Suppression of endogenous glucose production by mild hyperinsulinemia during exercise is determined predominantly by portal venous insulin

Hyperinsulinemia during exercise in people with diabetes requiring exogenous insulin is a major clinical problem. The aim of this study was to assess the significance of portal vein versus arterial insulin to hepatic effects of hyperinsulinemia during exercise. Dogs had sampling (artery, portal vein, and hepatic vein) and infusion (vena cava and portal vein) catheters and flow probes (hepatic artery and portal vein) implanted >16 days before a study. Protocols consisted of equilibration (-130 to -30 min), basal (-30 to 0 min), and treadmill exercise (0-150 min) periods. Somatostatin was infused and glucagon and insulin were replaced in the portal vein to achieve basal arterial and portal vein levels at rest and simulated levels during the first 60 min of exercise. From 60 to 150 min of exercise, the simulated insulin infusion was sustained (C; n = 7), modified to selectively create a physiologic increment in arterial insulin (Pe; n = 7), or altered to increase arterial insulin as in Pe but with a concomitant increase in portal insulin (PePo; n = 7). Euglycemic clamps were performed in all studies. Portal and arterial insulin were 15 +/- 2 and 4 +/- 1 micro U/ml (mean +/- SE of all groups), respectively, at t = 60 min in all groups. Insulin levels were unchanged for the remainder of the exercise period in C. Arterial insulin was increased from 3 +/- 1 to 14 +/- 2 micro U/ml, whereas portal insulin did not change in Pe after t = 60 min. Arterial insulin was increased from 3 +/- 1 to 15 +/- 2 micro U/ml, and portal insulin was increased from 16 +/- 3 to 33 +/- 3 micro U/ml in PePo after t = 60 min. Endogenous glucose production (R(a)) rose similarly from basal during the first 60 min of exercise in all groups (mean +/- SE of all groups was from 2.2 +/- 0.1 to 6.8 +/- 0.5 mg. kg(-1). min(-1)). The increase in R(a) was sustained for the remainder of the exercise period in C. R(a) was suppressed by approximately 40%, but only after 60 min of hyperinsulinemia, and by approximately 20% after 90 min of hyperinsulinemia in Pe. In contrast, the addition of portal venous hyperinsulinemia caused approximately 90% suppression of R(a) within 20 min and for the remainder of the experiment in PePo. Measurements of net hepatic glucose output were similar to R(a) responses in all groups. Arterial free fatty acids (FFAs), a stimulus of R(a), were increased to 1,255 +/- 258 micro mol/l in C but were only 459 +/- 67 and 312 +/- 42 micro mol/l in Pe and PePo, respectively, by 150 min of exercise. Thus, during exercise, the exquisite sensitivity of R(a) to hyperinsulinemia is due entirely to portal venous hyperinsulinemia during the first 60 min, after which peripheral hyperinsulinemia may control approximately 20-40%, possibly as a result of inhibition of the exercise-induced increase in FFA.     

Effects of antecedent hypoglycemia on subsequent counterregulatory responses to exercise

Antecedent hypoglycemia can blunt counterregulatory responses to subsequent hypoglycemia. It is uncertain, however, if prior hypoglycemia can blunt counterregulatory responses to other physiologic stresses. The aim of this study, therefore, was to determine whether antecedent hypoglycemia attenuates subsequent neuroendocrine and metabolic responses to exercise. Sixteen lean, healthy adults (eight men and eight women, ages 28+/-2 years, BMI 22+/-1 kg/m2, VO2max 43+/-3 ml x kg(-1) x min(-1)) were studied during 2-day protocols on two randomized occasions separated by 2 months. On day 1, subjects underwent morning and afternoon 2-h hyperinsulinemic (528+/-30 pmol/l) glucose clamp studies of 5.3+/-0.1 mmol/l (euglycemic control) or 2.9+/-0.1 mmol/l (hypoglycemic study). On day 2, subjects underwent 90 min of exercise on a static cycle ergometer at 80% of their anaerobic threshold (approximately 50% VO2max). Glycemia was equated during day 2 exercise studies via an exogenous glucose infusion. Day 1 hypoglycemia had significant effects on neuroendocrine and metabolic responses during day 2 exercise. The usual exercise-induced reduction in insulin, together with elevations of plasma epinephrine, norepinephrine, glucagon, growth hormone, pancreatic polypeptide, and cortisol levels, was significantly blunted after day 1 hypoglycemia (P<0.01). Commensurate with reduced neuroendocrine responses, key metabolic counterregulatory mechanisms of endogenous glucose production (EGP), lipolytic responses, and ketogenesis were also significantly attenuated (P<0.01) after day 1 hypoglycemia. Significantly greater rates of glucose infusion were required to maintain euglycemia during exercise after day 1 hypoglycemia compared with day 1 euglycemia (8.8+/-2.2 vs. 0.6+/-0.6 micromol x kg(-1) x min(-1); P<0.01). During the first 30 min of exercise, day 1 hypoglycemia had little effect on EGP, but during the latter 60 min of exercise, day 1 hypoglycemia was associated with a progressively smaller increase in EGP compared with day 1 euglycemia. Thus, by 90 min, the entire exercise-induced increment in EGP (8.8+/-1.1 micromol x kg(-1) x min(-1)) was abolished by day 1 hypoglycemia. We conclude that 1) antecedent hypoglycemia results in significant blunting of essential neuroendocrine (glucagon, insulin, catecholamines) and metabolic (endogenous glucose production, lipolysis, ketogenesis) responses to exercise; 2) antecedent hypoglycemia may play a role in the pathogenesis of exercise-related hypoglycemia in type 1 diabetic patients; and 3) antecedent hypoglycemia can blunt counterregulatory responses to other physiologic stresses in addition to hypoglycemia.     

