Citations

Exercise training elicits a metabolic and cardiovascular response that underlies fitness. The molecular mechanisms that orchestrate this adaptive response and secure the wide-ranging gains of a regimented exercise program are poorly understood. Formed through association of the Kir6.2 pore and the sulfonylurea receptor, the stress-responsive ATP-sensitive K+ channels (KATP channels), with their metabolic-sensing capability and broad tissue expression, are potential candidates for integrating the systemic adaptive response to repetitive exercise. Here, the responses of mice lacking functional Kir6.2-containing KATP channels (Kir6.2-KO) were compared with wild-type controls following a 28-day endurance swimming protocol. While chronic aquatic training resulted in lighter, leaner, and fitter wild-type animals, the Kir6.2-KO manifested less augmentation in exercise capacity and lacked metabolic improvement in body fat composition and glycemic handling with myocellular defects. Moreover, the repetitive stress of swimming unmasked a survival disadvantage in the Kir6.2-KO, associated with pathologic calcium-dependent structural damage in the heart and impaired cardiac performance. Thus, Kir6.2-containing KATP channel activity is required for attainment of the physiologic benefits of exercise training without injury.

Metabolic-sensing ATP-sensitive K+ channels (KATP channels) adjust membrane excitability to match cellular energetic demand. In the heart, KATP channel activity has been linked to homeostatic shortening of the action potential under stress, yet the requirement of channel function in securing cardiac electrical stability is only partially understood. Here, upon catecholamine challenge, disruption of KATP channels, by genetic deletion of the pore-forming Kir6.2 subunit, produced defective cardiac action potential shortening, predisposing the myocardium to early afterdepolarizations. This deficit in repolarization reserve, demonstrated in Kir6.2-knockout hearts, translated into a high risk for induction of triggered activity and ventricular dysrhythmia. Thus, intact KATP channel function is mandatory for adequate repolarization under sympathetic stress providing electrical tolerance against triggered arrhythmia.

Glucocorticoids are potent regulators of protein, fat, and carbohydrate metabolism. To determine if cortisol production occurs within the splanchnic bed in humans, 11 nondiabetic subjects were studied using the hepatic/leg catheterization method along with an infusion of [9,11,12,12-2H4] cortisol (D4-cortisol) as proposed by Andrews et al. In the fasting state, there was net release (P < 0.05) of cortisol from the splanchnic bed (6.1 ± 2.6 μg/min) and net uptake (P < 0.05) by the leg (1.7 ± 0.7 μg/min). This, along with cortisol production by other tissues (e.g., the adrenals), resulted in a total-body cortisol appearance rate of 18.1 ± 1.9 μg/min. Fractional splanchnic D4-cortisol extraction averaged 12.9 ± 1.3% (P < 0.001), splanchnic cortisol uptake 14.8 ± 2.0 μg/min (P < 0.001), and splanchnic cortisol production 22.2 ± 3.3 μg/min (P < 0.001). On the other hand, fractional leg D4-cortisol extraction averaged 5.6 ± 1.8% (P < 0.02), leg cortisol uptake 2.3 ± 0.7 μg/min (P < 0.01), and leg cortisol production 0.4 ± 0.4 μg/min, which did not differ from zero. Because D4-cortisol loses a deuterium during conversion to [9,12,12-2H3] cortisone (D3-cortisone), which in turn generates [9,12,122H3] cortisol (D3-cortisol) via 11-β hydroxysteroid dehydrogenase (11β-HSD) type 1, D3-cortisol production can be used as an index of 11β-HSD type 1 activity. Net splanchnic D3-cortisol release (3.9 ± 0.4 μg/min) and splanchnic D3-cortisol production (7.1 ± 0.7 μg/min) occurred (P < 0.01) in all subjects. In contrast, there was minimal leg D3-cortisol production (0.04 ± 0.01 μg/min), resulting in a strong correlation between splanchnic D3-cortisol production and total-body 3D-cortisol production in both the fasting state (r = 0.84; P < 0.02) and during an infusion of insulin (r = 0.97; P < 0.01). Thus, splanchnic production of cortisol occurs in nondiabetic humans at rates approximating that which occurs in the remainder of the body. These data support the possibility that alterations in splanchnic cortisol production contribute to visceral fat accumulation and the hepatic insulin resistance of obesity or type 2 diabetes.

