Perspectives for future research
In addition to being essential for locomotion, skeletal muscle from some animals is consumed as meat by man. A muscle's movement and meat quality are determined by the growth and composition of its composite fibres, as well as by the maintenance of ion gradients. The physiological and morphological properties of adult skeletal muscle are the combined result of genetic predisposition, diet, hormonal influences and the workload that the muscle has been exposed to.

During development skeletal muscles not only hypertrophy but also adapt to their required mechanical functions, such as rapid short-lasting movements (fast muscles) or prolonged actions (slow muscles). With respect to meat quality, slow muscles have better water holding capacity but lower colour stability than fast muscles [24].

A muscle's functional adaptation during development is evident through changes in the cation transport activity [8] and in the myosin heavy chain isoform expression during the postnatal period [9]. Both parameters are strongly affected by thyroid hormones and by exercise [5,32,35]. These effects are not easy to investigate independently since standardising exercise regimes is difficult and maintaining animals with relatively long growth periods is costly.

Muscle cell culture
The use of tissue culture techniques to study the adaptive behaviour and growth of muscle cells has obvious advantages [1]. Foetal myoblasts and adult muscle satellite cells are readily isolated and grown in vitro (Figure 7). After an initial phase of proliferation they fuse to form myotubes and then differentiate to become spontaneously contracting myofibres. During further growth, the satellite cells divide and their nuclei are added to the fibres, mimicking the processes of muscle growth and regeneration after injury.

A better understanding of the involvement of satellite cells in postnatal myogenesis and in muscle hypertrophy will be essential to improve the efficiency of muscle growth in meat producing animals [3]. Muscle cells from pigs [11] and cattle [3] have been cultured successfully and directed towards differentiation or proliferation by growth factors [15] and hormones [3]. Electrical and mechanical stimulation have also been applied to the cells in vitro, as a means of mimicking the application of a 'workload' [43]. Cultured human skeletal muscle cells have also been analysed for their degree of maturity by measuring Na⁺,K⁺-ATPase activity [2].

Future Research
We are currently developing an in vitro model to test the hypothesis that exercise, hormones and growth factors together determine the variations found in growth and fibre composition of skeletal muscle. Two fundamental questions are under consideration. First, do slow and fast muscle fibres respond differently to physical and hormonal stimulation? And second, when during embryonic and post-natal development are the growth and composition of skeletal muscle fibres most affected by these stimuli?

Concluding remarks
The concentration of Na⁺,K⁺ pumps in the skeletal muscle of cats, dogs, horses and cattle is regulated by mechanisms similar to those described in rodents and man. We already know that hyperthyroidism and physical training increase the number of Na⁺,K⁺ pumps in skeletal muscle and that hypothyroidism and immobility reduce their number. We know, too, that the rise in Na⁺,K⁺ pump concentration after training is associated with a blunted rise in plasma K⁺ during exercise. However, the question remains whether the mechanism responsible for the up-regulation of the Na⁺,K⁺ pump during hyperthyroidism is the same as that during training.

References...

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