Growth hormone increases mRNA levels of PPARdelta and Foxo1 in skeletal muscle of growth hormone deficient lit/lit mice

Adult individuals with congenital, untreated, severe isolated growth hormone deficiency have satisfactory muscular function

PURPOSE

While growth hormone (GH) and the insulin-like growth factor type I (IGF-I) are known to exert synergistic actions on muscle anabolism, the consequences of prolonged GH deficiency (GHD) on muscle function have not been well defined. We have previously described a large cohort of subjects with isolated GHD (IGHD) caused by a mutation in the GH-releasing hormone receptor gene, with low serum levels of GH and IGF-I. The aim of this study was to assess muscular function in these IGHD subjects.

METHODS

A total of 31 GH-naïve IGHD (16 males) and 40 control (20 males) subjects, matched by age and degree of daily physical activity, were enrolled. Fat free mass was measured by bioelectrical impedance; muscle strength by dynamometry of handgrip, trunk extension, and knee extension; myoelectric activity and muscle fatigue by fractal dimension; conduction velocity in vastus medialis, rectus femoris, and vastus lateralis muscles by surface electromyography.

RESULTS

The IGHD group showed higher knee extension strength both when corrected for weight and fat free mass, and higher handgrip and trunk extension strength corrected by fat free mass. They also exhibit higher conduction velocity of the muscles vastus medialis, rectus femoris, and vastus lateralis, but lower free fat mass and myoelectric activity of the vastus medialis, rectus femoris and vastus lateralis. There were no differences between the two groups in fractal dimension in all studied muscles.

CONCLUSION

Individuals with untreated IGHD have better muscle strength parameters adjusted for weight and fat free mass than controls. They also exhibit greater peripheral resistance to fatigue, demonstrating satisfactory muscle function.

Growth hormone potentiates thyroid hormone effects on post-exercise phosphocreatine recovery in skeletal muscle

OBJECTIVE

The aim of the study was to determine the respective impact of thyroxine and growth hormone on in vivo skeletal mitochondrial function assessed via post exercise phosphocreatine recovery.

DESIGN

The hind leg muscles of 32 hypophysectomized rats were investigated using (31)P nuclear magnetic resonance spectroscopy at rest and during the recovery period following a non tetanic stimulation of the sciatic nerve. Each rat was supplemented with hydrocortisone and was randomly assigned to one of the 4 groups: the group Hx was maintained in hypopituitarism., the group HxT was treated with 1 μg/100g/day of thyroxine (T4), the group HxG with 0.2 IU/kg/day of recombinant human GH (rGH) and the group HxGT by both thyroxine and rGH. Inorganic phosphate (Pi), phosphocreatine (PCr) and ATP were directly measured on the spectra, permitting the calculation of the phosphorylation potential (PP).

RESULTS

At rest, the rats treated with rGH or T4 exhibited higher PCr levels than rats Hx. The recovery rates of PCr and PP were higher in rats treated with T4 than in T4-deprivated rats, suggesting improved mitochondrial function. The rats treated by both T4 and rGH showed higher PCr and PP recovery than those maintained in hypopituitarism or treated with T4 or rGH alone.

CONCLUSIONS

The study demonstrates that in contrast to T4, GH given alone in hypophysectomized rats does not improve in vivo mitochondrial oxidative metabolism. Growth hormone potentiates T4 effects on oxidative metabolism.

The intricate role of growth hormone in metabolism

Growth hormone (GH), a master regulator of somatic growth, also regulates carbohydrate and lipid metabolism via complex interactions with insulin and insulin-like growth factor-1 (IGF-1). Data from human and rodent studies reveal the importance of GH in insulin synthesis and secretion, lipid metabolism and body fat remodeling. In this review, we will summarize the tissue-specific metabolic effects of GH, with emphasis on recent targets identified to mediate these effects. Furthermore, we will discuss what role GH plays in obesity and present possible mechanisms by which this may occur.

Biological effects of growth hormone on carbohydrate and lipid metabolism

This review will summarize the metabolic effects of growth hormone (GH) on the adipose tissue, liver, and skeletal muscle with focus on lipid and carbohydrate metabolism. The metabolic effects of GH predominantly involve the stimulation of lipolysis in the adipose tissue resulting in an increased flux of free fatty acids (FFAs) into the circulation. In the muscle and liver, GH stimulates triglyceride (TG) uptake, by enhancing lipoprotein lipase (LPL) expression, and its subsequent storage. The effects of GH on carbohydrate metabolism are more complicated and may be mediated indirectly via the antagonism of insulin action. Furthermore, GH has a net anabolic effect on protein metabolism although the molecular mechanisms of its actions are not completely understood. The major questions that still remain to be answered are (i) What are the molecular mechanisms by which GH regulates substrate metabolism? (ii) Does GH affect substrate metabolism directly or indirectly via IGF-1 or antagonism of insulin action?