MICROCIRCULATORY STASIS AND THE PRODUCTION OF TISSUE NECROSIS IN THE HAMSTER CHEEK POUCH INDUCED BY HISTAMINE OR A HISTAMINE LIBERATOR

Endothelial contraction induced by histamine-type mediators: an electron microscopic study

Previous work has shown that endogenous chemical mediators, of which histamine is the prototype, increase the permeability of blood vessels by causing gaps to appear between endothelial cells. In the present paper, morphologic and statistical evidence is presented, to suggest that endothelial cells contract under the influence of mediators, and that this contraction causes the formation of intercellular gaps. Histamine, serotonin, and bradykinin were injected subcutaneously into the scrotum of the rat, and the vessels of the underlying cremaster muscle were examined by electron microscopy. To eliminate the vascular collapse induced by routine fixation, in one series of animals (including controls) the root of the cremaster was constricted for 2-4 min prior to sacrifice, and the tissues were fixed under conditions of mild venous congestion. Electron micrographs were taken of 599 nuclei from the endothelium of small blood vessels representing the various experimental situations. Nuclear deformations were classified into four types of increasing tightness (notches, foldsl closing folds, and pinches. In the latter the apposed surfaces of the nuclear membrane are in contact). It was found that: (1) venous congestion tends to straighten the nuclei in al groups; (2) mediators cause a highly significant increase in the number of pinches (P < 0.001), also if the vessels are distended by venous congestion; (3) fixation without venous congestion causes vascular collapse. The degree of endothelial recoil, as measured by nuclear pinches, is very different from that caused by mediators (P < 0.001). (4) Pinched nuclei are more frequent in leaking vessels, and in cells adjacent to gaps (P < 0.001); (5) mediators also induce, in the endothelium, cytoplasmic changes suggestive of contraction, and similar to those of contracted smooth muscle; (6) there is no evidence of pericyte contraction under the conditions tested. Occasional pericytes appeared to receive fine nerve endings. Various hypotheses to explain nuclear pinching are discussed; the only satisfactory explanation is that which requires endothelial contraction.

On the mechanism of vascular leakage caused by histaminetype mediators. A microscopic study in vivo

A current hypothesis maintains that the vascular leakage induced by histamine, serotonin, and bradykinin is brought about by a hydrostatic mechanism: The larger veins are said to contract; the venules are therefore submitted to an increase in pressure, which causes their wall to give way. To test this hypothesis, vessels of an exposed striated muscle (rat and rabbit cremaster) were photographed while treated with the above-mentioned substances; the occurrence of vascular leakage was tested by the intravenous injection of carbon black (carbon labeling). All animals were under barbiturate anesthesia, given a neuromuscular blocking agent (gallamine triethiodid), and were maintained by artificial respiration, according to a technique previously described. In 72 rats and 4 rabbits, the first event observed was arteriolar dilatation. Constriction of a small vein was observed in only 1 in 76 experiments (in two other cases it was doubtful). It is concluded that venous spasm cannot account for the leakage induced by the mediators of the histamine type, though such leakage may be enhanced by venous spasm whenever it may occur. An alternative mechanism is proposed and discussed: Histamine-type mediators cause vascular leakage by a direct effect on the venular endothelium, which is induced to contract.