmol/L in the peripheral; mean +/- SEM; P = NS), and insulin concentrations were about 1.5-fold higher throughout the experiment in the peripheral vs. the portal insulin study due to the first pass extraction of insulin in the portal study. There was a much greater increment (P < 0.0001) in FFA in the portal vs. the peripheral study (area under the curve: portal, 19.5 +/- 3.9 mmol/L x 90 min; peripheral, 3.3 +/- 1.1 mmol/L x 90 min), whereas plasma glucagon and GH were higher in the peripheral study (P = 0.01 for glucagon; P = 0.015 for GH). There was no significant difference between studies in epinephrine and norepinephrine responses to hypoglycemia or stimulation of endogenous glucose production (area under the curve: portal, 636 +/- 103 micromol/kg x 90 min; peripheral, 705 +/- 69 micromol/kg x 90 min; P = NS). In summary, we have shown that the glucagon, GH, and FFA responses to hypoglycemia during insulin dissipation are affected by the route of insulin delivery and are not controlled exclusively by the nadir blood glucose level. The clinical importance of these observations in diabetic subjects as they relate to route of insulin delivery (portal or peripheral) during insulin dissipation remains to be determined.

Glucose rapidly decreases plasma membrane GLUT4 content in rat skeletal muscle

We have previously demonstrated that chronic hyperglycemia per se decreases GLUT4 glucose transporter expression and plasma membrane content in mildly streptozotocin- (STZ) diabetic rats (Biochem. J. 284, 341-348, 1992). In the present study, we investigated the effect of an acute rise in glycemia on muscle GLUT4 and GLUT1 protein contents in the plasma membrane, in the absence of insulin elevation. Four experimental groups of rats were analyzed in the postabsorptive state: 1. Control rats. 2. Hyperglycemic STZ-diabetic rats with moderately reduced fasting insulin levels. 3. STZ-diabetic rats made normoglycemic with phlorizin treatment. 4. Phlorizin-treated (normoglycemic) STZ-diabetic rats infused with glucose for 40 min. The uniqueness of the latter model is that glycemia can be rapidly raised without any concomitant increase in plasma insulin levels. Plasma membranes were isolated from hindlimb muscle and GLUT1 and GLUT4 proteins amounts determined by Western blot analysis. As predicted, STZ-diabetes caused a significant decrease in the abundance of GLUT4 in the isolated plasma membranes. Normalization of glycemia for 3 d with phlorizin treatment restored plasma membrane GLUT4 content in muscle of STZ-diabetic rats. A sudden rise in glycemia over a period of 40 min caused the GLUT4 levels in the plasma membrane fraction to decrease to those of nontreated STZ-diabetic rats. In contrast to the GLUT4 transporter, plasma membrane GLUT1 abundance was not changed by the acute glucose challenge. It is concluded that glucose can have regulatory effect by acutely reducing plasma membrane GLUT4 protein contents in rat skeletal muscle. We hypothesize that this glucose-induced downregulation of plasma membrane GLUT4 could represent a protective mechanism against excessive glucose uptake under hyperglycemic conditions accompanied by insulin resistance.

Mediation of glucoregulation at rest and during exercise by the glucose-fatty acid cycle: in vivo and in vitro studies

Himsworth (1934) demonstrated that increased fat consumption leads to decreased glucose tolerance due to decreased insulin sensitivity. Randle and colleagues (1964) named this interplay between fat and carbohydrate metabolism the glucose-fatty acid cycle (GFAC) and proposed a series of feedback mechanisms by which elevated levels of free fatty acids (FFAs) impair glucose uptake and oxidation in rat heart and diaphragm muscle. Numerous investigators have extended these studies to clarify the existence of GFAC and provide insight into the mechanisms and conditions under which it occurs. This paper reviews the literature and highlights other indirect means by which FFAs affect carbohydrate metabolism. Numerous in vitro studies are reviewed, emphasizing the importance of FFA concentration, carbon length, and degree of saturation. This article addresses evidence that the interplay between fat and carbohydrate metabolism is not a function of FFA concentration but a result of the impact that FFA levels have on insulin.