Interaction of insulin and prior exercise in control of hepatic metabolism of a glucose load

To determine if prior exercise enhances insulin-stimulated extraction of glucose by the liver, chronically catheterized dogs were submitted to 150 min of treadmill exercise or rest. After exercise or rest, dogs received portal glucose (18 micro mol x kg(-1) x min(-1)), peripheral somatostatin, and basal portal glucagon infusions from t = 0 to 150 min. A peripheral glucose infusion was used to clamp arterial blood glucose at 8.3 mmol/l. Insulin was infused into the portal vein to create either basal levels or mild hyperinsulinemia. Prior exercise did not increase whole-body glucose disposal in the presence of basal insulin (25.5 +/- 1.5 vs. 20.3 +/- 1.7 micro mol x kg(-1) x min(-1)), but resulted in a marked enhancement in the presence of elevated insulin (97.2 +/- 15.1 vs. 64.4 +/- 7.4 micro mol x kg(-1) x min(-1)). Prior exercise also increased net hepatic glucose uptake in the presence of both basal insulin (7.5 +/- 1.2 vs. 2.9 +/- 2.4 micro mol x kg(-1) x min(-1)) and elevated insulin (22.0 +/- 3.5 vs. 11.5 +/- 1.8 micro mol x kg(-1) x min(-1)). Likewise, net hepatic glucose fractional extraction was increased by prior exercise with both basal insulin (0.04 +/- 0.01 vs. 0.01 +/- 0.01 micro mol x kg(-1) x min(-1)) and elevated insulin (0.10 +/- 0.01 vs. 0.05 +/- 0.01). Hepatic glycogen synthesis was increased by elevated insulin, but was not enhanced by prior exercise. Although the increase in glucose extraction after exercise could be ascribed to increased insulin action, the increase in hepatic glycogen synthesis was independent of it.     

Hypoglycemia per se stimulates sympathetic neural as well as adrenomedullary activity, but, unlike the adrenomedullary response, the forearm sympathetic neural response is not reduced after recent hypoglycemia.

We tested the hypotheses that 1) hypoglycemia per se stimulates the sympathetic neural as well as the adrenomedullary component of the sympathochromaffin system, and 2) sympathetic neural responses to hypoglycemia, like adrenomedullary responses, are reduced after recent hypoglycemia. To this end, we studied 10 healthy young adults on 2 consecutive days on two separate occasions, on one occasion with euglycemia (5.0 mmol/l) and on the other occasion with hypoglycemia (2.8 mmol/l) from 1000 to 1200 and 1400 to 1600 on day 1 of each occasion. On day 2 of each occasion, plasma epinephrine and norepinephrine (NE) concentrations and rates of systemic NE spillover (SNESO) and forearm NE spillover (FNESO) were measured during hyperinsulinemic (12.0 pmol x kg(-1) x min(-1)) euglycemia (5.0 mmol/l) and hypoglycemia (2.8 mmol/l). Compared with values during euglycemia, plasma epinephrine and NE and rates of SNESO and FNESO all increased during hypoglycemia (P < 0.01). After day 1 hypoglycemia, there were reductions during hypoglycemia on day 2 in plasma epinephrine (2,050 +/- 500 vs. 2,960 +/- 400 pmol/l; P < 0.02), plasma NE (1.35 +/- 0.16 vs. 1.92 +/- 0.20 nmol/l; P < 0.01), and SNESO rates (5.13 +/- 0.84 vs. 6.87 +/- 0.81 nmol/min; P < 0.02). However, FNESO rates were unaltered (1.16 +/- 0.25 vs. 1.27 +/- 0.17 pmol x min(-1) x 100 ml tissue(-1). Thus we conclude that 1) hypoglycemia per se stimulates both the sympathetic neural and adrenomedullary components of the sympathochromaffin system and 2) adrenomedullary, but not forearm sympathetic neural, responses to hypoglycemia are reduced after recent hypoglycemia. The extent to which the lower plasma NE levels and reduced SNESO responses to hypoglycemia after day 1 hypoglycemia reflect reduced NE release from the adrenal medullae, sympathetic nerves other than those in the forearm, or both cannot be determined from these data.     

Inclusion of low amounts of fructose with an intraduodenal glucose load markedly reduces postprandial hyperglycemia and hyperinsulinemia in the conscious dog.

Intraportal infusion of small amounts of fructose markedly augmented net hepatic glucose uptake (NHGU) during hyperglycemic hyperinsulinemia in conscious dogs. In this study, we examined whether the inclusion of catalytic amounts of fructose with a glucose load reduces postprandial hyperglycemia and the pancreatic beta-cell response to a glucose load in conscious 42-h-fasted dogs. Each study consisted of an equilibration (-140 to -40 min), control (-40 to 0 min), and test period (0-240 min). During the latter period, glucose (44.4 micromol x kg(-1) x min(-1)) was continuously given intraduodenally with (2.22 micromol x kg(-1) x min(-1)) or without fructose. The glucose appearance rate in portal vein blood was not significantly different with or without the inclusion of fructose (41.3 +/- 2.7 vs. 37.3 +/- 8.3 micromol x kg(-1) x min(-1), respectively). In response to glucose infusion without the inclusion of fructose, the net hepatic glucose balance switched from output to uptake (from 10 +/- 2 to 11 +/- 4 micromol x kg(-1) x min(-1)) by 30 min and averaged 17 +/- 6 micromol x kg(-1) x min(-1). The fractional extraction of glucose by the liver during the infusion period was 7 +/- 2%. Net glycogen deposition was 2.44 mmol glucose equivalent/kg body wt; 49% of deposited glycogen was synthesized via the direct pathway. Net hepatic lactate production was 1.4 mmol/kg body wt. Arterial blood glucose rose from 4.1 +/- 0.2 to 7.3 +/- 0.4 mmol/l, and arterial plasma insulin rose from 42 +/- 6 to 258 +/- 66 pmol/l at 30 min, after which they decreased to 7.0 +/- 0.5 mmol/l and 198 +/- 66 pmol/l, respectively. Arterial plasma glucagon decreased from 54 +/- 7 to 32 +/- 3 ng/l. In response to intraduodenal glucose infusion in the presence of fructose, net hepatic glucose balance switched from 9 +/- 1 micromol x kg(-1) x min(-1) output to 12 +/- 3 and 28 +/- 5 micromol x kg(-1) x min(-1) uptake by 15 and 30 min, respectively. The average NHGU (28 +/- 5 micromol x kg(-1) x min(-1)) and fractional extraction during infusion period (12 +/- 2%), net glycogen deposition (3.68 mmol glucose equivalent/kg body wt), net hepatic lactate production (3.27 mmol/kg), and glycogen synthesis via the direct pathway (68%) were significantly higher (P < 0.05) compared to that in the absence of fructose. The increases in arterial blood glucose (from 4.4 +/- 0.1 to 6.4 +/- 0.2 mmol/l at 30 min) and arterial plasma insulin (from 48 +/- 6 to 126 +/- 30 pmol/l at 30 min) were significantly smaller (P < 0.05). In summary, the inclusion of small amounts of fructose with a glucose load augmented NHGU, increased hepatic glycogen synthesis via the direct pathway, and augmented hepatic glycolysis. As a result, postprandial hyperglycemia and insulin release by the pancreatic beta-cell were reduced. In conclusion, catalytic amounts of fructose have the ability to improve glucose tolerance.     

