We have previously established that approximately 30% of the endothelial junctions in the pericytic venules of the mouse diaphragm are open to a gap of approximately 30--60 A, and are fully permeated by hemeundecapeptide (H11P) (mol diam approximately 20 A). To estimate the size limit for molecules that can permeate these junctions, we have administered graded tracers intravenously and studied their behavior at the level of pericytic venules in bipolar microvascular fields (BMFs) in the mouse diaphragm. Horseradish peroxidase (HRP) (mol diam approximately 50 A) permeated only approximately 50% of the open junctions of the venular endothelium. Outflow through venular junctions appeared to be modest since the tracer remained restricted to the perivenular spaces. Hemoglobin (Hb, mol diam 64 x 55 x 50 A) permeated only a few (less than 5%), and ferritin (mol diam 110 A), practically none, of the endothelial junctions of the pericytic venules. The findings suggest that under normal conditions the size limit for permeant molecules for open venular junctions is approximately 60 A. Replicas of freeze-fracture preparations from appropriate regions in BMF showed that the intercellular junctions of the venular endothelium have the same organization as previously described for the corresponding segments of the microvasculature in the omentum and mesentery: discontinuous creases or grooves either free of or marked by few intramembrane particles only. Administration of histamine (topically or systemically) and 5-hydroxytryptamine (5-HT) (topically) resulted in typical focal separations of the endothelial junctions and intramural deposits of large tracer particles (carbon black) in the postcapillary venules.

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