Glucose infusion partially attenuates glucose production and increases uptake during intense exercise

Glucose infusion can prevent the increase in glucose production (Ra) and increase glucose uptake (Rd) during exercise of moderate intensity. We postulated that 1) because in postabsorptive intense exercise (>80% maximal O2 uptake) the eightfold increase in Ra may be mediated by catecholamines rather than by glucagon and insulin, exogenous glucose infusion would not prevent the Ra increment, and 2) such infusion would cause greater Rd. Fit young men were exercised at >85% maximal O2 uptake for 14 min in the postabsorptive state [controls (Con), n = 12] or at minute 210 of a 285-min glucose infusion. In seven subjects, the infusion was constant (CI; 4 mg . kg-1 . min-1), and in seven subjects it was varied (VI) to mimic the exercise Ra response in Con. Although glucose suppressed Ra to zero (with glycemia approximately 6 mM and insulin approximately 150 pM), an endogenous Ra response to exercise occurred, to peak increments two-thirds those in Con, in both CI and VI. Glucagon was unchanged, and very small increases in the glucagon-to-insulin ratio occurred in all three groups. Catecholamine responses were similar in all three groups, and correlation coefficients of Ra with plasma norepinephrine and epinephrine were significant in all. In all CI and VI, Rd at rest was 2x Con, increased earlier in exercise, and was higher for the 1 h of recovery with glucose infusion. Thus the Ra response was only partly attenuated, and the catecholamines are likely to be the regulators. This suggests that an acute endogenous Ra rise is possible even in the postprandial state. Furthermore, the fact that more circulating glucose is used by muscle during exercise and early recovery suggests that muscle glycogen is spared.

Role of free fatty acids and glucagon in the peripheral effect of insulin on glucose production in humans

We have shown previously that the greater suppression of endogenous glucose production (GP) with equimolar peripheral vs. portal insulin cannot be detected or is minimally reversed when the insulin-induced suppression of either free fatty acids (FFA) or glucagon alone is prevented. The present experiments were designed to minimize the insulin suppression of both glucagon and FFA in an attempt to further examine the mechanism of insulin's peripheral effect on GP. In nine healthy men, we investigated the effect of limiting the insulin suppression of both FFA and glucagon by infusing heparin (250 U/h), Intralipid 10% (25 ml/h), and glucagon (0.65 ng . kg-1 . min-1) during 1) portal (n = 9), 2) equimolar peripheral (n = 9), and 3) half-dose peripheral insulin delivery (n = 4) by use of our previously published tolbutamide infusion method, with calculation and matching of insulin secretion rate. GP decreased by 57.2 +/- 2. 6% with portal, 39.0 +/- 4.1% with equimolar peripheral, and 31.5 +/- 2.7% with half-dose peripheral insulin delivery (P < 0.001 for portal vs. peripheral and P < 0.001 for portal vs. half-dose peripheral). In contrast, in six control subjects in whom glucagon and FFA were not replaced, GP decreased by 62.6 +/- 2.4% with portal (n = 6), 75.7 +/- 3.0% with peripheral (n = 6), and 56.3 +/- 3.0% with half-dose peripheral (n = 4) insulin delivery (P < 0.01 for portal vs. peripheral and P = not significant for portal vs. half-dose peripheral). In summary, the greater suppression of GP with equimolar peripheral vs. portal insulin is eliminated and markedly reversed if the acute insulin-induced suppression of both plasma FFA and glucagon is minimized. This suggests that the insulin-induced suppression of glucagon and FFA has additive or cooperative effects in mediating the acute extrahepatic effect of insulin on GP.

Quantitative measurement of islet glucagon response to hypoglycemia by confocal fluorescence imaging in diabetic rats: effects of phlorizin treatment

We have shown that the glucagon irresponsiveness to hypoglycemia in diabetic rats is markedly improved by correction of hyperglycemia independent of insulin. In contrast, normalization of glycemia by insulin did not improve this response. To find out whether these glucagon responses reflect changes in islet glucagon, we directly quantified glucagon area and content in each pancreatic islet by using fluorescent immunostaining and computerized image analysis with confocal laser scanning microscopy (CLSM). The pancreases were analyzed in four groups of rats. 1. Normal controls (NC, n = 4), streptozotocin (65 mg/kg) diabetic rats. 2. Diabetic untreated (DU, n = 4). 3. Diabetic Phlorizin-treated, (0.4 g/kg), twice daily for 4 d (DP, n = 4). 4. Diabetic insulin-treated, using sustained release (2-3 U/d) insulin implant for 5 d (DI, n = 4). Basal plasma glucose was 7.4 +/- 0.3 mM in NC, increased to 14.5 +/- 2.2 mM in DU, which was normalized in DP (5.5 +/- 0.5) and DI (6.7 +/- 0.8). Acute hypoglycemia (H) was induced by i.v. insulin injection. The rats were sacrificed 2 h after insulin injection and the pancreas was removed. By imaging with CLSM, we quantified: 1. Percent of glucagon containing A-cell area/islet area, 2. Fluorescence intensity per islet area, which indicated glucagon content in the islet. 3. Fluorescence intensity per glucagon area indicating glucagon concentration in A-cells. In NC, glucagon containing A cell area was 21 +/- 2% of the islet area, and glucagon intensity and concentration was 11 +/- 1 U and 36 +/- 3.0 U, respectively, in basal (O) state and did not change in (H). In DU, glucagon area increased 183% (O) and 166% (H), and islet glucagon intensity increased by 235% (O) (p < 0.05), but decreased to 135% in H. Glucagon area in DP and DI did not differ significantly from DU. However, hypoglycemia in DP increased glucagon intensity in islet further to 306% of normal control (p < 0.05), suggesting marked increase in glucagon content indicating increased synthesis. In contrast, DI compared to DP showed a decrease in glucagon intensity in islet (46 +/- 3, DP to 22 +/- 2 DI; p < 0.05) in (H) state. Glucagon concentration followed the same pattern as its intensity.

