Occlusion of Lumina in Small Arterioles During Vasoconstriction

The microscopic appearance of small arterioles during epinephrine-induced vasoconstriction was studied in the dog mesentery. The tissues were quick-frozen in situ with liquid isopentane, freeze-substituted, and sectioned serially. Obliteration of the lumina of small arterioles during vasoconstriction was demonstrated in both random and serial sections. Endothelial cells, which were ordinarily flattened against the internal elastic membrane, were compressed and deformed during vasoconstriction, so that they protruded into the lumen and often coalesced in the center of the vessel to form a mechanical obstruction to blood flow. Vascular obliteration occurred commonly in very small arterioles, but was observed with diminishing frequency in progressively larger arterioles, and was not identified in vessels large enough to accommodate more than 10 endothelial cells in cross sections. In an ideal cylinder, tension is greatest at the innermost layer, and the gradient of tension across the wall falls off in a nonlinear fashion. These conditions probably do not hold in a functioning arteriole.