Effects of low and moderate antecedent exercise on counterregulatory responses to subsequent hypoglycemia in type 1 diabetes

Antecedent moderate-intensity exercise has been shown to blunt autonomic, neuroendocrine, and metabolic counterregulatory responses to subsequent hypoglycemia in nondiabetic individuals. The aims of the current study were to determine 1) whether this occurs in type 1 diabetic patients and 2) whether the degree of blunting is dependent on exercise intensity. Twenty-seven type 1 diabetic patients (13 women and 14 men) were studied during a single-step, 2-h hyperinsulinemic (9 pmol x kg(-1) x min(-1))-hypoglycemic (approximately 2.8 mmol/l) clamp 1 day after two 90-min exercise bouts at 30% (n = 11) or at 50% (n = 11) Vo(2max) or after no prior stress (control subjects, n = 25). After prior exercise at both 30 and 50% Vo(2max), epinephrine (1,959 +/- 553 and 1,528 +/- 424 vs. 3,420 +/- 424 pmol/l, respectively; P < 0.05) and pancreatic polypeptide (97 +/- 32 and 98 +/- 8 vs. 223 +/- 32 pmol/l, respectively; P < 0.05) responses to subsequent hypoglycemia were significantly lower compared with those of control subjects. Endogenous glucose production was significantly lower, while glucose utilization and, consequently, the exogenous glucose infusion rate needed to maintain hypoglycemia were significantly greater after both exercise intensities compared with that of control subjects. Muscle sympathetic nerve activity was significantly reduced by prior exercise of both intensities at baseline (16 +/- 4 and 22 +/- 4 vs. 31 +/- 3 bursts/min) and during hypoglycemia (22 +/- 4 and 27 +/- 5 vs. 41 +/- 3 bursts/min) compared with that of control subjects (P < 0.05). Total hypoglycemic symptoms were also significantly lower (P < 0.05) in both exercise groups compared with the control group. In summary, repeated episodes of prolonged exercise of both low and moderate intensities blunted key autonomic (epinephrine and pancreatic polypeptide) and metabolic (endogenous glucose production and peripheral glucose uptake) counterregulatory responses to next-day hypoglycemia in type 1 diabetes.     

The effects of dehydroepiandrosterone sulfate on counterregulatory responses during repeated hypoglycemia in conscious normal rats

We previously determined that both antecedent hypoglycemia and elevated cortisol levels blunt neuroendocrine and metabolic responses to subsequent hypoglycemia in conscious, unrestrained rats. The adrenal steroid dehydroepiandrosterone sulfate (DHEA-S) has been shown in several studies to oppose corticosteroid action. The purpose of this study was to determine if DHEA-S could preserve counterregulatory responses during repeated hypoglycemia. We studied 40 male Sprague-Dawley rats during a series of 2-day protocols. Day 1 consisted of two 2-h episodes of 1) hyperinsulinemic (30 pmol. kg(-1). min(-1)) euglycemia (6.2 +/- 0.2 mmol/l; n = 12; ANTE EUG), 2) hyperinsulinemic euglycemia (6.0 +/- 0.1 mmol/l; n = 8) plus simultaneous intravenous infusion of DHEA-S (30 mg/kg; ANTE EUG + DHEA-S), 3) hyperinsulinemic hypoglycemia (2.8 +/- 0.1 mmol/l; n = 12; ANTE HYPO), or 4) hyperinsulinemic hypoglycemia (2.8 +/- 0.1 mmol/l; n = 8) with simultaneous intravenous infusion of DHEA-S (30 mg/kg; ANTE HYPO + DHEA-S). Day 2 consisted of a single 2-h hyperinsulinemic hypoglycemic (2.8 +/- 0.1 mmol/l) clamp. During the final 30 min of day 2, hypoglycemia norepinephrine levels were significantly lower in the ANTE HYPO group versus the ANTE HYPO + DHEA-S group (2.0 +/- 0.2 vs. 3.3 +/- 0.6 nmol/l; P < 0.05). In addition, epinephrine (8 +/- 1 vs. 17 +/- 2, 14 +/- 3, and 15 +/- 3 nmol/l), glucagon (91 +/- 8 vs. 273 +/- 36, 231 +/- 42, and 297 +/- 48 ng/l), and corticosterone (1,255 +/- 193 vs. 1,915 +/- 212, 1,557 +/- 112, and 1,668 +/- 119 pmol/l) were significantly lower in the ANTE HYPO group versus the ANTE EUG, ANTE EUG + DHEA-S, and ANTE HYPO + DHEA-S groups (P < 0.05). Endogenous glucose production was also significantly less in the ANTE HYPO group versus the ANTE EUG, ANTE EUG + DHEA-S, and ANTE HYPO + DHEA-S groups (13 +/- 5 vs. 32 +/- 3, 38 +/- 7, and 29 +/- 8 micro mol/l. kg(-1). min(-1); P < 0.05). Consequently, the amount of exogenous glucose needed to maintain the glycemic level during the clamp studies was significantly higher in the ANTE HYPO versus the ANTE EUG, ANTE EUG + DHEA-S, and ANTE HYPO + DHEA-S groups (57 +/- 8 vs. 22 +/- 5, 18 +/- 6, and 18 +/- 3 micro mol/l. kg(-1). min(-1); P < 0.05). In summary, day-1 antecedent hypoglycemia blunted neuroendocrine and metabolic responses to next-day hypoglycemia. However, simultaneous DHEA-S infusion during antecedent hypoglycemia preserved neuroendocrine and metabolic counterregulatory responses during subsequent hypoglycemia in conscious rats.     