CONCLUSION

1. Increase in islet glucagon content in diabetic rats was associated with increase in glucagon containing A-cell area per islet. 2. Phlorizin-induced insulin independent correction of hyperglycemia increased glucagon content per islet in hypoglycemic state. This, in part, probably contributed to improved glucagon response to hypoglycemia observed earlier 3. Normalization of glycemia with insulin reduced glucagon content of each islet during hypoglycemia. This may explain, in part, unresponsiveness of glucagon to hypoglycemia often observed in insulin-dependent diabetes mellitus (IDDM) with intensive insulin therapy.

Insulin-independent acute restoration of euglycemia normalizes the impaired glucose clearance during exercise in diabetic dogs

At rest and during exercise, chronic hyperglycemia, high free fatty acid (FFA) oxidation, and insulin deficiency in diabetes are well known to impair glucose clearance (metabolic clearance rate [MCR]). The effect of acute restoration of glycemia per se on MCR has been less well characterized. We therefore studied normal and alloxan-diabetic dogs both at rest and during exercise, as diabetic hyperglycemic or after acutely induced euglycemia (<160 min) generated by infusion of either insulin or phlorizin. Glucose uptake was similar under hyperglycemic and normoglycemic conditions both at rest and during exercise, indicating a precise balance between the mass effect of glucose and decreased MCR. Rest and exercise MCR was fourfold lower under conditions of hyperglycemia, but insulin-independent restoration of euglycemia improved basal MCR threefold and normalized MCR during exercise. High FFA turnover did not affect glucose uptake but was correlated with plasma lactate concentrations (r = 0.72, P < 0.001), suggesting that muscle fuel requirements are controlled by glucose oxidation and not uptake. We conclude that in alloxan-diabetic dogs, the impaired MCR may be an adaptive phenomenon because correction of hyperglycemia corrects MCR despite partial insulin deficiency and high FFA turnover. We speculate that constant glucose uptake despite hyperglycemia in diabetes may protect the muscle from excessive exposure to glucose.

Fatty acids mediate the acute extrahepatic effects of insulin on hepatic glucose production in humans

We have shown previously in humans that insulin partly suppresses hepatic glucose production (HGP) by an extrahepatic (indirect) mechanism. In the present study, we investigated the role of free fatty acids (FFAs) in mediating the extrahepatic effects of insulin in humans and determined the extent to which insulin can regulate HGP by a non-FFA-mediated effect. Sixteen healthy men received an intravenous tolbutamide infusion for 3 h, and pancreatic insulin secretion was calculated by deconvolution of peripheral C-peptide levels. On a subsequent occasion, equimolar exogenous insulin was infused by peripheral vein. In both studies, glucose was clamped at euglycemia. We have previously validated this method and shown no independent insulin-like activity of tolbutamide. During the clamp, 9 of the 16 subjects received a low dose of heparin and Intralipid to prevent the insulin-induced suppression of FFAs, while 7 subjects received a high dose of heparin and Intralipid to raise FFAs approximately 2.5-fold. In both the high- and low-dose groups, peripheral insulin was higher and calculated portal insulin lower with peripheral versus portal insulin delivery. In the low-dose group, HGP decreased by 68.3 +/- 2.1% with portal insulin delivery and 64.7 +/- 3.7% with peripheral insulin delivery (NS). In the high-dose group, HGP decreased by 58.0 +/- 4.5% with portal insulin and 48.3 +/- 5.0% with peripheral insulin (P < 0.05). Four individuals who participated in the high-dose group underwent an additional peripheral insulin study in which the same dose of exogenous insulin was infused as in the high-dose group but in the absence of heparin and Intralipid. During this latter study, FFA levels declined by approximately 90% during hyperinsulinemia, and HGP was suppressed by 71.8 +/- 5.6%, which was a much greater suppression (P < 0.01) than when FFA levels were raised in these subjects during the equivalent rate insulin infusion. In summary, the previously observed greater suppression of HGP with equimolar peripheral versus portal insulin is eliminated or reversed, depending on plasma FFA levels, if FFAs are prevented from decreasing, suggesting an important role of FFAs in mediating the extrahepatic effects of insulin on HGP. However, the effect of FFA clamping is relatively small with a significant degree of suppression of HGP (by approximately 50%), which remains even when FFAs are elevated above basal levels, suggesting that in the physiological range FFAs only partially influence the suppression of HGP in humans. This suggests that other mechanisms, most likely hepatic, dominate the acute insulin-induced suppression of glucose production.

