Thomas B. Price

Mechanism of free fatty acid-induced insulin resistance in humans.

To examine the mechanism by which lipids cause insulin resistance in humans, skeletal muscle glycogen and glucose-6-phosphate concentrations were measured every 15 min by simultaneous 13C and 31P nuclear magnetic resonance spectroscopy in nine healthy subjects in the presence of low (0.18 +/- 0.02 mM [mean +/- SEM]; control) or high (1.93 +/- 0.04 mM; lipid infusion) plasma free fatty acid levels under euglycemic (approximately 5.2 mM) hyperinsulinemic (approximately 400 pM) clamp conditions for 6 h. During the initial 3.5 h of the clamp the rate of whole-body glucose uptake was not affected by lipid infusion, but it then decreased continuously to be approximately 46% of control values after 6 h (P < 0.00001). Augmented lipid oxidation was accompanied by a approximately 40% reduction of oxidative glucose metabolism starting during the third hour of lipid infusion (P < 0.05). Rates of muscle glycogen synthesis were similar during the first 3 h of lipid and control infusion, but thereafter decreased to approximately 50% of control values (4.0 +/- 1.0 vs. 9.3 +/- 1.6 mumol/[kg.min], P < 0.05). Reduction of muscle glycogen synthesis by elevated plasma free fatty acids was preceded by a fall of muscle glucose-6-phosphate concentrations starting at approximately 1.5 h (195 +/- 25 vs. control: 237 +/- 26 mM; P < 0.01). Therefore in contrast to the originally postulated mechanism in which free fatty acids were thought to inhibit insulin-stimulated glucose uptake in muscle through initial inhibition of pyruvate dehydrogenase these results demonstrate that free fatty acids induce insulin resistance in humans by initial inhibition of glucose transport/phosphorylation which is then followed by an approximately 50% reduction in both the rate of muscle glycogen synthesis and glucose oxidation.

Increased Glucose Transport–Phosphorylation and Muscle Glycogen Synthesis after Exercise Training in Insulin-Resistant Subjects

BACKGROUND Insulin resistance in the offspring of parents with non-insulin-dependent diabetes mellitus (NIDDM) is the best predictor of development of the disease and probably plays an important part in its pathogenesis. We studied the mechanism and degree to which exercise training improves insulin sensitivity in these subjects. METHODS Ten adult children of parents with NIDDM and eight normal subjects were studied before starting an aerobic exercise-training program, after one session of exercise, and after six weeks of exercise. Insulin sensitivity was measured by the hyperglycemic-hyperinsulinemic clamp technique combined with indirect calorimetry, and the rate of glycogen synthesis in muscle and the intramuscular glucose-6-phosphate concentration were measured by carbon-13 and phosphorus-31 nuclear magnetic resonance spectroscopy, respectively. RESULTS During the base-line study, the mean (+/-SE) rate of muscle glycogen synthesis was 63 +/- 9 percent lower in the offspring of diabetic parents than in the normal subjects (P < 0.001). The mean value increased 69 +/- 10 percent (P = 0.04) and 62 +/- 11 percent (P = 0.04) after the first exercise session and 102 +/- 11 percent (P = 0.02) and 97 +/- 9 percent (P = 0.008) after six weeks of exercise training in the offspring and the normal subjects, respectively. The increment in glucose-6-phosphate during hyperglycemic-hyperinsulinemic clamping was lower in the offspring than in the normal subjects (0.039 +/- 0.013 vs. 0.089 +/- 0.009 mmol per liter, P = 0.005), reflecting reduced glucose transport-phosphorylation, but this increment was normal in the offspring after the first exercise session and after exercise training. Basal and stimulated insulin secretion was higher in the offspring than the normal subjects and was not altered by the exercise training program. CONCLUSIONS Exercise increases insulin sensitivity in both normal subjects and the insulin-resistant offspring of diabetic parents because of a twofold increase in insulin-stimulated glycogen synthesis in muscle, due to an increase in insulin-stimulated glucose transport-phosphorylation.

