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Fusion of isolated sarcoplasmic reticulum membranes
Abstract:
Fragmented sarcoplasmic reticulum (FSR) vesicles from rabbit muscle were suspended in 1.5-5% glycerol solutions and were pelleted onto aluminum foil disks in a modified centrifuge tube. Examination of these pellets in the electron microscope after drying for 2--2.5, 4--5.5, and 21 hours revealed a progression of changes. First, distances between individual, round vesicles decreases. Next, somewhat flattened vesicles establish limited areas of contact with adjacent vesicles. Finally, vesicle fusion occurs and extended areas of double bilayers are formed. A water loss-time interaction appears to be needed for the fusion process. A Hg-phenyl azoferritin compound was used as a marker to identify intra- and extra-vesicular space in the fused samples. Quantitative measurements of birefringence during imbibition of pellet slices in a graded series (eta-1.41-1.43) is reached. The plateau seen in this part of the curve is again followed by steadily increasing birefringence at higher glycerol concentrations. This interruption in the birefringence curve is presumably due to a matching of the refractive indices of the glycerol solution and a lipid component in the membranes.
Insights
Fragmented sarcoplasmic reticulum (FSR) vesicles fuse over time as water is lost, forming double bilayers. This process, observed via electron microscopy, is crucial for understanding muscle membrane dynamics.
Area of Science:
- Biochemistry
- Cell Biology
- Muscle Physiology
Background:
- Fragmented sarcoplasmic reticulum (FSR) vesicles are critical for muscle contraction.
- Understanding vesicle dynamics and fusion is key to muscle function.
Purpose of the Study:
- To investigate the morphological changes and fusion process of FSR vesicles under controlled dehydration.
- To identify the role of water loss and time in vesicle fusion.
Main Methods:
- Rabbit muscle FSR vesicles were suspended in glycerol solutions and pelleted.
- Samples were examined using electron microscopy after varying drying times.
- Hg-phenyl azoferritin was used as a marker for intra- and extra-vesicular spaces.
- Birefringence measurements were taken during glycerol imbibition.
Main Results:
- Progressive changes observed: decreased vesicle distance, vesicle flattening with contact, and eventual vesicle fusion into double bilayers.
- Water loss and time were identified as necessary factors for vesicle fusion.
- Hg-phenyl azoferritin successfully marked intra- and extra-vesicular spaces in fused vesicles.
- Birefringence measurements showed an interruption correlating with refractive index matching.
Conclusions:
- Vesicle fusion in FSR is a time- and dehydration-dependent process.
- The study provides insights into membrane dynamics and structural changes during fusion.
- Birefringence changes suggest interactions between membrane lipids and the surrounding medium.