Stephen D. R. Harridge

Expression of IGF-I splice variants in young and old human skeletal muscle after high resistance exercise

The mRNA expression of two splice variants of the insulin‐like growth factor‐I (IGF‐I) gene, IGF‐IEa and mechano growth factor (MGF), were studied in human skeletal muscle. Subjects (eight young, aged 25–36 years, and seven elderly, aged 70–82 years) completed 10 sets of six repetitions of single legged knee extensor exercise at 80 % of their one repetition maximum. Muscle biopsy samples were obtained from the quadriceps muscle of both the control and exercised legs 2.5 h after completion of the exercise bout. Expression levels of the IGF‐I mRNA transcripts were determined using real‐time quantitative RT‐PCR with specific primers. The resting levels of MGF were significantly (≈100‐fold) lower than those of the IGF‐IEa isoform. No difference was observed between the resting levels of the two isoforms between the two subject groups. High resistance exercise resulted in a significant increase in MGF mRNA in the young, but not in the elderly subjects. No changes in IGF‐IEa mRNA levels were observed as a result of exercise in either group. The mRNA levels of the transcription factor MyoD were greater at rest in the older subjects (P < 0.05), but there was no significant effect of the exercise bout. Electrophoretic separation of myosin heavy chain (MHC) isoforms showed the older subjects to have a lower (P < 0.05) percentage of MHC‐II isoforms than the young subjects. However, no association was observed between the composition of the muscle and changes in the IGF‐I isoforms with exercise. The data from this study show an attenuated MGF response to high resistance exercise in the older subjects, indicative of age‐related desensitivity to mechanical loading. The data in young subjects indicate that the MGF and IGF‐IEa isoforms are differentially regulated in human skeletal muscle.

Knee extensor strength, activation, and size in very elderly people following strength training.

Muscle strength, activation, and size were studied in 11 very elderly subjects (8 women and 3 men; age range, 85–97 years) who completed 12 weeks of strength training of the knee extensor muscles. Training increased the maximum amount of weight that could be lifted once (134%; P < 0.05) and maximum voluntary isometric strength, measured as both force recorded at the ankle with the knee flexed 90° (17%, ns) and as torque with the knee flexed 60° (37%; P < 0.05). Anatomical lean quadriceps cross‐sectional area (LCSA) measured at midthigh using magnetic resonance imaging increased from 27.5 ± 9.6 cm2 to 30.2 ± 10.0 cm2 (9.8%; P < 0.05) after training. Both before and after training, isometric strength was closely related to LCSA, but training resulted in no significant change in muscle force per unit area of quadriceps muscle. Using the twitch interpolation technique, muscle activation during a maximal voluntary isometric contraction was shown to be incomplete in all subjects before training (ranging from 69% to 93%) and was not significantly increased after training. An increase in skeletal muscle mass may have important functional and metabolic benefits for very elderly people.

The effect of recombinant human growth hormone and resistance training on IGF‐I mRNA expression in the muscles of elderly men

The expression of two isoforms of insulin‐like growth factor‐I (IGF‐I): mechano growth factor (MGF) and IGF‐IEa were studied in muscle in response to growth hormone (GH) administration with and without resistance training in healthy elderly men. A third isoform, IGF‐IEb was also investigated in response to resistance training only. The subjects (age 74 ± 1 years, mean ±S.E.M) were assigned to either resistance training with placebo, resistance training combined with GH administration or GH administration alone. Real‐time quantitative RT‐PCR was used to determine mRNA levels in biopsies from the vastus lateralis muscle at baseline, after 5 and 12 weeks in the three groups. GH administration did not change MGF mRNA at 5 weeks, but significantly increased IGF‐IEa mRNA (237%). After 12 weeks, MGF mRNA was significantly increased (80%) compared to baseline. Five weeks of resistance training significantly increased the mRNA expression of MGF (163%), IGF‐IEa (68%) and IGF‐IEb (75%). No further changes were observed after 12 weeks. However, after 5 weeks of training combined with GH treatment, MGF mRNA increased significantly (456%) and IGF‐IEa mRNA by (167%). No further significant changes were noted at 12 weeks. The data suggest that when mechanical loading in the form of resistance training is combined with GH, MGF mRNA levels are enhanced. This may reflect an overall up‐regulation of transcription of the IGF‐I gene prior to splicing.

