Lower Fasting Muscle Mitochondrial Activity Relates to Hepatic Steatosis in Humans

OBJECTIVE Muscle insulin resistance has been implicated in the development of steatosis and dyslipidemia by changing the partitioning of postprandial substrate fluxes. Also, insulin resistance may be due to reduced mitochondrial function. We examined the association between mitochondrial activity, insulin sensitivity, and steatosis in a larger human population. RESEARCH DESIGN AND METHODS We analyzed muscle mitochondrial activity from ATP synthase flux (fATP) and ectopic lipids by multinuclei magnetic resonance spectroscopy from 113 volunteers with and without diabetes. Insulin sensitivity was assessed from M values using euglycemic-hyperinsulinemic clamps and/or from oral glucose insulin sensitivity (OGIS) using oral glucose tolerance tests. RESULTS Muscle fATP correlated negatively with hepatic lipid content and HbA1c. After model adjustment for study effects and other confounders, fATP showed a strong negative correlation with hepatic lipid content and a positive correlation with insulin sensitivity and fasting C-peptide. The negative correlation of muscle fATP with age, HbA1c, and plasma free fatty acids was weakened after adjustment. Body mass, muscle lipid contents, plasma lipoproteins, and triglycerides did not associate with fATP. CONCLUSIONS The association of impaired muscle mitochondrial activity with hepatic steatosis supports the concept of a close link between altered muscle and liver energy metabolism as early abnormalities promoting insulin resistance

A Single Nucleotide Polymorphism Associates With the Response of Muscle ATP Synthesis to Long-Term Exercise Training in Relatives of Type 2 Diabetic Humans

OBJECTIVE-Myocellular ATP synthesis (fATP) associates with insulin sensitivity in first-degree relatives of subjects with type 2 diabetes. Short-term endurance training can modify their fATP and insulin sensitivity. This study examines the effects of moderate long-term exercise using endurance or resistance training in this cohort. RESEARCH DESIGN AND METHODS-A randomized, parallel-group trial tested 16 glucose-tolerant nonobese relatives (8 subjects in the endurance training group and 8 subjects in the resistance training group) before and after 26 weeks of endurance or resistance training. Exercise performance was assessed from power output and oxygen uptake (VO2) during incremental tests and from maximal torque of knee flexors (MaxT(flex)) and extensors (MaxT(ext)) using isokinetic dynamometry. fATP and ectopic lipids were measured with H-1/P-31 magnetic resonance spectroscopy. RESULTS Endurance training increased power output and VO2 by 44 and 30%, respectively (both P < 0.001), whereas resistance training increased MaxT(ext) and MaxT(flex) by 23 and 40%, respectively (both P < 0.001). Across all groups, insulin sensitivity (382 +/- 90 vs. 389 +/- 40 mL.min.m(-2)) and ectopic lipid contents were comparable after exercise training. However, 8 of 16 relatives had 26% greater fATP, increasing from 9.5 +/- 2.3 to 11.9 +/- 2.4 mu mol.mL(-1).m(-1) (P < 0.05). Six of eight responders were carriers of the G/G single nucleotide polymorphism rs540467 of the NDUFB6 gene (P = 0.019), which encodes a subunit of mitochondrial complex I. CONCLUSIONS-Moderate exercise training for 6 months does not necessarily improve insulin sensitivity but may increase ATP synthase flux. Genetic predisposition can modify the individual response of the ATP synthase flux independently of insulin sensitivity

Reduced basal ATP synthetic flux of skeletal muscle in patients with previous acromegaly.

BACKGROUND: Impaired mitochondrial function and ectopic lipid deposition in skeletal muscle and liver have been linked to decreased insulin sensitivity. As growth hormone (GH) excess can reduce insulin sensitivity, we examined the impact of previous acromegaly (AM) on glucose metabolism, lipid storage and muscular ATP turnover. PARTICIPANTS AND METHODS: Seven AM (4f/3 m, age: 46+/-4 years, BMI: 28+/-1 kg/m(2)) and healthy volunteers (CON: 3f/4 m, 43+/-4 years, 26+/-2 kg/m(2)) matched for age and body mass underwent oral glucose testing for assessment of insulin sensitivity (OGIS) and ss-cell function (adaptation index, ADAP). Whole body oxidative capacity was measured with indirect calorimetry and spiroergometry. Unidirectional ATP synthetic flux (fATP) was assessed from (31)P magnetic resonance spectroscopy (MRS) of calf muscle. Lipid contents of tibialis anterior (IMCLt) and soleus muscles (IMCLs) and liver (HCL) were measured with (1)H MRS. RESULTS: Despite comparable GH, insulin-like growth factor-1 (IGF-I) and insulin sensitivity, AM had approximately 85% lower ADAP (p<0.01) and approximately 21% reduced VO(2)max (p<0.05). fATP was similarly approximately 25% lower in AM (p<0.05) and related positively to ADAP (r = 0.744, p<0.01), but negatively to BMI (r = -0.582, p<0.05). AM had approximately 3 fold higher HCL (p<0.05) while IMCLt and IMCLs did not differ between the groups. CONCLUSIONS: Humans with a history of acromegaly exhibit reduced insulin secretion, muscular ATP synthesis and oxidative capacity but elevated liver fat content. This suggests that alterations in ss-cell function and myocellular ATP production may persist despite normalization of GH secretion after successful treatment of acromegaly

Relationship between ectopic lipid content of tibialis muscle (IMCLt) and plasma concentrations of insulin-like growth factor-1 (IGF-I) in 7 subjects with previous acromegaly (AM, black squares) and 7 age- and body mass index-matched controls (CON, grey squares) (r = 0.726, p<0.05).

<p>Relationship between ectopic lipid content of tibialis muscle (IMCLt) and plasma concentrations of insulin-like growth factor-1 (IGF-I) in 7 subjects with previous acromegaly (AM, black squares) and 7 age- and body mass index-matched controls (CON, grey squares) (r = 0.726, p<0.05).</p

Expired gas analysis during resting (indirect calorimetry) and exercise (spiroergometry).

<p>Fasting substrate oxidation and parameters of physical fitness (means±SEM) in subjects with prior acromegaly (AM) and controls (CON).</p>§<p>P<0.05 vs. AM.</p

Ectopic lipid deposition: (A) in M.soleus (IMCLs), (B) M.tibialis ant. (IMCLt) and (C) in the liver (HCL) (p<0.05).

<p>Ectopic lipid deposition: (A) in M.soleus (IMCLs), (B) M.tibialis ant. (IMCLt) and (C) in the liver (HCL) (p<0.05).</p

Anthropometric and laboratory data (means±SEM) in subjects with prior acromegaly (AM) and controls (CON).

*<p>P<0.01.</p>§<p>P<0.05 vs. AM.</p