Alterations in postprandial hepatic glycogen metabolism in type 2 diabetes

Decreased skeletal muscle glucose disposal and increased endogenous glucose production (EGP) contribute to postprandial hyperglycemia in type 2 diabetes, but the contribution of hepatic glycogen metabolism remains uncertain. Hepatic glycogen metabolism and EGP were monitored in type 2 diabetic patients and nondiabetic volunteer control subjects (CON) after mixed meal ingestion and during hyperglycemic-hyperinsulinemic-somatostatin clamps applying 13C nuclear magnetic resonance spectroscopy (NMRS) and variable infusion dual-tracer technique. Hepatocellular lipid (HCL) content was quantified by 1H NMRS. Before dinner, hepatic glycogen was lower in type 2 diabetic patients (227 +/- 6 vs. CON: 275 +/- 10 mmol/l liver, P < 0.001). After meal ingestion, net synthetic rates were 0.76 +/- 0.16 (type 2 diabetic patients) and 1.36 +/- 0.15 mg x kg(-1) x min(-1) (CON, P < 0.02), resulting in peak concentrations of 283 +/- 15 and 360 +/- 11 mmol/l liver. Postprandial rates of EGP were approximately 0.3 mg x kg(-1) x min(-1) (30-170 min; P < 0.05 vs. CON) higher in type 2 diabetic patients. Under clamp conditions, type 2 diabetic patients featured approximately 54% lower (P < 0.03) net hepatic glycogen synthesis and approximately 0.5 mg x kg(-1) x min(-1) higher (P < 0.02) EGP. Hepatic glucose storage negatively correlated with HCL content (R = -0.602, P < 0.05). Type 2 diabetic patients exhibit 1) reduction of postprandial hepatic glycogen synthesis, 2) temporarily impaired suppression of EGP, and 3) no normalization of these defects by controlled hyperglycemic hyperinsulinemia. Thus, impaired insulin sensitivity and/or chronic glucolipotoxicity in addition to the effects of an altered insulin-to-glucagon ratio or increased free fatty acids accounts for defective hepatic glycogen metabolism in type 2 diabetic patients.     

Enhanced stimulation of glucose uptake by insulin increases exercise-stimulated glucose uptake in skeletal muscle in humans: studies using [15O]O2, [15O]H2O, [18F]fluoro-deoxy-glucose, and positron emission tomography

In vitro studies have shown that insulin and exercise stimulate glucose uptake in part via distinct mechanisms. We determined whether a high rate of insulin-stimulated glucose uptake (good insulin sensitivity) is associated with an enhanced ability of exercise to increase glucose uptake in vivo in humans. In our study, 22 normal subjects performed one-legged isometric exercise for 105 min (45-150 min) under intravenously maintained euglycemic-hyperinsulinemic conditions (0-150 min). Rates of oxygen consumption, blood flow, and glucose uptake were quantitated simultaneously in skeletal muscle of both legs using [15O]O2, [15O]H2O, [18F]fluoro-deoxy-glucose, and positron emission tomography. The one-legged exercise, performed at an intensity of 11% of maximal isometric force, was designed to induce similar increases in oxygen consumption in both groups. In the entire group, exercise increased oxygen consumption from 2.3 +/- 0.3 ml x kg(-1) muscle x min(-1) (insulin) to 34.2 +/- 3. ml x kg(-1) muscle x min(-1) (insulin and exercise) (P < 0.001) and muscle glucose uptake from 60 +/- 6 pmol x kg(-1) muscle x min(-1) (insulin) to 220 +/- 22 micromol x kg(-1) muscle x min(-1) (insulin and exercise) (P < 0.001). The exercise-induced increase in glucose uptake was due to marked increases in blood flow (36 +/- 5 ml x kg(-1) muscle x min(-1) [insulin] vs. 262 +/- 20 ml x kg(-1) muscle x min(-1) [insulin and exercise], P < 0.001) rather than glucose extraction, which decreased from 2.0 +/- 0.2 mmol/l (insulin) to 1.0 +/- 0.1 mmol/1 (insulin and exercise) (P < 0.001). The subjects were classified according to their mean rate of whole-body insulin-stimulated glucose uptake into those with high (49 +/- 3 micromol x kg(-1) x min(-1)) and normal (27 +/- 2 micromol x kg(-1) x min(-1)) rates of insulin-stimulated glucose uptake. Both insulin-stimulated (2.4 +/- 1.1 vs. 2.3 +/- 1.2 ml x kg(-1) muscle x min(-1), normal vs. high insulin sensitivity) and exercise- and insulin-stimulated (33 +/- 6 vs. 34 +/- 4 ml x kg(-1) muscle x min(-1)) rates of oxygen consumption were comparable between the groups. Exercise increased glucose uptake more in the group with high insulin sensitivity (195 +/- 25 pmol x kg(-1) muscle x min(-1)) than in the group with normal insulin sensitivity (125 +/- 19 micromol x kg(-1) muscle x min(-1)) (P < 0.05). Muscle blood flow was closely correlated with the rate of oxygen consumption (r = 0.91, P < 0.0001), and insulin-stimulated (30 +/- 5 vs. 35 +/- 6 ml x kg(-1) muscle x min(-1)) and exercise-induced increments (222 +/- 31 vs. 228 +/- 23 ml x kg(-1) muscle x min(-1)) in muscle blood flow were similar between the groups. Glucose extraction remained higher in the group with high insulin sensitivity (1.2 +/- 0.2 mmol/l) than in the group with normal insulin sensitivity (0.7 +/- 0.1 mmol/l, P < 0.05). We conclude that whereas acute exercise per se increases glucose uptake via increasing glucose delivery, good insulin sensitivity modulates exercise-induced increases in glucose uptake by enhancing cellular glucose extraction.     

