Two Years of Growth Hormone (GH)
Gudmundur Johannsson, Gunnar Grimby, Katharina Stibrant Sunnerhagen and Bent-Ake Bengtsson
THE SYNDROME of GH deficiency in adults is associated with reduced muscle mass and muscle strength. In addition, the discontinuation of GH treatment in young adults with GH deficiency reduced maximal voluntary isometric muscle strength, muscle size, and muscle fiber area, thereby indicating the importance of GH in adults for the maintenance of muscle mass and strength.
Furthermore, the contractile properties of the muscle in GH-deficient adults could be disrupted in a manner suggesting a higher proportion of fast twitch, type 2 muscle fibers.
The administration of GH to GH-deficient adults for 6-18 months has been shown to increase the lean body mass, muscle volume, and maximal voluntary isometric muscle strength.
In hypophysectomized rats, the administration of GH increased and restored the proportion of slow twitch, fatigue-resistant, type 1 muscle fibers, but this effect has not been demonstrated in two small studies of GH-deficient adults.
The contractile properties of the muscle in GH-deficient adults may, however, change in response to GH in a manner that suggests an increase in the proportion of type 1 muscle fibers.
The anabolic action of GH is not only restricted to GH deficiency, as GH treatment increased fat-free mass in athletes, healthy young untrained men, and elderly men. Combined with resistance exercise training in young men, GH enhanced the increase in fat-free mass, total body water, and positive whole body protein balance compared with the effect of exercise alone.
However, the fractional quadriceps muscle protein synthesis and isometric and concentric knee flexor and knee extensor muscle strength were not enhanced by combining GH treatment and training.
This indicates that the additional lean tissue in response to GH was not skeletal muscle and that factors other than anabolic ones, stimulated by exercise, may be of more importance to muscle strength in GH-sufficient adults.
Muscle endurance is rarely studied in GH deficiency. Cuneo et al. demonstrated that the shoulder abduction fatigue index, calculated from 10 maximal voluntary shoulder abductions performed within 1 min, was lower than expected in GH-deficient adults and was not affected by 6 months of GH treatment.
In contrast, general endurance measured by bicycle ergonometry increased in response to GH treatment. Knowledge of the long term effects of GH treatment on muscle function in GH-deficient adults is limited.
This information is important, as patients with acromegaly have muscles that appear to be hypertrophied but may be functionally impaired, indicating that exposure to overly high levels of GH for a long period of time may have a deleterious effect on muscle function.
The primary objective of this trial was to study the effects of long term GH treatment in GH-deficient adults on isometric and isokinetic muscle strength and local muscle endurance.
This was achieved by performing repeated measurements of muscle function for the quadriceps, hamstrings, and hand-grip for a period of 2 yr in a large heterogeneous cohort of GH-deficient adults in terms of age and sex.
Results from the GH-deficient patients were compared with muscle function in a reference population. The secondary objective of this trial was to explore possible predictive background factors for the treatment response.
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