Comparison of MRI with EMG to study muscle activity associated with dynamic plantar flexion

This study compared magnetic resonance imaging (MRI) and surface electromyography (EMG) to evaluate the effect of knee angle upon plantar flexion activity in the triceps surae muscles [medial & lateral gastrocnemius (MG, LG) and the soleus (SOL)]. Two weight & height matched groups performed identical protocols, twelve (6M, 6F) in the MRI group, twelve (8M, 4F) in the EMG group. Subjects plantar flexed dynamically for 2 min at 25% of 1-repetition maximum voluntary contraction (1-RM). Exercise was performed with the knee extended (0 degrees flexion), flexed (90 degrees ), and partially flexed (45 degrees ). In the MRI group spin-echo images were acquired before and immediately following each exercise session. T(2) times, calculated at rest and after exercise by fitting the echoes to a monoexponential decay pattern with a least-squares algorithm, were compared with EMG data. In the EMG group a bipolar electrode was used to collect samples were from the MG, LG, SOL, and anterior tibialis (TA) during exercise at each knee angle, MRI also examined the peroneus (PER). At 0 degrees flexion MRI demonstrated a significant post-exercise T(2) increase in the MG (p < or = 0.001), LG (p < or = 0.001), and PER (p < or = 0.01), with no T(2) change in the SOL or TA. At 90 degrees flexion there was a significant T(2) increase in the SOL (p < or = 0.001) with no significant T(2) change in the MG, LG, PER, or TA. At 45 degrees T(2) increased significantly in the SOL (p < or = 0.001) and LG (p < or = 0.05), but not the MG, PER, or TA. EMG produced similar results with the exception that there was significant activity in the TA during the relaxation cycle of the 90 degrees protocol. We conclude that: 1) Soleus activity is measurable by MRI; and 2) MRI and EMG produce similar results from different physiological sources, and are therefore complementary tools for evaluating muscle activity.

PPARα L162V underlies variation in serum triglycerides and subcutaneous fat volume in young males

BackgroundOf the five sub-phenotypes defining metabolic syndrome, all are known to have strong genetic components (typically 50–80% of population variation). Studies defining genetic predispositions have typically focused on older populations with metabolic syndrome and/or type 2 diabetes. We hypothesized that the study of younger populations would mitigate many confounding variables, and allow us to better define genetic predisposition loci for metabolic syndrome.MethodsWe studied 610 young adult volunteers (average age 24 yrs) for metabolic syndrome markers, and volumetric MRI of upper arm muscle, bone, and fat pre- and post-unilateral resistance training.ResultsWe found the PPARα L162V polymorphism to be a strong determinant of serum triglyceride levels in young White males, where carriers of the V allele showed 78% increase in triglycerides relative to L homozygotes (LL = 116 ± 11 mg/dL, LV = 208 ± 30 mg/dL; p = 0.004). Men with the V allele showed lower HDL (LL = 42 ± 1 mg/dL, LV = 34 ± 2 mg/dL; p = 0.001), but women did not. Subcutaneous fat volume was higher in males carrying the V allele, however, exercise training increased fat volume of the untrained arm in V carriers, while LL genotypes significantly decreased in fat volume (LL = -1,707 ± 21 mm3, LV = 17,617 ± 58 mm3 ; p = 0.002), indicating a systemic effect of the V allele on adiposity after unilateral training. Our study suggests that the primary effect of PPARα L162V is on serum triglycerides, with downstream effects on adiposity and response to training.ConclusionOur results on association of PPARα and triglycerides in males showed a much larger effect of the V allele than previously reported in older and less healthy populations. Specifically, we showed the V allele to increase triglycerides by 78% (p = 0.004), and this single polymorphism accounted for 3.8% of all variation in serum triglycerides in males (p = 0.0037).

