Segmental differentiation of endothelial intercellular junctions in intra-acinar arteries and veins of the rat lung.

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Abstract
The present study examines by freeze fracture the structure of endothelial intercellular junctions in pulmonary intra-acinar arteries and veins. Because of the complexity of the pulmonary vascular bed, it was necessary to devise a means to distinguish venous from arterial vessels in freeze fracture replicas. By modifying a recently described technique, which entails occluding venous outflow during intravascular perfusion fixation of the lung, the arteries were emptied of and the veins filled with erythrocytes. Tight junctions (TJ) of intra-acinar arteries, ranging in internal diameter from 17-125 .mu.m, consisted of 2-6 continuous, interconnected rows of particles 100-500 .ANG. in length, present in grooves on the E face and complementary low, particle-poor ridges on the P face. Numerous large gap junctions (GJ) were present in the meshwork of TJ and formed rows parallel to the TJ elements. In intra-acinar veins, measuring 32-284 .mu.m in internal diameter, TJ consisted of 1-5 sparsely interconnected rows or particles on the E face and low, virtually particle free ridges on the P face. Gap junctions were smaller and fewer in veins than in arteries. In both arteries and veins, special endothelial junctions were observed. These consisted of closely spaced, alternating rows of GJ and TJ particles and were similar to those observed in systemic vessels. The presence of GJ suggests that there is extensive electronic and/or metabolic coupling in arterial endothelium, and that this is less extensive in venous endothelium. The continuous network of multiple TJ particles in both arteries and veins suggests that thase segments are less permeable than the venular ones.