Direct and indirect effects of insulin in suppressing glucose production in depancreatized dogs: role of glucagon

We have previously shown that during glucose clamps in moderately hyperglycemic depancreatized dogs: 1) peripheral insulin infusion, resulting in greater systemic insulinemia and greater suppression of glucagon than equidose portal infusion, inhibited glucose production (GP) to a greater extent; and 2) portal and half-dose peripheral infusions, resulting in matched peripheral insulinemia and similar suppression of glucagon, inhibited GP equally. These findings are consistent with an indirect effect of insulin in suppressing GP in diabetic dogs, which might be partly mediated by the differential suppression of glucagon. To address this question, we performed the experimental protocols of the previous study under conditions of constant glucagon levels (approximately 550 ng/liter), achieved by a high rate portal glucagon infusion (5 ng/kg.min). As in the previous study (basal glucagon levels, approximately 170 ng/liter), we used depancreatized dogs and assessed GP with HPLC-purified [6(-3)H]glucose. After obtaining constant basal hyperglycemia (approximately 10 mM) with portal infusions of insulin (4.8 +/- 0.5 pmol/kg.min) and glucagon, an additional infusion of insulin was administered for 180 min, either portally (portal; n = 7) or peripherally (peripheral; n = 8) at the same rate (5.4 pmol/kg.min) or at half that rate peripherally (1/2 periph; n = 5). Plasma glucose and glucose specific activities were clamped at basal levels. Systemic insulin levels increased by 215 +/- 16,310 +/- 26, and 184 +/- 15 pM, and estimated hepatic insulin levels increased by 398 +/- 20, 310 +/- 26, and 184 +/- 15 pM with portal, peripheral, and 1/2 periph, respectively. GP was suppressed to the same extent with portal and peripheral (53 +/- 6% and 50 +/- 6%), but less with 1/2 periph (35 +/- 5%). FFA levels were suppressed to a greater extent with peripheral than portal or 1/2 periph, whereas the responses of lactate alanine and glycerol to insulin infusion were similar in the three groups. Thus, in the present report, unlike in our previous study, 1) suppression of GP was proportional to the hepatic insulin levels; and 2) systemic insulin levels did not dominate suppression of GP. We, therefore, conclude that in hyperglycemic depancreatized dogs 1) glucagon, at concentrations seen in poorly controlled diabetes, can unmask a direct effect of hepatic insulin levels on GP; and 2) the suppression of glucagon may play a role in the peripheral effect of exogenously delivered insulin on GP. This is the first in vivo study to show that the main direct effect of insulin on the liver is to counteract the effect of glucagon.

Glucagon enhances the direct suppressive effect of insulin on hepatic glucose production in humans

The present study examines the role of glucagon in modulating the hepatic and extrahepatic effects of insulin on hepatic glucose production (HGP). We infused glucagon at a constant rate (0.65 ng x kg(-1) x min(-1)) during equimolar portal and peripheral insulin delivery in seven healthy males by our previously published tolbutamide infusion method. In contrast to our previous study, in which glucagon fell by approximately 30% during hyperinsulinemia and suppression of HGP was significantly greater with equimolar peripheral than with portal insulin delivery, HGP was actually suppressed to a lesser extent with peripheral insulin delivery (69 +/- 10%) than when insulin was delivered portally (76 +/- 5%, P < 0.05). To further examine whether glucagon was enhancing the effect of portal insulin, in four additional individuals HGP was suppressed to a greater extent during a tolbutamide infusion when glucagon was administered continuously throughout the basal and hyperinsulinemic periods than when glucagon was infused during the basal period only; HGP suppressed by 63 +/- 3 vs. 52 +/- 3%, respectively, P = 0.02). Tolbutamide had no effect on HGP when infused into three C-peptide-negative individuals with type I diabetes during a low-dose insulin and glucagon infusion. These data suggest that glucagon levels are an important determinant of the balance between insulin's direct and indirect effects on HGP, with glucagon likely potentiating the direct hepatic effect of insulin.