Turnover of human muscle glycogen with low-intensity exercise

To determine whether glycogen turnover occurs during prolonged low-intensity exercise, five subjects performed plantar flexion of the right leg at 15% MVC for 5 h. At rest and during the initial 2.5 h of exercise gastrocnemius glycogen was monitored in both legs with natural abundance 13C NMR. At 2.5 h exercise, a step-up infusion of 99% enriched 1-13C glucose was begun and maintained over the next 1.5 h of continued exercise to monitor 1-13C glucose incorporation into the exercising muscles glycogen pool. Exercise was continued for an hour following the infusion, and NMR scans were performed throughout the session. During the first 2 h of exercise, glycogen 1-13C signal amplitudes dropped approximately 30% and remained there at 2.5 h, indicating that glycogen concentration had leveled. Following infusion, glycogen signal amplitudes rose to 123% of resting values, remaining there during an hour of subsequent exercise. There was no change of glycogen 1-13C signal in the nonexercising leg. Venous glucose levels remained stable until the infusion was begun and then rose < 7% (5.57-5.96 mmol.l-1) during the infusion. Venous insulin and C-peptide levels did not change during the infusion. We conclude that the human gastrocnemius can degrade and synthesize glycogen simultaneously during prolonged low-intensity exercise.

Myostatin and Follistatin Polymorphisms Interact with Muscle Phenotypes and Ethnicity

PURPOSE We examined associations among myostatin (MSTN) 2379 A > G and 163 G > A and follistatin (FST) -5003 A > T and -833 G > T single nucleotide polymorphisms (SNP) on the muscle size and the strength response to resistance training (RT). METHODS Subjects (n = 645, age = 24.1 +/- 0.2 yr, body mass index [BMI] = 24.2 +/- 0.2 kg x m(-2)) self-disclosed themselves as Caucasian (78.9%), African American (3.6%), Asian (8.4%), Hispanic (5.0%), or Other (4.2%). They were genotyped for MSTN 2379 A > G (n = 645), MSTN 163 G > A (n = 639), FST -5003 A > T (n = 580), and FST -833 G > T (n = 603). We assessed dynamic (one repetition maximum [1RM]) and isometric (maximum voluntary contraction [MVC]) muscle strength and size (cross-sectional area [CSA]) of the elbow flexors before and after 12 wk of unilateral upper-arm RT. Repeated-measures ANCOVA tested associations among genetic variants and muscle phenotypes with age and BMI as covariates. RESULTS Baseline MVC was greater among African Americans who were carriers of the MSTN G(2379) allele (AG/GG, n = 15) than the A2379A homozygotes (n = 8; 64.2 +/- 6.8 vs 49.8 +/- 8.7 kg). African Americans who were carriers of the FST T(-5003) allele (n = 12) had greater baseline 1RM (11.9 +/- 0.7 vs 8.8 +/- 0.5 kg) and CSA (24.4 +/- 1.3 vs 19.1 +/- 1.2 cm(2)) than African Americans with the A-5003A genotype (n = 14; P < 0.05). No MSTN or FST genotype and muscle phenotype associations were found among the other ethnic groups (P >or= 0.05). CONCLUSION MSTN 2379 A > G and FST -5003 A > T were associated with baseline muscle strength and size among African Americans only. These ethnic-specific associations are hypothesis generating and should be confirmed in a larger sample of African Americans.

Resistin Polymorphisms Are Associated with Muscle, Bone, and Fat Phenotypes in White Men and Women

Objective: The biological function of resistin (RST) is unknown, although it may have roles in obesity, diabetes, and insulin resistance. The objective of this study was to examine the effects of single nucleotide polymorphisms (SNPs) in the human RST gene on muscle, bone, and adipose tissue phenotypes and in response to resistance training (RT).

The Muscle Strength and Size Response to Upper Arm, Unilateral Resistance Training Among Adults Who Are Overweight and Obese

