RAT DIAPHRAGM DURING POSTNATAL-DEVELOPMENT .1. CHANGES IN DISTRIBUTION OF MUSCLE-FIBER TYPE AND IN OXIDATIVE POTENTIAL

The changes occurring in the histochemical characteristics of the rat diaphragm during the postnatal period were examined. Fibre-type distri bution, fibre oxidative capacity, i.e. succinate-dehydrogenase (SDH) a ctivity, and cross-sectional area were compared in the costal (COS) an d crural (CRU) regions, and across their abdominal and thoracic surfac es. The proportions of type I and IIb fibres in both COS and CRU incre ased with age, while the proportion of type IIa fibres progressively d ecreased. For COS, fibre distribution was homogeneous over the entire muscle and did not change after 4 weeks. For CRU, it was heterogeneous with a higher proportion of type I fibres on the thoracic surface as from the first week. All fibre types significantly increased in cross- sectional area between 1 and 8 weeks, with no significant differences in COS and CRU. Mean SDH activity did not differ between COS and CRU o r across the muscles. Mean SDH activities were low and identical in al l fibre types at birth, and then increased, peaking at the 6th week in type I and IIa fibres. When total muscle fibre oxidative capacity was calculated from an index including fibre-type proportion, cross-secti onal area and mean SDH activity, it was significantly higher at 1 than at 8 weeks after birth; this might have functional implications for t he newborn.