The effect of pioglitazone on hepatic glucose uptake measured with indirect and direct methods in alloxan-induced diabetic dogs

Pioglitazone, a thiazolidinedione derivative, ameliorates hyperglycemia by augmenting peripheral glucose disposal and suppressing hepatic glucose production in diabetic animals. However, the effect of this agent on hepatic glucose uptake has not been explored. To determine this, experiments were conducted in alloxan-induced diabetic dogs with (pioglitazone group, n = 7) or without (control group, n = 5) a 10-day oral treatment with pioglitazone (1 mg x kg(-1) x day(-1)). A euglycemic-hyperinsulinemic (insulin infusion rate 25.2 pmol x kg(-1) x min(-1)) clamp was maintained by adjusting the peripheral glucose infusion rate (GIR). After a 60-min basal period (period I), portal glucose infusion (Pinf, 33.3 micromol x kg(-1) x min(-1)) was administered for 120 min (period II). This was followed by a 60-min recovery period (period III). Arterial insulin levels were kept stable in the supraphysiological range throughout the experiment (1,623 +/- 52, pioglitazone group; 1,712 +/- 52 pmol/l, C group). There was no significant difference in whole-body glucose utilization determined by [3-3H]glucose between the pioglitazone and C groups in period I (68.4 +/- 2.8 vs. 70.1 +/- 2.8 micromol x kg(-1) x min(-1), respectively) and period III (81.2 +/- 5.0 vs. 74.5 +/- 3.3 micromol x kg(-1) x min(-1), respectively). Net hepatic glucose uptake (NHGU) determined by arteriovenous difference method was approximately zero in the basal period (-0.7 +/- 1.1, pioglitazone group; 0.1 +/- 1.2 micromol x kg(-1) x min(-1), C group). In period II, hepatic glucose uptake, determined by the changes in GIR, was significantly higher in the pioglitazone group (6.5 +/- 0.6 micromol x kg(-1) x min(-1)) than in the C group (-0.4 +/- 0.6 micromol x kg(-1) x min(-1), P < 0.001). This observation was also confirmed by NHGU during portal glucose infusion (6.9 +/- 1.4 vs. 2.1 +/- 1.8 micromol x kg(-1) x min(-1), pioglitazone vs. C, respectively; P < 0.025). We conclude that pioglitazone treatment enhances hepatic glucose uptake during portal glucose loading in alloxan-induced diabetic dogs. However, in hyperinsulinemic conditions, pioglitazone does not enhance the already high peripheral glucose uptake.

Normal hepatic insulin sensitivity in lean, mild noninsulin-dependent diabetic patients

We studied hepatic and extrahepatic insulin sensitivity and insulin release in seven nonobese patients with mild noninsulin-dependent diabetes mellitus (NIDDM) and 10 control subjects, matched for age, body mass index, and physical fitness. Glucose turnover was studied during sequential hyperinsulinemic euglycemic clamps (insulin infusion, 0.25 and 1.0 mU/kg BW.min), applying the hot-GINF (tracer-enriched glucose infusion) technique and using [6-3H]glucose. Hepatic glucose production was lower in hyperglycemic NIDDM patients during the basal period (P < 0.01), but was equivalent at similar glucose and insulin levels attained during both clamps. In contrast, during the low and high insulin clamps, glucose utilization was lower in NIDDM [14.90 +/- 1.00 vs. 17.24 +/- 0.83 (P < 0.01) and 41.37 +/- 3.05 vs. 50.54 +/- 3.61 mumol/kg BW.min (P < 0.01)]. Accordingly, the glucose infusion rate necessary to maintain euglycemia was lower in NIDDM [7.72 +/- 2.00 vs. 10.68 +/- 1.17 (P < 0.05) and 42.14 +/- 4.50 vs. 51.60 +/- 4.28 mumol/kg BW.min (P < 0.01)]. There was, however, a considerable overlap between patients and controls in the parameters describing insulin sensitivity. The insulin response to orally administered glucose as well as that to a standardized glucose infusion test (GIT) were diminished in NIDDM [average incremental insulin secretion during an oral glucose tolerance test, 88 +/- 28 vs. 251 +/- 50 pmol/L.min (P < 0.05); during first 10 min of GIT, 7 +/- 16 vs. 234 +/- 29 pmol/L.min (P < 0.001)]. There was no overlap in acute phase insulin secretion during the GIT between the groups. In conclusion, nonobese, mild NIDDM patients showed no impairment in hepatic, but a slight reduction in extrahepatic insulin sensitivity, with extensive overlap between diabetic and control subjects. In contrast, impairment of insulin release was very pronounced and without overlap.