To determine whether the insulin dose-response curves for suppression of endogenous glucose production (EGP) and stimulation of splanchnic glucose uptake (SGU) differ in nondiabetic humans and are abnormal in type 2 diabetes, 14 nondiabetic and 12 diabetic subjects were studied. Glucose was clamped at ∼9.5 mmol/l and endogenous hormone secretion inhibited by somatostatin, while glucagon and growth hormone were replaced by an exogenous infusion. Insulin was progressively increased from ∼150 to ∼350 and ∼700 pmol/l by means of an exogenous insulin infusion, while EGP, SGU, and leg glucose uptake (LGU) were measured using the splanchnic and leg catheterization methods, combined with a [3-3H]glucose infusion. In nondiabetic subjects, an increase in insulin from ∼150 to ∼350 pmol/l resulted in maximal suppression of EGP, whereas SGU continued to increase (P < 0.001) when insulin was increased to ∼700 pmol/l. In contrast, EGP progressively decreased (P < 0.001) and SGU progressively increased (P < 0.001) in the diabetic subjects as insulin increased from ∼150 to ∼700 pmol/l. Although EGP was higher (P < 0.01) in the diabetic than nondiabetic subjects only at the lowest insulin concentration, SGU was lower (P < 0.01) in the diabetic subjects at all insulin concentrations tested. On the other hand, in contrast to LGU and overall glucose disposal, the increment in SGU in response to both increments in insulin did not differ in the diabetic and nondiabetic subjects, implying a right shifted but parallel dose-response curve. These data indicate that the dose-response curves for suppression of glucose production and stimulation of glucose uptake differ in nondiabetic subjects and are abnormal in people with type 2 diabetes. Taken together, these data also suggest that agents that enhance SGU in diabetic patients (e.g. glucokinase activators) are likely to improve glucose tolerance.

Little is known about the fate of the lipolytic products produced by the action of lipoprotein lipase (LPL) on circulating triglyceride-rich lipoproteins in humans. We studied eight lean, healthy male subjects after an overnight fast. Subjects received infusions of lipid emulsions containing triolein labeled with 3H on both the glycerol backbone and the fatty acid portion of the molecule; 14C glycerol and 14C oleate were coinfused to quantify the systemic and forearm release of 3H glycerol and 3H oleate resulting from LPL action. There was significant forearm uptake of both whole plasma triglyceride (presumed to represent primarily VLDL; extraction fraction 2.6 ± 0.6%, P < 0.005 vs. zero) and radiolabeled triglyceride derived from the lipid emulsion (a surrogate for chylomicrons; extraction fraction 31 ± 4%, P < 0.005 vs zero). Systemic clearance and forearm fractional extraction of glycerol was greater than that of oleate (P < 0.001 and P < 0.02, respectively). The systemic and forearm fractional release of LPL-generated glycerol were similar at 51 ± 4 and 59 ± 1%, respectively (NS). In contrast, the forearm fractional release of LPL-generated oleate was less than systemic fractional release (14 ± 2 vs. 36 ± 4%, P < 0.0001). These results indicate that there is escape, or spillover, of the lipolytic products of LPL action on triglyceride-rich lipoproteins in humans. They further suggest that LPL-mediated fatty acid uptake is an inefficient process, but may be more efficient in muscle than in adipose tissue.