Plasticity of human skeletal muscle: gene expression to in vivo function

Human skeletal muscle is a highly heterogeneous tissue, able to adapt to the different challenges that may be placed upon it. When overloaded, a muscle adapts by increasing its size and strength through satellite‐cell‐mediated mechanisms, whereby protein synthesis is increased and new nuclei are added to maintain the myonuclear domain. This process is regulated by an array of mechanical, hormonal and nutritional signals. Growth factors, such as insulin‐like growth factor I (IGF‐I) and testosterone, are potent anabolic agents, whilst myostatin acts as a negative regulator of muscle mass. Insulin‐like growth factor I is unique in being able to stimulate both the proliferation and the differentiation of satellite cells and works as part of an important local repair and adaptive mechanism. Speed of movement, as characterized by maximal velocity of shortening (Vmax), is regulated primarily by the isoform of myosin heavy chain (MHC) contained within a muscle fibre. Human fibres can express three MHCs: MHC‐I, ‐IIa and ‐IIx, in order of increasing Vmax and maximal power output. Training studies suggest that there is a subtle interplay between the MHC‐IIa and ‐IIx isoforms, with the latter being downregulated by activity and upregulated by inactivity. However, switching between the two main isoforms appears to require significant challenges to a muscle. Upregulation of fast gene programs is caused by prolonged disuse, whilst upregulation of slow gene programs appears to require significant and prolonged activity. The potential mechanisms by which alterations in muscle composition are mediated are discussed. The implications in terms of contractile function of altering muscle phenotype are discussed from the single fibre to the whole muscle level.

Muscle function in elite master weightlifters.

PURPOSE To determine whether explosive power and isometric strength of the lower-limb muscles in elite master Olympic weightlifters declines at a similar rate to nontrained healthy controls with increasing age. METHODS 54 elite level masters weightlifters (aged 40-87), who were competitors at the World Masters Weightlifting Championships (1999), were compared with a similar number of aged-matched, healthy untrained individuals. Isometric knee extensor strength and lower-limb explosive power were tested. Extent of antagonist co-contraction during isometric knee extension was determined by EMG and power loading characteristics by using a variable inertial system. Muscle volume was estimated using anthropometry. RESULTS On average, the weightlifters were able to generate 32% more peak power (P < 0.05) in the lower limbs and 32% more isometric knee extensor force (P < 0.05) than the control subjects. No significant differences in lower-leg volume were observed between the two groups. Peak power declined at a similar rate with increasing age in the weightlifters and controls (1.2 and 1.3% of a 45-yr-olds value per year), as did strength, but at a lower rate (0.6 and 0.5% per year). The inertial load at which the weightlifters achieved their maximal peak power output was greater (P < 0.05) than the controls. The torque generated at this optimal inertia was also greater in the weightlifters (P < 0.05), whereas the time taken for the weightlifters to reach their maximal peak power was on average 13% shorter (P < 0.05). No differences in antagonist co-contraction during isometric knee extension were observed between the two groups. CONCLUSIONS Muscle power and isometric strength decline at a similar rate with increasing age in elite master weightlifters and healthy controls. In spite of inertial load optimization, muscle power declined in both groups at approximately twice the rate of isometric strength. Although similar rates of decline were observed, the absolute differences between the weightlifters and controls were such that an 85-yr-old weightlifter was as powerful as a 65-yr-old control subject. This would therefore represent an apparent age advantage of approximately 20 yr for the weightlifters.

Skeletal muscle dysfunction in critical care: wasting, weakness, and rehabilitation strategies.

Understanding the trajectory of skeletal muscle loss, evaluating its relationship to the subsequent functional impairment, and understanding the underlying mechanisms of skeletal muscle wasting will provide goals for novel treatment strategies in the intensive care setting. A focused approach on the effect of critical illness on muscle morphology, muscle protein turnover, and the associated muscle-signaling pathways during the early and recovery stages of critical illness is required. This could potentially lead to targeted pharmacologic and nonpharmacologic strategies to treat, or even prevent, peripheral muscle wasting and weakness.

Sarcopenia and hypertrophy: A role for insulin-like growth factor-1 in aged muscle?