Glucagon response to hypoglycemia is improved by insulin-independent restoration of normoglycemia in diabetic rats

The aim of this study was to determine whether the impaired glucagon response to insulin-induced hypoglycemia in the diabetic rat can be improved by correction of hyperglycemia independent of insulin. Four groups of age-matched male Sprague-Dawley rats (246 +/- 13 g BW) were studied: 1) normal controls (NC; n = 7); 2) diabetic, untreated (DU; n = 6); 3) diabetic, treated for 5-7 days using sustained release (2-3 U/day) insulin implants (DI; n = 6); and 4) diabetic, treated for 3-4 days with phlorizin (0.4 g/kg), given sc twice daily (DP; n = 7). Diabetes was induced by a single injection of streptozotocin (65 mg/kg). Basal plasma glucose was 7.4 +/- 0.3 mM in NC, but rose to 14.5 +/- 2.2 mM in DU. Basal hyperglycemia was corrected with phlorizin and insulin treatments (5.5 +/- 0.5 and 6.7 +/- 0.8 mM, respectively). NC rats responded to insulin-induced hypoglycemia with a rapid and marked increase in glucagon (peak, 2059 +/- 311 pg/ml). The glucagon response was blunted in DU (635 +/- 180 pg/ml) and was partially improved by prolonged normalization of glycemia in DP (1335 +/- 295 pg/ml; P < 0.05). Plasma somatostatin levels in all diabetic groups were 2- to 3-fold higher in the basal state, but were not different during hypoglycemia, than those in NC rats. Compared to levels in NC rats, diabetes resulted in decreased insulin, but elevated glucagon and somatostatin concentrations in the pancreatic tissue. Treatment with both insulin and phlorizin reversed the changes in the pancreatic content of both glucagon and somatostatin. Pancreatic proglucagon messenger RNA did not show significant differences among the four groups in either state. Insulin treatment in the DI group resulted in a delayed and much smaller increase in the glucagon response (740 +/- 138 pg/ml) to hypoglycemia despite normalization of glycemia. We, therefore, conclude that in streptozotocin-diabetic rats, the impaired glucagon responsiveness to hypoglycemia is significantly improved by insulin-independent correction of hyperglycemia, suggesting the importance of normoglycemia per se in maintaining, at least in part, the glucose sensitivity of pancreatic alpha-cells.

A moderate decline in specific activity does not lead to an underestimation of hepatic glucose production during a glucose clamp

We have previously shown that modeling errors lead to underestimation of hepatic glucose production (HGP) during glucose clamps when specific activity (SA) declines markedly. We wished to assess whether the failure to keep SA constant substantially affects calculation of HGP during insulin infusion when glucose requirements to maintain the glucose clamp are moderate. Therefore, 150-minute hyperinsulinemic (5.4 pmol - kg (-1) - min (-1) clamps were performed in depancreatized dogs that were maintained hyperglycemic (approximately 10 mmol/L with either (l) unlabeled glucose infusate (COLD Ginf, n = 5) or (2) labeled glucose infusate (HOT Ginf, n = 6) containing high-performance liquid chromatography (HPLC purified [6-3H]glucose. Insulinemia and glucagonemia were similar between the two groups. Additionally, glucose infusion rates were equivalent with COLD and HOT Ginf, indicating comparable insulin effects on overall glucose metabolism. The SA decreased a maximum of 32% with COLD Ginf, but remained constant with HOT Ginf. HGP was suppressed equally with COLD or HOT Ginf treatments at each time point during the clamp (mean suppression during last hour of clamp, 69% +/- 4% and 69% +/- 5%, P = NS, COLD and HOT Ginf, respectively). We conclude that when glucose requirements are moderate and SA changes slowly, as in the diabetic dog, it is not necessary to keep SA perfectly constant to avoid significant modeling errors when calculating HPG during hyperinsulinemic clamps.

