Development and physiological characteristics of avian fast and slow contracting muscles
1. The ALD and PLD muscles from the chicken.were studied throughout ex ovo development from 3 days of age until early adulthood. These two muscles provided a model for studying phasic fast fibres and tonic slow fibres. 2. Development over the first 30 days was compared in control groups and groups immobilised in two positions, "resting" and shortened. Analysis included histochemically staining for Myosin ATPase, Phosphorylase, Succinic Dehydrogenase and histological staining for nuclei/cytoplasm and nerve endings. The ALD was shown to have 2 tonic fibre types described by both contractile and metabolic enzyme markers. The PLD was shown to have different fibre types when stained for phosphorylase activity. These types were not observed with the traditional Myosin ATPase stain. The PLD also exhibited a few, p9ssibly foetal fibres during the first two weeks of ex ovo growth. Immobilisation in both positions caused the histochemically displayed activities of these enzymes to become reduced. 3. Biochemically assays for Mg 2+-activated myofibrillar ATPase (both a alone and total activity a+ b) activity were developed for chicken muscle. These assays were used to study the changes in the ALD and PLD muscles across development and with immobilisation in both positions. Both enzymes’ activities increased during normal development with peaks in activity shown at 15 days. The PLD was shown to have 4 times the Mg2+-activated myofibrillar ATPase activity than the ALD at 51 days age. For the same age, total phosphorylase activity was 3 times higher in the PLD muscles than the ALD muscles. The higher activities 161 exhibited by the fast-phasic PLD muscle over the slow-tonic ALD muscle is in agreement with the theory that the PLD-is a faster contracting, highly anaerobic, muscle and. the ALD a slow-contracting) aerobic muscle. Immobilisation in both positions showed an initial reduction in both enzyme's activities followed by a recovery despite longer periods of immobilisation. The growth peaks exhibited by these enzymes appeared to be displaced in time when inactivity through immobilisation was imposed. 4. Chickens were thyroidectomized from 6 weeks age for 4 weeks and the ALD, PLD and ST muscles were analysed for Mg 2+ -activated myofibrillar ATPase and phosphorylase (a and a+ b) activity. Sham-operated birds acted as controls. The muscles were also stained histochemically for Myosin ATPase and phosphorylase. Thyroidectomy caused a differential effect in the PLD and ST with the ALD in terms of changes in these enzymes, activities. The PLD and ST showed a significant reduction in activity of both main enzymes (phosphorylase a activity was unchanged). The ALD however, exhibited no change in activity of the two main enzymes. In conclusion the reduced presence of thyroid hormone slows down fastphasic chicken muscles but has little effect on slow-tonic chicken muscles. 5. A study was made on. the pCa-tension relationship for skinned single fibres from the ALD and PLD muscles from 9 week old chickens. The threshold for calcium activation of contraction was found to at a pCa-of 7.5 for the ALD and of 6.63 for the PLD. The ALD showed a minimum calcium binding site number of 2 and the PLD of 3. The maximum isometric tension observed was approximately 8.3 Nom- 2 for 162 both muscles. The ALD-therefore will contract at extremely low free calcium levels whereas the PLD needs almost 0.2 micro M higher free calcium to contract. Possible explanations for this difference and the differences in physiology and. function of the two muscles are presented.