Weight gain induced by an energy-dense diet is hypothesized to arise in part from defects in the neuronal response to circulating adiposity negative feedback signals, such as insulin. Peripheral tissue insulin resistance involves cellular inflammatory responses thought to be invoked by excess lipid. Therefore, we sought to determine whether similar signaling pathways are activated in the brain of rats fed a high-fat (HF) diet. The ability of intracerebroventricular (icv) insulin to reduce food intake and activate hypothalamic signal transduction is attenuated in HF-fed compared with low-fat (LF)-fed rats. This effect was accompanied by both hypothalamic accumulation of palmitoyl- and stearoyl-CoA and activation of a marker of inflammatory signaling, inhibitor of kappaB kinase-beta (IKKbeta). Hypothalamic insulin resistance and inflammation were observed with icv palmitate infusion or HF feeding independent of excess caloric intake. Last, we observed that central IKKbeta inhibition reduced food intake and was associated with increased hypothalamic insulin sensitivity in rats fed a HF but not a LF diet. These data collectively support a model of diet-induced obesity whereby dietary fat, not excess calories, induces hypothalamic insulin resistance by increasing the content of saturated acyl-CoA species and activating local inflammatory signals, which result in a failure to appropriately regulate food intake.

Previous work suggests that normal GLUT4 content is sufficient for increases in muscle glucose uptake (MGU) during exercise because GLUT4 overexpression does not increase exercise-stimulated MGU. Instead of glucose transport, glucose phosphorylation is a primary limitation of exercise-stimulated MGU. It was hypothesized that a partial ablation of GLUT4 would not impair exercise-stimulated MGU when glucose phosphorylation capacity is normal but would do so when glucose phosphorylation capacity was increased. Thus, C57BL/6J mice with hexokinase II (HKII) overexpression (HK Tg ), a GLUT4 partial knock-out (G4 ؉/؊ ), or both (HK Tg ؉ G4 ؉/؊ ) and wild-type (WT) littermates were implanted with carotid artery and jugular vein catheters for sampling and infusions at 4 months of age. After a 7-day recovery, 5-h fasted mice remained sedentary or ran on a treadmill at 0.6 mph for 30 min (n ‫؍‬ 9 -12 per group) and received a bolus of 2-deoxy[ 3 H]glucose to provide an index of MGU (R g ). Arterial blood glucose and plasma insulin concentrations were similar in WT, G4 ؉/؊ , HK Tg , and HK Tg ؉ G4 ؉/؊ mice. Sedentary R g values were the same in all genotypes in all muscles studied, confirming that glucose transport is a significant barrier to basal glucose uptake. Gastrocnemius and soleus R g were greater in exercising compared with sedentary mice in all genotypes. During exercise, G4 ؉/؊ mice had a marked increase in blood glucose that was corrected by the addition of HK II overexpression. Exercise R g (mol/100g/min) was not different between WT and G4 ؉/؊ mice in the gastrocnemius (24 ؎ 5 versus 21 ؎ 2) or the soleus (54 ؎ 6 versus 70 ؎ 7). In contrast, the enhanced exercise R g observed in HK Tg mice compared with that in WT mice was absent in HK Tg ؉ G4 ؉/؊ mice in both the gastrocnemius (39 ؎ 7 versus 22 ؎ 6) and the soleus (98 ؎ 13 versus 65 ؎ 13). Thus, glucose transport is not a significant barrier to exercise-stimulated MGU despite a 50% reduction in GLUT4 content when glucose phosphorylation capacity is normal. However, when glucose phosphorylation capacity is increased by HK II overexpression, GLUT4 availability becomes a marked limitation to exercise-stimulated MGU.Muscle glucose uptake (MGU) 1 can be separated into three sequential steps, i.e. delivery of glucose from the blood to the muscle, transport across the sarcolemma by a GLUT, and irreversible phosphorylation to glucose-6-phosphate by an HK isozyme. Each of these steps can serve as a barrier to MGU and, thus, are important in regulating glucose influx. During resting conditions, the transport step exerts the most control in regulating MGU, as GLUT1 (1-4) or GLUT4 (5, 6) overexpression augments basal MGU. Previous work suggests that normal GLUT4 content is sufficient for increases in MGU during exercise, because GLUT4 overexpression alone does not further increase exercise-stimulated MGU (7). Instead of glucose transport, glucose phosphorylation is a primary limitation of exercise-stimulated MGU (7-9). Heterozygous GLUT4 knock-out mice serve as a useful ...