Insulin action on muscle protein kinetics and amino acid transport during recovery after resistance exercise.

We have determined the individual and combined effects of insulin and prior exercise on leg muscle protein synthesis and degradation, amino acid transport, glucose uptake, and alanine metabolism. Normal volunteers were studied in the postabsorptive state at rest and about 3 h after a heavy leg resistance exercise routine. The leg arteriovenous balance technique was used in combination with stable isotopic tracers of amino acids and biopsies of the vastus lateralis muscle. Insulin was infused into a femoral artery to increase the leg insulin concentrations to high physiologic levels without substantively affecting the whole-body level. Protein synthesis and degradation were determined as rates of intramuscular phenylalanine utilization and appearance, and muscle fractional synthetic rate (FSR) was also determined. Leg blood flow was greater after exercise than at rest (P<0.05). Insulin accelerated blood flow at rest but not after exercise (P<0.05). The rates of protein synthesis and degradation were greater during the postexercise recovery (65+/-10 and 74+/-10 nmol x min(-1) x 100 ml(-1) leg volume, respectively) than at rest (30+/-7 and 46+/-8 nmol x min(-1) x 100 ml(-1) leg volume, respectively; P<0.05). Insulin infusion increased protein synthesis at rest (51+/-4 nmol x min(-1) x 100 ml(-1) leg volume) but not during the postexercise recovery (64+/-9 nmol x min(-1) x 100 ml(-1) leg volume; P<0.05). Insulin infusion at rest did not change the rate of protein degradation (48+/-3 nmol x min(-1) 100 ml(-1) leg volume). In contrast, insulin infusion after exercise significantly decreased the rate of protein degradation (52+/-9 nmol x min(-1) x 100 ml(-1) leg volume). The insulin stimulatory effects on inward alanine transport and glucose uptake were three times greater during the postexercise recovery than at rest (P<0.05). In contrast, the insulin effects on phenylalanine, leucine, and lysine transport were similar at rest and after exercise. In conclusion, the ability of insulin to stimulate glucose uptake and alanine transport and to suppress protein degradation in skeletal muscle is increased after resistance exercise. Decreased amino acid availability may limit the stimulatory effect of insulin on muscle protein synthesis after exercise.     

Resistance to exercise-induced increase in glucose uptake during hyperinsulinemia in insulin-resistant skeletal muscle of patients with type 1 diabetes

Insulin and exercise have been shown to activate glucose transport at least in part via different signaling pathways. However, it is unknown whether insulin resistance is associated with a defect in the ability of an acute bout of exercise to enhance muscle glucose uptake in vivo. We compared the abilities of insulin and isometric exercise to stimulate muscle blood flow and glucose uptake in 12 men with type 1 diabetes (age 24 +/- 1 years, BMI 23.0 +/- 0.4 kg/m(2)) and in 11 age- and weight-matched nondiabetic men (age 25 +/- 1 years, BMI 22.3 +/- 0.6 kg/m(2)) during euglycemic hyperinsulinemia (1 mU. kg(-1). min(-1) insulin infusion for 150 min). One-legged exercise was performed at an intensity of 10% of maximal isometric force for 105 min (range 45-150). Rates of muscle blood flow, oxygen consumption, and glucose uptake were quantitated simultaneously in both legs using [(15)O]water, [(15)O]oxygen, [(18)F]-2-fluoro-2-deoxy-D-glucose, and positron emission tomography. Resting rates of oxygen consumption were similar during hyperinsulinemia between the groups (2.4 +/- 0.3 vs. 2.0 +/- 0.5 ml. kg(-1) muscle. min(-1); normal subjects versus patients with type 1 diabetes, NS), and exercise increased oxygen consumption similarly in both groups (25.3 +/- 4.3 vs. 20.1 +/- 3.0 ml. kg(-1) muscle. min(-1), respectively, NS). Rates of insulin-stimulated muscle blood flow and the increments in muscle blood flow induced by exercise were also similar in normal subjects (129 +/- 14 ml. kg(-1). min(-1)) and in patients with type 1 diabetes (115 +/- 12 ml. kg(-1). min(-1)). The patients with type 1 diabetes exhibited resistance to both insulin stimulation of glucose uptake (34 +/- 6 vs. 76 +/- 9 micromol. kg(-1) muscle. min(-1), P < 0.001) and also to the exercise-induced increment in glucose uptake (82 +/- 15 vs. 162 +/- 29 micromol. kg(-1) muscle. min(-1), P < 0.05). We conclude that the ability of exercise to increase insulin-stimulated glucose uptake in vivo is blunted in patients with insulin-resistant type 1 diabetes compared with normal subjects. This could be caused by either separate or common defects in exercise- and insulin-stimulated pathways.     

Overactivation of S6 kinase 1 as a cause of human insulin resistance during increased amino acid availability