Determinants of glucose turnover in the pathophysiology of diabetes: an in vivo analysis in diabetic dogs

Hyperglycaemia in diabetes results from a combination of increased hepatic glucose production and decreased metabolic clearance of glucose. Our report summarizes recent work conducted in our laboratory to investigate the regulatory factors involved in the control of glucose turnover in diabetes. The action of insulin, both directly and indirectly, in regulating glucose turnover in diabetic dogs is considered. 1) In the depancreatized diabetic dog, peripheral rather than portal insulin levels determine the suppression of hepatic glucose production via indirect mechanisms such as limiting, precursors for gluconeogenesis and/or inhibiting glucagon secretion. 2) The differential effects of insulin and insulin-like growth factor I on glucose turnover may be dependent on a decline in glycaemia since previously observed differential effects on glucose turnover were masked under conditions of clamped hyperglycaemia in the depancreatized dog. 3) In a paradoxical dichotomous fashion, hyperglycaemia both contributes to, and compensates for, defective glucose clearance in diabetes. Acute restoration of euglycaemia significantly improves glucose clearance at rest and normalizes the exercise-induced increment in clearance in alloxan-diabetic dogs. 4) Our model of centrally-induced stress also shows that an increase in glucose utilization and clearance is largely independent of changes in insulin and that the combined effects of catecholamines and glucagon are responsible for increasing glucose production.

Hepatic glucose production is regulated both by direct hepatic and extrahepatic effects of insulin in humans

The present study examines the effect of the route of insulin delivery on glucose turnover in humans. By using a new noninvasive in vivo method, the acute effect of insulin secreted by the pancreas can be compared with that of insulin delivered by a peripheral vein. Three euglycemic-hyperinsulinemic studies were performed in lean healthy men. In the first study (n = 10), constant portal hyperinsulinemia was produced using a programmed intravenous tolbutamide infusion algorithm, and the insulin secretion rate was mathematically derived by deconvolution from peripheral plasma C-peptide levels. In the second study (n = 10), exogenous insulin was infused by peripheral vein at the same rate as that determined in the first study. In the third study (n = 7), the peripheral insulin levels in the first study were matched by infusing exogenous insulin into a peripheral vein at half that rate. Peripheral insulin levels were higher (P < 0.001) with the full-rate peripheral insulin infusion (266.3 +/- 28.1 pmol/l) than with the portal delivery of insulin (171.1 +/- 30.4 pmol/l) or the half-rate peripheral insulin infusion (158.6 +/- 7.4 pmol/l) (portal versus half-rate peripheral insulin infusion, NS). Calculated hepatic insulin levels were higher (P < 0.001) in the portal insulin study (443.1 +/- 52.6 pmol/l) than in the full-rate peripheral insulin study (303.6 +/- 30.9 pmol/l) or in the half-rate peripheral insulin study (204.5 +/- 9.8 pmol/l). Hepatic glucose production (HGP) was suppressed to a greater extent with the full-rate peripheral insulin infusion (69.3 +/- 7.8%, P < 0.001 vs. portal or half-rate peripheral insulin) than portal (50.3 +/- 9.8%) or half-rate peripheral insulin infusion (36.8 +/- 3.8%). In the portal insulin study, however, suppression was greater than in the half-rate peripheral insulin study (P < 0.01), in spite of equal peripheral insulin levels. The assumption that tolbutamide, when used in this fashion, has no independent effect on glucose turnover, glucagon, or gluconeogenic precursor and energy substrates for gluconeogenesis was validated in five C-peptide-negative patients with IDDM. We conclude that in nondiabetic humans, 1) peripheral effects of insulin are important in suppressing HGP, as evidenced by the greater suppression of HGP with equivalent rate peripheral versus portal insulin delivery, and 2) because HGP was suppressed to a greater extent with portal verus peripheral insulin delivery at half the rate when peripheral insulin levels were matched, insulin-induced suppression of HGP is also partly mediated by a direct hepatic effect.

Exercise increases the plasma membrane content of the Na+ -K+ pump and its mRNA in rat skeletal muscles

Muscle fibers adapt to ionic challenges of exercise by increasing the plasma membrane Na+-K+ pump activity. Chronic exercise training has been shown to increase the total amount of Na+-K+ pumps present in skeletal muscle. However, the mechanism of adaptation of the Na+-K+ pump to an acute bout of exercise has not been determined, and it is not known whether it involves alterations in the content of plasma membrane pump subunits. Here we examine the effect of 1 h of treadmill running (20 m/min, 10% grade) on the subcellular distribution and expression of Na+-K+ pump subunits in rat skeletal muscles. Red type I and IIa (red-I/IIa) and white type IIa and IIb (white-IIa/IIb) hindlimb muscles from resting and exercised female Sprague-Dawley rats were removed for subcellular fractionation. By homogenization and gradient centrifugation, crude membranes and purified plasma membranes were isolated and subjected to gel electrophoresis and immunoblotting by using pump subunit-specific antibodies. Furthermore, mRNA was isolated from specific red type I (red-I) and white type IIb (white-IIb) muscles and subjected to Northern blotting by using subunit-specific probes. In both red-I/IIa and white-IIa/IIb muscles, exercise significantly raised the plasma membrane content of the alpha1-subunit of the pump by 64 +/- 24 and 55 +/- 22%, respectively (P < 0.05), and elevated the alpha2-polypeptide by 43 +/- 22 and 94 +/- 39%, respectively (P < 0.05). No significant effect of exercise could be detected on the amount of these subunits in an internal membrane fraction or in total membranes. In addition, exercise significantly increased the alpha1-subunit mRNA in red-I muscle (by 50 +/- 7%; P < 0.05) and the beta2-subunit mRNA in white-IIb muscles (by 64 +/- 19%; P < 0.01), but the alpha2- and beta1-mRNA levels were unaffected in this time period. We conclude that increased presence of alpha1- and alpha2-polypeptides at the plasma membrane and subsequent elevation of the alpha1- and beta2-subunit mRNAs may be mechanisms by which acute exercise regulates the Na+-K+ pump of skeletal muscle.