Alzheimer disease and type 2 diabetes are characterized by increased prevalence with aging, a genetic predisposition, and comparable pathological features in the islet and brain (amyloid derived from amyloid β protein in the brain in Alzheimer disease and islet amyloid derived from islet amyloid polypeptide in the pancreas in type 2 diabetes). Evidence is growing to link precursors of amyloid deposition in the brain and pancreas with the pathogenesis of Alzheimer disease and type 2 diabetes, respectively. Given these similarities, we questioned whether there may be a common underlying mechanism predisposing to islet and cerebral amyloid. To address this, we first examined the prevalence of type 2 diabetes in a community-based controlled study, the Mayo Clinic Alzheimer Disease Patient Registry (ADPR), which follows patients with Alzheimer disease versus control subjects without Alzheimer disease. In addition to this clinical study, we performed a pathological study of autopsy cases from this same community to determine whether there is an increased prevalence of islet amyloid in patients with Alzheimer disease and increased prevalence of cerebral amyloid in patients with type 2 diabetes. Patients who were enrolled in the ADPR (Alzheimer disease n = 100, non-Alzheimer disease control subjects n = 138) were classified according to fasting glucose concentration (FPG) as nondiabetic (FPG <110 mg/dl), impaired fasting glucose (IFG, FPG 110–125 mg/dl), and type 2 diabetes (FPG >126 mg/dl). The mean slope of FPG over 10 years in each case was also compared between Alzheimer disease and non-Alzheimer disease control subjects. Pancreas and brain were examined from autopsy specimens obtained from 105 humans (first, 28 cases of Alzheimer disease disease vs. 21 non-Alzheimer disease control subjects and, second, 35 subjects with type 2 diabetes vs. 21 non-type 2 diabetes control subjects) for the presence of islet and brain amyloid. Both type 2 diabetes (35% vs. 18%; P < 0.05) and IFG (46% vs. 24%; P < 0.01) were more prevalent in Alzheimer disease versus non-Alzheimer disease control subjects, so 81% of cases of Alzheimer disease had either type 2 diabetes or IFG. The slope of increase of FPG with age over 10 years was also greater in Alzheimer disease than non-Alzheimer disease control subjects (P < 0.01). Islet amyloid was more frequent (P < 0.05) and extensive (P < 0.05) in patients with Alzheimer disease than in non-Alzheimer disease control subjects. However, diffuse and neuritic plaques were not more common in type 2 diabetes than in control subjects. In cases of type 2 diabetes when they were present, the duration of type 2 diabetes correlated with the density of diffuse (P < 0.001) and neuritic plaques (P < 0.01). In this community cohort from southeast Minnesota, type 2 diabetes and IFG are more common in patients with Alzheimer disease than in control subjects, as is the pathological hallmark of type 2 diabetes, islet amyloid. However, there was no increase in brain plaque formation in cases of type 2 diabetes, although when it was present, it correlated in extent with duration of diabetes. These data support the hypothesis that patients with Alzheimer disease are more vulnerable to type 2 diabetes and the possibility of linkage between the processes responsible for loss of brain cells and β-cells in these diseases.

Insulin resistance increases and muscle oxidative capacity decreases during aging, but lifestyle changes—especially physical activity—may reverse these trends. Here we report the effect of a 16-week aerobic exercise program (n = 65) or control activity (n = 37) performed by men and women aged 21–87 years on insulin sensitivity and muscle mitochondria. Insulin sensitivity, measured by intravenous glucose tolerance test, decreased with age (r = −0.32) and was related to abdominal fat content (r = −0.65). Exercise increased peak oxygen uptake (Vo2peak; 10%), activity of muscle mitochondrial enzymes (citrate synthase and cytochrome c oxidase, 45–76%) and mRNA levels of mitochondrial genes (COX4, ND4, both 66%) and genes involved in mitochondrial biogenesis (PGC-1α, 55%; NRF-1, 15%; TFAM, 85%). Exercise also increased muscle GLUT4 mRNA and protein (30–52%) and reduced abdominal fat (5%) and plasma triglycerides (25%). None of these changes were affected by age. In contrast, insulin sensitivity improved in younger people but not in middle-aged or older groups. Thus, the muscle mitochondrial response to 4 months of aerobic exercise training was similar in all age-groups, although the older people did not have an improvement in insulin sensitivity.