To examine the molecular mechanisms by which plasma amino acid elevation impairs insulin action, we studied seven healthy men twice in random order during infusion of an amino acid mixture or saline (total plasma amino acid approximately 6 vs. approximately 2 mmol/l). Somatostatin-insulin-glucose clamps created conditions of low peripheral hyperinsulinemia ( approximately 100 pmol/l, 0-180 min) and prandial-like peripheral hyperinsulinemia ( approximately 430 pmol/l, 180-360 min). At low peripheral hyperinsulinemia, endogenous glucose production (EGP) did not change during amino acid infusion but decreased by approximately 70% during saline infusion (EGP(150-180 min) 11 +/- 1 vs. 3 +/- 1 mumol . kg(-1) . min(-1), P = 0.001). Prandial-like peripheral hyperinsulinemia completely suppressed EGP during both protocols, whereas whole-body rate of glucose disappearance (R(d)) was approximately 33% lower during amino acid infusion (R(d) (330-360 min) 50 +/- 4 vs. 75 +/- 6 mumol . kg(-1) . min(-1), P = 0.002) indicating insulin resistance. In skeletal muscle biopsies taken before and after prandial-like peripheral hyperinsulinemia, plasma amino acid elevation markedly increased the ability of insulin to activate S6 kinase 1 compared with saline infusion ( approximately 3.7- vs. approximately 1.9-fold over baseline). Furthermore, amino acid infusion increased the inhibitory insulin receptor substrate-1 phosphorylation at Ser312 and Ser636/639 and decreased insulin-induced phosphoinositide 3-kinase activity. However, plasma amino acid elevation failed to reduce insulin-induced Akt/protein kinase B and glycogen synthase kinase 3alpha phosphorylation. In conclusion, amino acids impair 1) insulin-mediated suppression of glucose production and 2) insulin-stimulated glucose disposal in skeletal muscle. Our results suggest that overactivation of the mammalian target of rapamycin/S6 kinase 1 pathway and inhibitory serine phosphorylation of insulin receptor substrate-1 underlie the impairment of insulin action in amino acid-infused humans.     

Insulin sensitively controls the glucagon response to mild hypoglycemia in the dog

In the present study, we examined how the arterial insulin level alters the alpha-cell response to a fall in plasma glucose in the conscious overnight fasted dog. Each study consisted of an equilibration (-140 to -40 min), a control (-40 to 0 min), and a test period (0 to 180 min), during which BAY R 3401 (10 mg/kg), a glycogen phosphorylase inhibitor, was administered orally to decrease glucose output in each of four groups (n = 5). In group 1, saline was infused. In group 2, insulin was infused peripherally (3.6 pmol. kg(- 1). min(-1)), and the arterial plasma glucose level was clamped to the level seen in group 1. In group 3, saline was infused, and euglycemia was maintained. In group 4, insulin (3.6 pmol. kg(-1). min(-1)) was given, and euglycemia was maintained by glucose infusion. In group 1, drug administration decreased the arterial plasma glucose level (mmol/l) from 5.8 +/- 0.2 (basal) to 5.2 +/- 0.3 and 4.4 +/- 0.3 by 30 and 90 min, respectively (P < 0.01). Arterial plasma insulin levels (pmol/l) and the hepatic portal-arterial difference in plasma insulin (pmol/l) decreased (P < 0.01) from 78 +/- 18 and 90 +/- 24 to 24 +/- 6 and 12 +/- 6 over the first 30 min of the test period. The arterial glucagon levels (ng/l) and the hepatic portal-arterial difference in plasma glucagon (ng/l) rose from 43 +/- 5 and 5 +/- 2 to 51 +/- 5 and 10 +/- 5 by 30 min (P < 0.05) and to 79 +/- 16 and 31 +/- 15 (P < 0.05) by 90 min, respectively. In group 2, in response to insulin infusion, arterial insulin (pmol/l) was elevated from 48 +/- 6 to 132 +/- 6 to an average of 156 +/- 6. The hepatic portal-arterial difference in plasma insulin was eliminated, indicating a complete inhibition of endogenous insulin release. The arterial glucagon level (ng/l) and the hepatic portal-arterial difference in plasma glucagon (ng/l) did not rise significantly (40 +/- 5 and 7 +/- 4 at basal, 44 +/- 4 and 9 +/- 4 at 90 min, and 44 +/- 8 and 15 +/- 7 at 180 min). In group 3, when euglycemia was maintained, the insulin and glucagon levels and the hepatic portal-arterial difference remained constant. In group 4, the arterial plasma glucose level remained basal (5.9 +/- 1.1 mmol/l) throughout, whereas insulin infusion increased the arterial insulin level to an average of 138 +/- 6 pmol/l. The hepatic portal-arterial difference in plasma insulin was again eliminated. Arterial glucagon level (ng/l) and the hepatic portal-arterial difference in plasma glucagon (ng/l) did not change significantly (43 +/- 2 and 9 +/- 2 at basal, 39 +/- 3 and 9 +/- 2 at 90 min, and 37 +/- 3 and 7 +/- 2 at 180 min). Thus, a difference of approximately 120 pmol/l in arterial insulin completely abolished the response of the alpha-cell to mild hypoglycemia.     

Exercise and thiazolidinedione therapy normalize insulin action in the obese Zucker fatty rat

Thiazolidinediones and exercise are both known to improve insulin action independently. Therefore, we determined whether combined therapy could normalize insulin action in the Zucker fatty (ZF) rat. Rats were fed troglitazone as a 0.2% food admixture over a 3-week exercise training period (treadmill running 5 days/week, 20 m/min, 0% grade, 60 min/day). Subsequent to drug and/or exercise therapy, animals were chronically cannulated in the carotid artery (sampling) and jugular vein (infusion). After a 4-day recovery from surgery, animals were exposed to a hyperinsulinemic (40 mU x kg(-1) x min(-1)) euglycemic clamp (8.5 +/- 0.12 mmol/l; P = 0.45 between groups). Independently, exercise (n = 7) and troglitazone (n = 7) improved the glucose disposal rate 20% (P = 0.04) and 76% (P = 0.001), respectively, when compared with untreated ZF controls (n = 11). In combination, exercise and troglitazone therapy (n = 6) produced significant increments in the following: tracer-determined glucose disposal rate (combined therapy, 52.4 +/- 2.9 mg x kg(-1) x min(-1), vs. untreated ZF, 25.8 +/- 0.8 mg x kg(-1) x min(-1); P = 0.0001), total GLUT4 protein (twofold increase; P = 0.001), insulin receptor substrate (IRS)-1 protein (fourfold increase; P = 0.0001), and Akt phosphorylation (2.9-fold increase; P = 0.002). In conclusion, 1) exercise and troglitazone therapy each improved insulin action in the ZF rat, whereas the combination of the two led to complete normalization of insulin sensitivity, and 2) combination treatment also resulted in normalization of GLUT4 total protein, IRS-1 protein, and Akt phosphorylation compared with lean littermates.     

