Fourteen GH-deficient (GHD) adults were compared with 12 age-, sex-, and body mass index-matched control subjects using a baseline tritiated glucose equilibration period and euglycemic-hyperinsulinemic (approximately 55 mU/L) clamp in conjunction with paired muscle biopsies for measurement of glycogen synthase fractional velocity (FV0.1). Despite similar basal rates of total glucose disposal (Rd), there was a 64% reduction in the insulin-stimulated rise (delta) in Rd in the GHD adults compared to that in controls [16.6 +/- 2.8 vs. 44.7 +/- 6.0 mumol/kg fat free mass (FFM)/min; P < 0.001], which was mainly due to a decreased glucose storage (GS) rate (delta GS, 12.6 +/- 2.9 vs. 39.5 +/- 7.5 mumol/kg FFM/min; P < 0.01). Furthermore, the insulin sensitivity indexes of Rd (0.39 +/- 0.07 vs. 0.85 +/- 0.11; P < 0.05) and GS (0.25 +/- 0.07 vs. 0.72 +/- 0.13 mumol/kg FFM/min per mU/L; P < 0.02) were reduced in GHD adults compared to the control values. The insulin sensitivity of the glycolytic pathway was also reduced by approximately 50% in GHD adults (P = 0.07 vs. controls). Insulin-stimulated FV0.1 was decreased in GHD adults (0.31 +/- 0.02 vs. 0.47 +/- 0.03; P < 0.005) despite similar basal FV0.1. Using multiple and stepwise regression analysis, duration of GH deficiency, fasting triglycerides and fasting insulin accounted for 67% of the variance in the insulin sensitivity index of Rd. In conclusion, the severe insulin resistance in GHD adults is mainly due to the inhibition of the GS pathway and glycogen synthase activity in peripheral tissues, which is related to the duration of GH deficiency, fasting triglycerides, and fasting insulin.

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To determine the impact of variable plasma insulin concentrations and glycemia on the partitioning of whole body glucose metabolism between glycolysis and glucose storage, we estimated endogenous hepatic glucose production and rates of in vivo glycolytic flux (GF) and glucose storage (GS) in six normal dogs from the generation of plasma tritiated water (3H2O) and [3-3H]glucose specific activity during 150 min of somatostatin euglycemic (E) and hyperglycemic (H) clamps at hypoinsulinemic, basal, intermediate, and high insulin levels. During both E and H clamps, overall rates of GF and GS increased with the rising insulin levels, but the relative contributions to in vivo glucose disposal of GF decreased, whereas GS rose progressively with increasing insulin levels. The relative contribution of GS during H to overall glucose disposal was greater at the lower insulin level. In addition, in absolute terms, GF and GS were significantly higher (P < 0.05) during H than during E at all insulin levels. Moreover, the incremental rise in GF induced by H was equal for the low to intermediate insulin levels tested, independent of the prevailing free fatty acid (FFA) levels. However, when whole body glucose disposal rates were matched, GF and GS rates were independent of the coexisting glycemia, insulin, and/or FFA levels. We conclude that 1) insulin has a major impact on the intracellular fate of infused glucose, with a lesser but significant effect of hyperglycemia per se on these processes; 2) the magnitude of the hyperglycemia-induced increase in GF is independent of the prevailing insulin level from low to intermediate levels; and 3) in vivo GF and GS are dependent on the net rate of glucose uptake into cells but independent of absolute FFA levels or whether glucose uptake is stimulated by raised insulin or glucose levels.

OBJECTIVE-Fructose-1,6-bisphosphatase (FBPase) is a gluconeogenic enzyme that is upregulated in islets or pancreatic ␤-cell lines exposed to high fat. However, whether specific ␤-cell upregulation of FBPase can impair insulin secretory function is not known. The objective of this study therefore is to determine whether a specific increase in islet ␤-cell FBPase can result in reduced glucose-mediated insulin secretion. RESEARCH DESIGN AND METHODS-To test this hypothesis, we have generated three transgenic mouse lines overexpressing the human FBPase (huFBPase) gene specifically in pancreatic islet ␤-cells. In addition, to investigate the biochemical mechanism by which elevated FBPase affects insulin secretion, we made two pancreatic ␤-cell lines (MIN6) stably overexpressing huFBPase.RESULTS-FBPase transgenic mice showed reduced insulin secretion in response to an intravenous glucose bolus. Compared with the untransfected parental MIN6, FBPase-overexpressing cells showed a decreased cell proliferation rate and significantly depressed glucose-induced insulin secretion. These defects were associated with a decrease in the rate of glucose utilization, resulting in reduced cellular ATP levels.CONCLUSIONS-Taken together, these results suggest that upregulation of FBPase in pancreatic islet ␤-cells, as occurs in states of lipid oversupply and type 2 diabetes, contributes to insulin secretory dysfunction.

