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Changes in the structure, composition and function of sarcoplasmic-reticulum membrane during development
Abstract:
The structure, chemical composition and function of the microsomal fraction, isolated by differential centrifugation and purified on sucrose gradients, from muscle of fetal, newborn and young rabbits were characterized and compared with those of sarcoplasmic reticulum vesicles from adult muscle. Negative staining shows that the microsomal vesicles isolated from muscles of embryos and newborn animals are smooth, in contrast to vesicles obtained from adult muscle which contain 4-nm particles on their surface. The particles appear first in the microsomal vesicles from muscles of 5--8-day-old rabbits. Their number increases with the age of the animals. Ca2+-pump protein, with molecular weight about 100000, accounts for 10% of the total protein content in sarcoplasmic reticulum membrane, isolated at the earliest stages of development analysed. Its amount increases continuously with the rabbit's age to the adult value of about 70% of total sarcoplasmic reticulum protein. The low amount of 100000-dalton protein and lack of 4-nm surface particles in sarcoplasmic reticulum vesicles obtained from fetal and newborn rabbits are strictly correlated with the low activity of Ca2+-dependent ATPase and the ability to take up Ca2+. These activities rise in parallel with the age of the rabbits. On the other hand, Mg2+-dependent ATPase activity is very high at the early stages of development and declines continuously to a low value in sarcoplasmic reticulum from adult muscle. The sarcoplasmic reticulum membrane from fetal and newborn rabbits contains a higher amount of lipids as compared with the membrane present in the muscle of adult animals. The ratio of both phospholipid to protein and neutral lipid to protein decreases with the age of the rabbits. The composition of sarcoplasmic reticulum phospholipids also changes during development.
Insights
Sarcoplasmic reticulum development in rabbits shows immature vesicles in newborns lacking Ca2+ pump protein and surface particles. These features mature with age, correlating with increased Ca2+ uptake and ATPase activity.
Area of Science:
- Muscle physiology
- Developmental biology
- Biochemistry
Background:
- Sarcoplasmic reticulum (SR) is crucial for muscle contraction, regulating intracellular calcium levels.
- Understanding SR development is key to comprehending muscle maturation and function.
Purpose of the Study:
- To characterize and compare the structural, chemical, and functional properties of SR vesicles during rabbit muscle development.
- To investigate the developmental changes in Ca2+-pump protein, ATPase activity, and lipid composition of SR.
Main Methods:
- Differential centrifugation and sucrose gradient purification of microsomal fractions from rabbit muscle at various developmental stages.
- Negative staining electron microscopy to visualize vesicle surface morphology.
- Biochemical assays to determine protein content (Ca2+-pump), Ca2+-dependent and Mg2+-dependent ATPase activities, and lipid composition.
Main Results:
- Immature SR vesicles from fetal and newborn rabbits are smooth, lacking the 4-nm surface particles seen in adult muscle.
- The Ca2+-pump protein (100,000-dalton) is low in early development and increases with age, paralleling Ca2+ uptake and Ca2+-dependent ATPase activity.
- Mg2+-dependent ATPase activity is high in early development and decreases with age, while lipid content is higher in immature SR.
Conclusions:
- SR maturation involves the gradual acquisition of Ca2+-pump protein and surface particles, correlating with enhanced Ca2+ handling capabilities.
- Developmental changes in SR composition, including protein and lipid content, are essential for functional maturation of muscle.
- The study highlights significant differences in SR structure and function between immature and adult rabbit muscle.