Ventilatory response to exercise and kinetics of oxygen recovery are similar in cardiac transplant recipients and patients with mild chronic heart failure.

BACKGROUND: Exercise capacity, assessed by cardiopulmonary exercise treadmill testing (CPET), does not return to normal following heart transplantation. This study evaluated the ventilatory response to exercise and the kinetics of oxygen (O(2)) recovery in heart transplant recipients (HTR) compared to healthy volunteers (HV) and heart failure patients. METHODS: Eighteen patients with end-stage heart failure (ESHF), 12 with mild heart failure (MHF) matched for peak oxygen consumption (Vo(2)) with the HTR, 12 HTR and 12 HV underwent CPET for measurements of peak Vo(2), Vo(2) at anaerobic threshold (AT), first-degree slope of Vo(2) decline during early recovery (Vo(2)/t-slope), time required for a 50% fall from peak Vo(2) (T(1/2) of Vo(2)) and the slopes of VE/Vco(2) and VE/Vo(2). RESULTS: The MHF and HTR groups had similar ventilatory responses to exercise and O(2) recovery kinetics. Peak Vo(2) (18.5 +/- 5.7 vs 9.4 +/- 0.9 ml/kg/min, p < 0.001), AT (13.8 +/- 4.8 vs 6.7 +/- 1.8 ml/kg/min, p < 0.001) and Vo(2)/t-slope (0.6 +/- 0.2 vs 0.3 +/- 0.2 liter/min/min, p = 0.055) were higher in the HTR than in the ESHF group. In contrast, HTR had lower VE/Vco(2)-slope (31.4 +/- 3.8 vs 39.2 +/- 9.9, p = 0.015) and T(1/2) Vo(2) (1.5 +/- 0.3 vs 2.4 +/- 1.1 minute, p = 0.014) than the ESHF group. Compared to HV, HTR had lower Vo(2) peak (18.5 +/- 5.7 vs 28.4 +/- 6.9 ml/kg/min, p < 0.001), AT (13.8 +/- 4.8 vs 19.8 +/- 4.5 ml/kg/min, p = 0.04), Vo(2)/t-slope (0.6 +/- 0.2 vs 1.0 +/- 0.4 liter/min/min, p = 0.005) and steeper VE/Vco(2) slope (31.4 +/- 3.8 vs 23.6 +/- 2.7, p = 0.062). Heart rate deceleration during recovery was significantly slower in HTR than in all other groups. CONCLUSIONS: Exercise intolerance and delayed O(2) recovery kinetics were only partially reversed after heart transplantation. This finding suggests that some of the pathophysiologic mechanisms of heart failure persist after heart transplantation.     

Evidence that carotid bodies play an important role in glucoregulation in vivo

The carotid bodies are sensitive to glucose in vitro and can be stimulated to cause hyperglycemia in vivo. The aim of this study was to determine if the carotid bodies are involved in basal glucoregulation or the counterregulatory response to an insulin-induced decrement in arterial glucose in vivo. Dogs were surgically prepared >16 days before the experiment. The carotid bodies and their associated nerves were removed (carotid body resected [CBR]) or left intact (Sham), and infusion and sampling catheters were implanted. Removal of carotid bodies was verified by the absence of a ventilatory response to NaCN. Experiments were performed in 18-h fasted conscious dogs and consisted of a tracer ([3-3H]glucose) equilibration period (-120 to -40 min), a basal period (-40 to 0 min), and an insulin infusion (1 mU x kg(-1) x min(-1)) period (0-150 min) during which glucose was infused as needed to clamp at mildly hypoglycemic (65 mg/dl) or euglycemic (105 mg/dl) levels. Basal (8 microU/ml) and clamp (40 microU/ml) insulin levels were similar in both groups. Basal arterial glucagon was reduced in CBR compared with Sham (30 + 2 vs. 40 +/- 2 pg/ml) and remained reduced in CBR during hypoglycemia (peak levels of 36 +/- 3 vs. 52 +/- 7 pg/ml). Cortisol levels were not significantly different between the 2 groups in the basal state, but were reduced during the hypoglycemic clamp in CBR. Catecholamine levels were not significantly different between the 2 groups in the basal and hypoglycemic periods. The glucose infusion rate required to clamp glucose at 65 mg/dl was 2.5-fold greater in CBR compared with Sham (4.0 +/- 0.4 vs. 1.6 +/- 0.4 mg x kg(-1) x min(-1)). Basal endogenous glucose appearance (R(a)) was equal in CBR and Sham (2.5 +/- 0.1 vs. 2.5 +/- 0.2 mg x kg(-1) x min(-1)). During the hypoglycemic clamp, insulin suppressed R(a) in CBR but not Sham (1.1 +/- 0.2 vs. 2.5 +/- 0.2 mg x kg(-1) x min(-1) during the last 30 min of the clamp), reflecting impaired counterregulation. Glucose disappearance (R(d)) in the basal state was similar in CBR and Sham, whereas it was elevated in CBR during the hypoglycemic clamp (4.8 +/- 0.1 vs. 3.9 +/- 0.1 mg x kg(-1) x min(-1) during the last 30 min of the clamp). R(d) was also elevated in euglycemic clamp studies, indicating an effect of carotid body resection independent of hypoglycemia. There were no other measured systematic endocrine or metabolic effects of carotid body resection during euglycemic clamps. In conclusion, we found that the carotid bodies (or receptors anatomically close by) play an important role in the insulin-induced counterregulatory response to mild hypoglycemia.     

