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The permeability of muscle capillaries to horseradish peroxidase
Abstract:
The main objective of this study was to determine the pathways by which horseradish peroxidase (HRP) can cross the endothelium of muscle capillaries. Specimens of mouse diaphragm were fixed for cytochemical analysis at various intervals after intervenous injection of 0.5 mg HRP, at 4 min after intervenous injection of varied amounts of HRP, and at 4 min after intervenous injections in various volumes of isotonic NaCl. Our findings indicate that endothelial junctions serve as a barrier which may allow passage of very limited amounts of HRP. They also suggest that endothelial vesicles transfer HRP from the capillary lumen to the pericapillary interstitium as well as in the reverse direction. Increasing the volume of solution injected to approximately 30% of total blood volume did not increase the amount of HRP that left the capillary lumen. Our results with HRP do not provide clearcut evidence that endothelial junctions are the site of the small pore.
Insights
Horseradish peroxidase (HRP) crosses muscle capillaries via endothelial vesicles, with limited passage through junctions. Injecting larger fluid volumes did not enhance HRP transfer from capillaries.
Area of Science:
- Endothelial transport mechanisms
- Capillary permeability studies
- Muscle microcirculation physiology
Background:
- Understanding how molecules like horseradish peroxidase (HRP) traverse the endothelial barrier is crucial for studying microvascular function.
- The endothelium forms a selective barrier, and the pathways for macromolecule passage are not fully elucidated.
- Previous research has explored various routes, including vesicular transport and intercellular junctions.
Purpose of the Study:
- To investigate the specific pathways utilized by horseradish peroxidase (HRP) to cross the endothelium of muscle capillaries.
- To evaluate the role of endothelial junctions and vesicles in HRP transport.
- To determine the effect of injected fluid volume on capillary HRP leakage.
Main Methods:
- Cytochemical analysis of mouse diaphragm specimens.
- Timed intravenous injections of horseradish peroxidase (HRP) at varying doses and intervals.
- Injections of varying volumes of isotonic saline to assess fluid volume effects.
Main Results:
- Endothelial junctions permit only minimal passage of HRP, acting as a restrictive barrier.
- Endothelial vesicles facilitate bidirectional transfer of HRP between the capillary lumen and pericapillary interstitium.
- Increasing injected fluid volume up to 30% of total blood volume did not significantly increase HRP efflux from capillaries.
Conclusions:
- Endothelial vesicles are a primary route for HRP transport across muscle capillaries.
- Endothelial junctions are not the main pathway for significant HRP passage.
- The study did not find conclusive evidence supporting endothelial junctions as the site of the 'small pore' pathway for HRP.