The aim of this study was to test whether in fact glucose transport is rate-limiting in control of muscle glucose uptake (MGU) under physiological hyperinsulinaemic conditions in the conscious, unrestrained mouse. C57Bl/6J mice overexpressing GLUT4 (GLUT4 Tg ), hexokinase II (HK Tg ), or both (GLUT4 Tg + HK Tg ), were compared to wild-type (WT) littermates. Catheters were implanted into a carotid artery and jugular vein for sampling and infusions at 4 month of age. After a 5-day recovery period, conscious mice underwent one of two protocols (n = 8-14/group) after a 5-h fast. Saline or insulin (4 mU kg −1 min −1 ) was infused for 120 min. All mice received a bolus of 2-deoxy[ 3 H]glucose (2-3 HDG) at 95 min. Glucose was clamped at ∼165 mg dl −1 during insulin infusion and insulin levels reached ∼80 µU ml −1 . The rate of disappearance of 2-3 HDG from the blood provided an index of whole body glucose clearance. Gastrocnemius, superficial vastus lateralis and soleus muscles were excised at 120 min to determine 2-3 HDG-6-phosphate levels and calculate an index of MGU (R g ). Results show that whole body and tissue-specific indices of glucose utilization were: (1) augmented by GLUT4 overexpression, but not HKII overexpression, in the basal state; (2) enhanced by HKII overexpression in the presence of physiological hyperinsulinaemia; and (3) largely unaffected by GLUT4 overexpression during insulin clamps whether alone or combined with HKII overexpression. Therefore, while glucose transport is the primary barrier to MGU under basal conditions, glucose phosphorylation becomes a more important barrier during physiological hyperinsulinaemia in all muscles. The control of MGU is distributed rather than confined to a single rate-limiting step such as glucose transport as glucose transport and phosphorylation can both become barriers to skeletal muscle glucose influx.

Hexokinase (HK) II content is elevated in fatigue resistant muscle fibres and exercise trained muscle. The aim of this study was to determine if exercise capacity is dependent on muscle HK protein content. C57Bl/6 mice with a 50% HK knockout (HK +/− ), no genetic manipulation (wild-type, WT) and an ∼3-fold HK overexpression (HK Tg ) were tested. Mice (n = 12/group) completed both a maximal oxygen consumption (V O 2 ,max ) test and an endurance capacity test (run at ∼75%V O 2 ,max ) on an enclosed treadmill equipped to measure gas exchange. Arterial and venous catheters were surgically implanted into separate groups of mice (n = 9-11/group) in order to measure an index of muscle glucose uptake (R g ) during 30 min of treadmill exercise. Maximum work rate (0.95 ± 0.05, 1.00 ± 0.04 and 1.06 ± 0.07 kg m min −1 ),V O 2 ,max (137 ± 3, 141 ± 4 and 141 ± 5 ml kg −1 min −1 ) and maximal respiratory exchange ratio (1.04 ± 0.02, 1.00 ± 0.03 and 1.04 ± 0.04) were similar in HK +/− , WT and HK Tg , respectively. Exercise endurance capacity (measured as time to exhaustion) increased as HK content increased (55 ± 11, 77 ± 5 and 98 ± 9 min) and this was related to R g measured in mice during 30 min of exercise (13 ± 2, 24 ± 5 and 42 ± 5 µmol (100 g) −1 min −1 ). Muscle glycogen in sedentary HK +/− mice and HK +/− mice following 30 min of exercise were significantly lower than in HK Tg and WT mice. However, the net exercise-induced muscle glycogen breakdown was equal in the three genotypes. In summary, HK protein content within the range studied (a) was not associated with a difference in the capacity to perform maximal intensity exercise, (b) was a powerful determinant of the ability to sustain moderate intensity exercise, as reducing HK content impaired endurance and increasing HK content enhanced endurance, and (c) although directly related to exercise endurance, was not a determinant of net muscle glycogen usage during exercise. In conclusion, adaptations that increase HK protein content and/or functional activity such as regular exercise contribute to increased muscular endurance.

Previous work suggests that normal GLUT4 content is sufficient for increases in muscle glucose uptake (MGU) during hyperinsulinemia, because glucose phosphorylation is the more formidable barrier to insulin-stimulated MGU. It was hypothesized that a partial ablation of GLUT4 would not impair insulin-stimulated MGU when glucose phosphorylation capacity is normal but would do so when glucose phosphorylation capacity is increased. Thus, chow-fed C57BL/6J mice with a GLUT4 partial knockout (GLUT4(+/-)), hexokinase II overexpression (HK(Tg)), or both (HK(Tg) + GLUT4(+/-)) and wild-type littermates were studied. Carotid artery and jugular vein catheters were implanted for sampling and infusions at 4 months of age. After a 5-d recovery, 5-h fasted mice (n = 8-11/group) underwent a 120-min saline infusion or insulin clamp (4 mU/kg.min insulin with glucose maintained at 165 mg/dl) and received a 2-deoxy[(3)H]glucose bolus to provide an index of MGU (R(g)) for the soleus, gastrocnemius, and superficial vastus lateralis. Basal R(g) from all muscles studied from saline-infused mice were not changed by any of the genetic modifications. HK(Tg) mice had augmented insulin-stimulated R(g) in all muscles studied compared with remaining genotypes. Insulin-stimulated R(g) was not impaired in any of the muscles studied from GLUT4(+/-) mice. However, the enhanced insulin-stimulated R(g) created by HK overexpression was ablated in HK(Tg) + GLUT4(+/-) mice. Thus, a 50% reduction of normal GLUT4 content in the presence of normal HK activity does not impair insulin-stimulated MGU. However, when the glucose phosphorylation barrier is lowered by HK overexpression, GLUT4 availability becomes a limitation to insulin-stimulated MGU.