HAMEED, M., S.D.R. HARRIDGE, and G. GOLDSPINK. Sarcopenia and hypertrophy: A role for insulin-like growth factor-1 in aged muscle? Exerc. Sport Sci. Rev., Vol. 30, No. 1, pp 15–19, 2002. Sarcopenia is the loss of muscle mass associated with the aging process. Although systemic or circulating growth hormone and insulin-like growth factor-1 levels fall as we age, this is likely to be of lesser importance in regard to muscle mass than the role of locally produced insulin-like growth factor-1s generated in the muscle in response to exercise.

Ageing and local growth factors in muscle

Muscle responds to mechanical overload by increasing its size. In contrast, as a muscle gets older it atrophies. The mechanisms regulating these differing responses are not fully understood. Animal studies have shown that older muscles are less well able to repair following contraction‐induced injury than young muscles. It is becoming clear that local growth factors produced within the muscle may play important roles in both repair, adaptation and ageing. The growth hormone/insulin like growth factor 1 (GH/IGF‐I) axis is important during growth and development, but circulating levels of these hormones decline in later life. However, many tissues including muscle, produce IGF‐I for autocrine and paracrine actions. Genetic manipulation of IGF‐I in muscle has shown that it has considerable anabolic affects on muscle both in young and old animals. Insulin like growth factor 1 exists in multiple isoforms and one isoform, which differs from the systemic or liver type (IGF‐IEa), appears to be particularly sensitive to mechanical signals and to muscle damage. This isoform (IGF‐IEc) has been termed mechano growth factor (MGF). The anabolic actions of IGF‐I and MGF are through stimulating protein synthesis and by playing a role in the activation, proliferation and differentiation of satellite cells. These effects are discussed in relation to human studies of muscle adaptation to strength training in older people who seem to retain an ability to increase muscle mass and strength through this type of exercise.

High‐throughput ultra‐high‐performance liquid chromatography/tandem mass spectrometry quantitation of insulin‐like growth factor‐I and leucine‐rich α‐2‐glycoprotein in serum as biomarkers of recombinant human growth hormone administration

Insulin-like growth factor-I (IGF-I) is a known biomarker of recombinant human growth hormone (rhGH) abuse, and is also used clinically to confirm acromegaly. The protein leucine-rich alpha-2-glycoprotein (LRG) was recently identified as a putative biomarker of rhGH administration. The combination of an ACN depletion method and a 5-min ultra-high-performance liquid chromatography/tandem mass spectrometry (uHPLC/MS/MS)-based selected reaction monitoring (SRM) assay detected both IGF-I and LRG at endogenous concentrations. Four eight-point standard addition curves of IGF-I (16-2000 ng/mL) demonstrated good linearity (r(2) = 0.9991 and coefficients of variance (CVs) <13%). Serum samples from two rhGH administrations were extracted and their uHPLC/MS/MS-derived IGF-I concentrations correlated well against immunochemistry-derived values. Combining IGF-I and LRG data improved the separation of treated and placebo states compared with IGF-I alone, further strengthening the hypothesis that LRG is a biomarker of rhGH administration. Artificial neural networks (ANNs) analysis of the LRG and IGF-I data demonstrated an improved model over that developed using IGF-I alone, with a predictive accuracy of 97%, specificity of 96% and sensitivity of 100%. Receiver operator characteristic (ROC) analysis gave an AUC value of 0.98. This study demonstrates the first large scale and high throughput uHPLC/MS/MS-based quantitation of a medium abundance protein (IGF-I) in human serum. Furthermore, the data we have presented for the quantitative analysis of IGF-I suggest that, in this case, monitoring a single SRM transition to a trypsin peptide surrogate is a valid approach to protein quantitation by LC/MS/MS.

Structure to function: muscle failure in critically ill patients

Impaired physical function and reduced physical activity are common findings in intensive care unit (ICU) survivors. More importantly, reduced muscle strength during critical illness is an independent predictor of survival. Skeletal muscle wasting as a direct consequence of critical illness has been suggested as the cause. However, data on the physiological processes regulating muscle mass, and function, in these critically ill patients are limited as this is not only a technically challenging research area, but also the heterogeneity of the patient group adds complexity to the interpretation of results. Despite this, clinical and research interest in this area is growing. This article highlights the issues involved in measurement of muscle function and mass in critically ill patients and the physiological complexities involved in studying these patients. Although the data are limited, this article reviews the animal and healthy human data providing a rational approach to the potential pathophysiological mechanisms involved in muscle mass regulation in critically ill patients, including the established muscle wasting ‘risk factors’ such as ageing, immobility and systemic inflammation, all of which are common findings in the general critical care population.