Exercise-induced changes in insulin and glucagon are not required for enhanced hepatic glucose uptake after exercise but influence the fate of glucose within the liver

To test whether pancreatic hormonal changes that occur during exercise are necessary for the postexercise enhancement of insulin-stimulated net hepatic glucose uptake, chronically catheterized dogs were exercised on a treadmill or rested for 150 min. At the onset of exercise, somatostatin was infused into a peripheral vein, and insulin and glucagon were infused in the portal vein to maintain basal levels (EX-Basal) or simulate the response to exercise (EX-Sim). Glucose was infused as needed to maintain euglycemia during exercise. After exercise or rest, somatostatin infusion was continued in exercised dogs and initiated in dogs that had remained sedentary. In addition, basal glucagon, glucose, and insulin were infused in the portal vein for 150 min to create a hyperinsulinemic-hyperglycemic clamp in EX-Basal, EX-Sim, and sedentary dogs. Steady-state measurements were made during the final 50 min of the clamp. During exercise, net hepatic glucose output (mg x kg(-1) x min(-1)) rose in EX-Sim (7.6 +/- 2.8) but not EX-Basal (1.9 +/- 0.3) dogs. During the hyperinsulinemic-hyperglycemic clamp that followed either exercise or rest, net hepatic glucose uptake (mg x kg(-1) x min(-1)) was elevated in both EX-Basal (4.0 +/- 0.7) and EX-Sim (4.6 +/- 0.5) dogs compared with sedentary dogs (2.0 +/- 0.3). Despite this elevation in net hepatic glucose uptake after exercise, glucose incorporation into hepatic glycogen, determined using [3-3H]glucose, was not different in EX-Basal and sedentary dogs, but was 50 +/- 30% greater in EX-Sim dogs. Exercise-induced changes in insulin and glucagon, and consequent glycogen depletion, are not required for the increase in net hepatic glucose uptake after exercise but result in a greater fraction of the glucose consumed by the liver being directed to glycogen.     

Intraislet hyperinsulinemia prevents the glucagon response to hypoglycemia despite an intact autonomic response

Because absence of the glucagon response to falling plasma glucose concentrations plays a key role in the pathogenesis of iatrogenic hypoglycemia in patients with insulin-deficient diabetes and the mechanism of this defect is unknown, and given evidence in experimental animals that a decrease in intraislet insulin is a signal to increased glucagon secretion, we examined the role of endogenous insulin in the physiological glucagon response to hypoglycemia. We tested the hypothesis that intraislet hyperinsulinemia prevents the glucagon response to hypoglycemia despite an intact autonomic-adrenomedullary, sympathetic neural, and parasympathetic neural-response and a low alpha-cell glucose concentration. Twelve healthy young adults were studied on three separate occasions. Insulin was infused in hourly steps in relatively low doses (1.5, 3.0, 4.5, and 6.0 pmol.kg(-1).min(-1)) from 60 through 300 min on all three occasions. Plasma glucose levels were clamped at euglycemia ( approximately 5.0 mmol/l, approximately 90 mg/dl) on one occasion and at hourly steps of approximately 4.7, 4.2, 3.6, and 3.0 mmol/l ( approximately 85, 75, 65, and 55 mg/dl) from 60 through 300 min on the other two occasions. On one of the latter occasions, the beta-cell secretagogue tolbutamide was infused in a dose of 1.0 g/h from 60 through 300 min. Hypoglycemia with tolbutamide infusion, compared with similar hypoglycemia alone, was associated with higher (P < 0.0001) C-peptide levels (final values of 1.0 +/- 0.2 vs. 0.1 +/- 0.0 nmol/l), higher (P < 0.0001) rates of insulin secretion (final values of 198 +/- 60 vs. 15 +/- 4 pmol/min), and higher (P < 0.0001) insulin levels (final values of 325 +/- 30 vs. 245 +/- 20 pmol/l) as expected. The glucagon response to hypoglycemia was prevented during tolbutamide infusion (P < 0.0001). Glucagon levels were 17 +/- 1 pmol/l at baseline on both occasions, 14 +/- 1 vs. 15 +/- 1 pmol/l, respectively, during the initial hyperinsulinemic euglycemia, and 15 +/- 1 vs. 22 +/- 2 pmol/l, respectively, during hypoglycemia with and without tolbutamide infusion. Autonomic-adrenomedullary (plasma epinephrine), sympathetic neural (plasma norepinephrine), and parasympathetic neural (plasma pancreatic polypeptide)-responses to hypoglycemia were not reduced during tolbutamide infusion. We conclude that intraislet hyperinsulinemia prevents the glucagon response to hypoglycemia despite an intact autonomic response and a low alpha-cell glucose concentration.     

WISE-2005: supine treadmill exercise within lower body negative pressure and flywheel resistive exercise as a countermeasure to bed rest-induced bone loss in women during 60-day simulated microgravity

Bone loss associated with disuse during bed rest (BR), an analog of space flight, can be attenuated by exercise. In previous studies, the efficacy of either aerobic or resistive exercise countermeasures has been examined separately. We hypothesized that a regimen of combined resistive and aerobic exercise during BR would prevent bone resorption and promote bone formation. After a 20-day ambulatory adaptation to controlled confinement and diet, 16 women participated in a 60-day, 6 degrees head-down-tilt BR and were assigned randomly to one of the two groups. Control subjects (CON, n=8) performed no countermeasure. Exercise subjects (EX, n=8) participated in an exercise program during BR, alternating between supine treadmill exercise within lower body negative pressure (3-4 d wk(-1)) and flywheel resistive exercise (2-3 d wk(-1)). By the last week of BR, excretion of helical peptide (CON, 79%+/-44 increase; EX, 64%+/-50, mean+/-SD) and N-terminal cross-linking telopeptide (CON, 51%+/-34; EX, 43%+/-56), markers of bone resorption, were greater than they were before BR in both groups (P<0.05). However, serum concentrations of the bone formation marker procollagen type I N propeptide were greater in EX than CON throughout and after bed rest (P<0.05), while concentrations of the bone formation marker bone alkaline phosphatase tended to be greater in EX than CON. Dual-energy X-ray absorptiometry results indicated that the exercise treatment significantly (P<0.05) attenuated loss of hip and leg bone mineral density in EX compared to CON. The combination of resistive and aerobic exercise did not prevent bone resorption but did promote bone formation, and helped mitigate the net bone loss associated with simulated microgravity.