. Prevailing hyperglycemia is critical in the regulation of glucose metabolism during exercise in poorly controlled alloxan-diabetic dogs. J Appl Physiol 98: 930 -939, 2005; doi:10.1152/japplphysiol.00687.2004.-The separate impacts of the chronic diabetic state and the prevailing hyperglycemia on plasma substrates and hormones, in vivo glucose turnover, and ex vivo skeletal muscle (SkM) during exercise were examined in the same six dogs before alloxan-induced diabetes (prealloxan) and after 4 -5 wk of poorly controlled hyperglycemic diabetes (HGD) in the absence and presence of ϳ300-min phlorizin-induced (glycosuria mediated) normoglycemia (NGD). For each treatment state, the ϳ15-h-fasted dog underwent a primed continuous 150-min infusion of [3-3 H]glucose, followed by a 30-min treadmill exercise test (ϳ65% maximal oxygen capacity), with SkM biopsies taken from the thigh (vastus lateralis) before and after exercise. In the HGD and NGD states, preexercise hepatic glucose production rose by 130 and 160%, and the metabolic clearance rate of glucose (MCRg) fell by 70 and 37%, respectively, compared with the corresponding prealloxan state, but the rates of glucose uptake into peripheral tissues (Rdtissue) and total glycolysis (GF) were unchanged, despite an increased availability of plasma free fatty acid in the NGD state. Exercise-induced increments in hepatic glucose production, Rdtissue, and plasma-derived GF were severely blunted by ϳ30 -50% in the NGD state, but increments in MCRg remained markedly reduced by ϳ70 -75% in both diabetic states. SkM intracellular glucose concentrations were significantly elevated only in the HGD state. Although Rdtissue during exercise in the diabetic states correlated positively with preexercise plasma glucose and insulin and GF and negatively with preexercise plasma free fatty acid, stepwise regression analysis revealed that an individual's preexercise glucose and GF accounted for 88% of Rdtissue during exercise. In conclusion, the prevailing hyperglycemia in poorly controlled diabetes is critical in maintaining a sufficient supply of plasma glucose for SkM glucose uptake during exercise. During phlorizin-induced NGD, increments in both Rdtissue and GF are impaired due to a diminished fuel supply from plasma glucose and a sustained reduction in increments of MCRg. glucose uptake; plasma-derived glycolysis; skeletal muscle; free fatty acids; metabolic clearance rate of glucose; phlorizin-induced normoglycemia THE REGULATION OF GLUCOSE METABOLISM in skeletal muscle (SkM) during exercise occurs predominantly via an insulinindependent pathway (13,16,22) and involves the recruitment of SkM GLUT4 transporters from a distinct insulin-independent GLUT-4 pool (13). It is known that the prior insulin sensitivity of an individual is important in determining the rate of glucose uptake into SkM (Rd tissue ) in response to acute exercise (28). Furthermore, the intensity and duration of the exercise are also critical in determining the relative importance of carbohydrate and fat substrates as ...