The roles of catecholamines in glucoregulation in intense exercise as defined by the islet cell clamp technique

Exercise at > 85% VO2max causes the greatest known physiological increases in glucose production rates (Ra). To define the relative roles of catecholamine versus glucagon/insulin responses in stimulating Ra, normal subjects in the postabsorptive state exercised at 87 +/- 2% VO2max during an islet cell clamp (IC): intravenous octreotide (somatostatin analog), 30 ng.kg-1.min-1; glucagon, 0.8 ng.kg-1.min-1; growth hormone, 10 ng.kg-1.min-1; and insulin adjusted to achieve euglycemia, then constant 56 +/- 7 min before exercise. Seven control subjects exercised without an IC. In four subjects (IC-1) with hormone infusions held constant during exercise, plasma insulin rose 76% and glucagon 35%, perhaps because of altered hemodynamics. In seven subjects (IC-2), hormone infusions were decreased stepwise during exercise and returned stepwise to initial rates during early recovery. Ra increased sixfold in control and both IC groups. Plasma norepinephrine and epinephrine likewise increased > 12-fold with no differences among groups; both catecholamines correlated closely with Ra. Because mixed venous blood plasma insulin declined and glucagon did not change in control subjects, the glucagon-to-insulin ratio increased from 0.20 to 0.26 (P = 0.02). In IC subjects, plasma insulin increased and glucagon was either constant (IC-2) or increased less than insulin, resulting in nonsignificant declines in the immunoreactive glucose-to-immunoreactive insulin ratio. Although a rise in insulin would have been expected to attenuate the Ra increment, this effect was overridden. The strong correlations of Ra with catecholamines and the similar Ra responses despite divergent glucagon-to-insulin responses are consistent with the primacy of catecholamines in regulation of Ra in intense exercise.

Multiple roles of phosphatidylinositol 3-kinase in regulation of glucose transport, amino acid transport, and glucose transporters in L6 skeletal muscle cells

Phosphatidylinositol 3-kinase (PI3k) activity is required for the insulin stimulation of glucose transport in adipocytes and Chinese hamster ovary cells. Wortmannin (WM), an inhibitor of PI3k, inhibits the stimulation of glucose transport by insulin and the gain of glucose transporters at the cell surface. However, the effect of inhibition of PI3k on the maintenance of the basal and the insulin-stimulated glucose transport and on the intracellular donor pool of glucose transporters has not been clarified. Here we show that in L6 skeletal muscle cells in culture WM significantly inhibits the basal PI3k activity (by 40%), decreases the levels of phosphatidylinositol 3,4-phosphate and 3,4,5-phosphate (by about 50%) and abolishes the activation of the enzyme by insulin. WM inhibited the basal rate of transport of glucose (by 45%) and of amino acids through system A (by 25%) and abolished their stimulation by insulin. Insulin caused a transient increase in PI3k activity and PI3k products that returned to basal levels within 40 min, whereas glucose and amino acid transport remained elevated. Under these conditions, WM reduced the rate of glucose and amino acid transport back to basal levels. In unstimulated cells, WM decreased significantly the GLUT4 glucose transporter content at the plasma membrane and prevented the ability of insulin to recruit transporters to this membrane. Interestingly, the intracellular pools of the GLUT3 and GLUT4 glucose transporters were significantly reduced in response to WM treatment alone. We conclude that in muscle cells PI3k activity is required to maintain basal and insulin-stimulated glucose and amino acid transport, as well as to develop the stimulation of the two transport processes in response to the hormone. We hypothesize that PI3k, likely through production of phosphatidylinositol 3,4-phosphate and 3,4,5-phosphate, regulates the basal plasma membrane glucose transporter recycling and the organization of the transporter intracellular pool, in addition to being an insulin signal.