Ageing and local growth factors in muscle.
Harridge SD.
Department of Physiology, Royal Free & University College Medical School, Rowland Hill
Street, London, NW3 2PF, UK.
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.
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