To determine the in vivo effect of amino acids (AAs) alone or in combination with insulin on splanchnic and muscle protein dynamics, we infused stable isotope tracers of AAs in 36 healthy subjects and sampled from femoral artery and vein and hepatic vein. The subjects were randomized into six groups and were studied at baseline and during infusions of saline (group 1), insulin (0.5 mU · kg−1 · min−1) (group 2), insulin plus replacement of AAs (group 3) insulin plus high-dose AAs (group 4), or somatostatin and baseline replacement doses of insulin, glucagon and GH plus high dose of AAs (group 5) or saline (group 6). Insulin reduced muscle release of AAs mainly by inhibition of protein breakdown. Insulin also enhanced AA-induced muscle protein synthesis (PS) and reduced leucine transamination. The main effect of AAs on muscle was the enhancement of PS. Insulin had no effect on protein dynamics or leucine transamination in splanchnic bed. However, AAs reduced protein breakdown and increased synthesis in splanchnic bed in a dose-dependent manner. AAs also enhanced leucine transamination in both splanchnic and muscle beds. Thus insulin’s anabolic effect was mostly on muscle, whereas AAs acted on muscle as well as on splanchnic bed. Insulin achieved anabolic effect in muscle by inhibition of protein breakdown, enhancing AA-induced PS, and reducing leucine transamination. AAs largely determined protein anabolism in splanchnic bed by stimulating PS and decreasing protein breakdown.

The major predisposing genetic component in type 1 diabetes maps to the major histocompatibility complex locus in both mice and humans. To verify the HLA class II association with disease pathogenesis, we adopted the transgenic approach. Expression of HLA-DQ8, the molecule showing the strongest association with human type 1 diabetes, in the diabetes-predisposing milieu of NOD mice in the absence of the endogenous class II molecule I-Ag7 did not render susceptibility to type 1 diabetes. To study if providing a local proinflammatory environment would lead to diabetes in these mice, Aβo.NOD.DQ8 were bred with C57BL/6 mice expressing tumor necrosis factor (TNF)-α in the β-cells of the islets of Langerhans. Surprisingly, although diabetes was evident in the F1 intercross expressing rat insulin promoter (RIP)-TNF, offspring lacking either endogenous or transgenic class II molecules developed accelerated diabetes with high frequency in both sexes. Moreover, expression of any functional class II molecule seemed to confer significant protection from diabetes in this model. Thus, neonatal expression of TNF-α in an islet-specific manner bypassed the requirement of CD4+ T-cells and resulted in diabetes that could be mediated by CD8+ T-cells. We also show for the first time that diabetes in NOD.RIP-TNF mice can occur independent of inheritance of NOD-derived idd1.

The present study sought to determine whether elevated plasma free fatty acids (FFAs) alter the splanchnic and muscle glucose metabolism in women. To do so, FFAs were increased in seven women by an 8-h Intralipid/heparin (IL/hep) infusion, and the results were compared with those observed in nine women who were infused with glycerol alone. Glucose was clamped at ∼8.3 mmol/l and insulin was increased to ∼300 pmol/l to stimulate both muscle and hepatic glucose uptake. Insulin secretion was inhibited with somatostatin. Leg and splanchnic glucose metabolism were assessed using a combined catheter and tracer dilution approach. The glucose infusion rates required to maintain target plasma glucose concentrations were lower (P < 0.01) during IL/hep than glycerol infusion (30.8 ± 2.6 vs. 65.0 ± 7.9 μmol · kg−1 · min−1). Whole-body glucose disappearance (37.0 ± 2.2 vs. 70.9 ± 8.7 μmol · kg−1 · min−1; P < 0.001) and leg glucose uptake (24.3 ± 4.2 vs. 59.6 ± 10.0 μmol · kg fat-free mass of the leg−1 · min−1; P < 0.02) were also lower, whereas splanchnic glucose production (8.2 ± 0.8 vs. 4.3 ± 0.7 μmol · kg−1 · min−1; P < 0.01) was higher during IL/hep than glycerol infusion. We conclude that in the presence of combined hyperinsulinemia and hyperglycemia, elevated FFAs impair glucose metabolism in women by inhibiting whole- body glucose disposal, muscle glucose uptake, and suppression of splanchnic glucose production.