Overweight and obesity result in musculoskeletal impairments that limit exercise capacity. We examined if the muscle strength and size response to resistance training (RT) differed among 687 young (mean ± SEM, 24.2 ± 0.2 years) overweight and obese (OW) compared to normal weight (NW) adults as denoted by the body mass index (BMI). Subjects were 449 NW (22.0 ± 0.1 kg·m-2, 23.4 ± 0.3 years) and 238 OW (29.2 ± 0.2 kg·m-2, 25.6 ± 0.4 years) men (n = 285) and women (n = 402) who underwent 12 weeks (2 d·wk-1) of RT of the non- dominant arm. Maximum voluntary contraction (MVC) and 1 repetition maximum (1RM) assessed peak elbow flexor strength. Magnetic resonance imaging measured the biceps muscle cross sectional area (CSA). Multiple dependent variable analysis of covariance tested if muscle strength and size differed among BMI groups pre-, post-, and pre-to-post–RT. Overweight and obese had greater MVC, 1RM, and CSA than NW pre- and post- RT (p < 0.001). Maximum voluntary contraction and 1RM gains were not different between BMI groups pre- to post-RT (p ≥ 0.05). When adjusted for baseline values, NW had greater relative MVC (21.2 ± 1.0 vs. 17.4 ± 1.4%) and 1RM (54.3 ± 1.5 vs. 49.0 ± 2.0%) increases than OW (p > 0.05). Normal weight also had greater allometric MVC (0.48 ± 0.02 kg·kg-0.67 vs. 0.40 ± 0.03 kg·kg-0.67) and 1RM (0.25 ± 0.00 vs. 0.22 ± 0.01 kg·kg-0.67) gains than OW (p > 0.05). CSA gains were greater among OW than NW (3.6 ± 0.2 vs. 3.2 ± 0.1 cm2)(p > 0.001); however, relative CSA increases were not different between BMI groups (19.4 ± 0.5 vs. 18.4 ± 0.7%) (p ≥ 0.05). Despite similar relative muscle size increases, relative and allometic strength gains were less among OW than NW. These findings indicate the short-term relative and allometric muscle strength response to RT may be attenuated among adults who are overweight and obese.

Changes in magnetic resonance transverse relaxation times of two muscles following standardized exercise

Magnetic resonance (MR) imaging studies of exercising leg muscles were performed to compare the changes in MR transverse relaxation times (T2) that result from exercise of the anterior tibialis (AT) and extensor digitorum/hallicus longus (E) in the anterior compartment of the lower leg with those T2 changes in the medial and lateral gastrocnemius (G) in the posterior compartment. Spin-echo MR images were obtained at 1.5 Tesla before and during the first 14 min of recovery from dynamic exercise. In order to normalize the exercise, workloads for each subject were set at 25% of the measured maximum voluntary contraction (MVC) of the anterior and posterior compartments. In separate exercise sessions, a nonmagnetic, pneumatic exercise apparatus was employed for either dorsiflexion or plantarflexion against a fixed constant resistance for two different exercise durations (1 min 45 s or 5 min). Transaxial MR images (TR = 1000 ms, TE = 30, 60, 90, 120 ms, 128 x 256 matrix, 1.5 cm slice) were used to calculate T2 values. Although subjects performed approximately 7-fold more work (P < or = 0.001, dorsiflexion vs plantarflexion) during plantarflexion than during dorsiflexion at both exercise durations, the exercise induced T2, while being greater than those at rest (P < or = 0.001), were not significantly different in the different compartments. We conclude that, when exercised at the same workload (25% of MVC), these two muscles produce T2 changes that are not significantly different from each other.

NMR of glycogen in exercise.

Natural-abundance 13C NMR spectroscopy is a non-invasive technique that enables in vivo assessments of muscle and/or liver glycogen concentrations. Over the last several years, 13C NMR has been developed and used to obtain information about human glycogen metabolism with diet and exercise. Since NMR is non-invasive, more data points are available over a specified time course, dramatically improving the time resolution. This improved time resolution has enabled the documentation of subtleties of muscle glycogen re-synthesis following severe glycogen depletion that were not previously observed. Muscle and liver glycogen concentrations have been tracked in several different human populations under conditions that include: (1) muscle glycogen recovery from intense localized exercise with normal insulin and with insulin suppressed; (2) muscle glycogen recovery in an insulin-resistant population; (3) muscle glycogen depletion during prolonged low-intensity exercise; (4) effect of a mixed meal on postprandial muscle and liver glycogen synthesis. The present review focuses on basic 13C NMR and gives results from selected studies.