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Permeability of muscle capillaries to microperoxidase
1Department of Anatomy, School of Medicine, University of California, San Francisco 94143, USA.
Abstract:
In this study we attempted to identify a morphologic counterpart of the small pore of muscle capillaries. The existence of such a pore has been postulated by physiologists to explain the permeability of muscle capillaries to small macromolecules. We injected mice intravenously with microperoxidase (MP) and fixed specimens of diaphragm at intervals of 0-250 s after the injection to localize the tracer by electron microscopy. The small size of MP (1,900 mol wt and 20 A molecular diameter [MD]) ensures its ready passage through the small pore since the latter is thought to be either a cylindrical channel 90 A in diameter or a slit 55 A wide. MP appears in the pericapillary interstitium within 30 s of initiation of its intravenous injection. The patterns of localization of MP observed within clefts between adjacent capillary endothelial cells indicate that some endothelial junctions are permeable to this tracer. Although small vesicles transfer MP across the endothelium, we do not believe that the vesicles transfer substantial amounts of MP into the pericapillary interstitium. We did not obtain evidence that MP crosses the endothelium of capillaries through channels formed either by a single vesicle or by a series of linked vesicles opening simultaneously at both surfaces of the endothelial cell. From our observations we conclude that some endothelial junctions of capillaries are permeable to MP, and that these permeable junctions are a plausible morphologic counterpart of the small pore.
Insights
Researchers identified the small pore in muscle capillaries, which explains how small macromolecules pass through. Microperoxidase tracer studies revealed that endothelial junctions, not vesicles, are permeable, acting as the capillary small pore.
Area of Science:
- Cell biology
- Physiology
- Microscopy
Background:
- Muscle capillary permeability to macromolecules is a long-standing physiological question.
- A postulated 'small pore' is hypothesized to explain this phenomenon.
Purpose of the Study:
- To identify the morphologic basis of the muscle capillary 'small pore'.
- To investigate the pathway of small macromolecule passage across capillary endothelium.
Main Methods:
- Intravenous injection of microperoxidase (MP) tracer in mice.
- Electron microscopy of diaphragm capillary specimens at various time points post-injection.
- Localization of MP to determine its passage across the endothelium.
Main Results:
- Microperoxidase (1,900 mol wt, 20 A MD) appeared in the interstitium within 30 seconds.
- MP was observed within clefts between endothelial cells, indicating junctional permeability.
- Vesicular transport of MP was minimal; no evidence of transcellular channels formed by vesicles was found.
Conclusions:
- Endothelial junctions in muscle capillaries are permeable to small tracers like MP.
- These permeable junctions represent a plausible morphologic counterpart of the postulated 'small pore'.