We determined the impact of variable insulinemia and glycemia on the in vivo partitioning of glucose effectiveness (GE) and insulin sensitivity (SI) and the in vitro intracellular processing of glucose metabolism. Six somatostatin- and [3-3H]glucose-infused dogs underwent euglycemic and hyperglycemic clamps at four physiological insulin (Ins) levels before a muscle biopsy. From the rates of glucose infusion (GINF), total glucose disposal (Rd), total glycolysis (GF), and glucose storage (GS), plots of delta GINF, delta Rd, delta GS vs. delta log Ins concentration were found to be linear for each dog, allowing calculation of the partitioning of GE and SI into their major in vivo sites (periphery vs. liver) and intracellular metabolic pathways (GS vs. GF). Insulinopenia induced a significant reduction in total GE. From insulinopenia to high insulinemia, the 2.3-fold increase in total GE was due to the increased peripheral glucose responsiveness of the GS pathway. Hyperglycemia induced a significant reduction in total SI, with approximately one-half of this reduction due to the decreased peripheral insulin responsiveness of the GF pathway. In skeletal muscle, both glycogen content and glycogen synthase fractional activity were positively correlated with log Ins concentration, Rd, and GS but negatively correlated with glucose 6-phosphate concentration. Moreover, both Rd and GS were negatively correlated with lactate concentration. We conclude that 1) the inhibition of GE and SI induced by insulinopenia and hyperglycemia, respectively, is due mainly to the reduced peripheral responsiveness of contrasting intracellular metabolic pathways; and 2) hyperinsulinemia and/or hyperglycemia stimulates glycogen synthesis and GF but not nonoxidative glycolysis.

. Skeletal muscle basal AMP-activated protein kinase activity is chronically elevated in alloxan-diabetic dogs: impact of exercise. J Appl Physiol 95: 1523-1530. First published June 27, 2003 10.1152/japplphysiol.00199.2003.-The effect of diabetes and exercise on skeletal muscle (SkM) AMP-activated protein kinase (AMPK)␣1 and -␣2 activities and site-specific phosphorylation of acetyl-CoA carboxylase was examined in the same six dogs before alloxan (35 mg/kg)-induced diabetes (C) and after 4-5 wk of suboptimally controlled hyperglycemic and hypoinsulinemic diabetes (DHG) in the presence and absence of 300-min phlorizin (50 g ⅐ kg Ϫ1 ⅐ min Ϫ1 )-induced "normoglycemia" (DNG). In each study, the dog underwent a 150-min [3-3 H]glucose infusion period, followed by a 30-min treadmill exercise test (60-70% maximal oxygen capacity) to measure the rate of glucose disposal into peripheral tissues (Rd tissue). SkM biopsies were taken from the thigh (vastus lateralis) before and immediately after exercise. In the C and DHG states, the rise in plasma free fatty acids (FFA) with exercise (ϳ40%) was similar. In the DNG group, preexercise FFA were significantly higher, but the absolute rise in FFA with exercise was similar. However, the exercise-induced increment in Rd tissue was significantly blunted (by ϳ40-50%) in the DNG group compared with the other states. In SkM, preexercise AMPK␣1 and -␣2 activities were significantly elevated (by ϳ60-125%) in both diabetic states, but unlike the C group these activities did not rise further with exercise. Additionally, preexercise acetyl-CoA carboxylase phosphorylation in both diabetic states was elevated by ϳ70-80%, but the increases with exercise were similar to the C group. Preexercise AMPK␣1 and -␣2 activities were negatively correlated with Rd tissue during exercise for the combined groups (both P Ͻ 0.02). In conclusion, the elevated preexercise SkM AMPK␣1 and -␣2 activities contribute to the ongoing basal supply of glucose and fatty acid metabolism in suboptimally controlled hypoinsulinemic diabetic dogs; but whether they also play a permissive role in the metabolic stress response to exercise remains uncertain. hyperglycemia; hypoinsulinemia; acetyl coenzyme A carboxylase; glucose and fatty acid metabolism; phlorizin-induced normoglycemia THE REGULATION OF GLUCOSE and fatty acid (FA) metabolism in skeletal muscle (SkM) during exercise is currently the focus of intense investigation (21, 41). Transmembrane GLUT-4-mediated glucose transport is a major rate-determining site for exercise and insulinstimulated SkM glucose uptake (48), although several other factors, including enhanced capillary recruitment (19) and intracellular enzyme activities (such as pyruvate dehydrogenase) (31), also contribute to the increased glucose uptake and processing observed in exercising SkM. In addition, as the energy demanded by the exercising muscle rises, FA uptake and oxidation are also increased (34, 37). Thus heavily exercising SkM is potentially at risk of outstripping its rising energy needs a...

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