Transitions in membrane composition during postnatal development of rabbit fast muscle

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Abstract
Early postnatal changes (4–5 days to 15 days after birth) in the biochemical composition of microsomes were investigated in rabbit skeletal muscles destined to become fast-twitch muscles. During this period, a steady decrease in the microsomal content of cholesterol and of ouabain-sensitive Na+/K+-ATPase activity, as well as a decrease in protein electrophoretic components in the 80 000–70 000 molecular weight range, were observed. These changes are probably due to a diminishing yield of microsomal membranes derived from T-tubules, as the age of the animals increases, and are indicated from a knowledge of the mixed composition of muscle microsomes and previous biochemical data on isolated T-tubules. The content of cytochromeb5, which was found to be high in muscle microsomes of new born animals, decreased strikingly as the amount of membrane-bound Ca2+-ATPase protein increased, with a crossing-over point at about 7–10 days after birth. These changes, possibly corresponding to a transition from precursor sarcoplasmic reticulum (SR) to mature SR, were found to be temporally correlated with changes in [3H]α-tocopherol binding ability of the microsomes and in the mitochondrial content of glycerol phosphate dehydrogenase. At the same critical periods, coincident with the onset of motile activity, the immunological cross-reactivity of the Ca2+-ATPase protein of microsomal vesicles, with antibody specific for the Ca2+-ATPase of adult fast SR, was found to increase markedly, as tested by competitive enzyme-linked immunosorbent assay (ELISA). The immunological data are consistent with data in the literature demonstrating an increase in the concentration of Ca2+-ATPase molecules in the SR membranes during ontogenic development. Both these data and catalytic data, however, suggest that the Ca2+-ATPase protein is present in the same form in the SR of immature and of adult fast muscle and, in an antigenically different form, in slow muscle SR.

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