Summary: Injection of carrageenan into the rat cremaster causes immediate vascular labelling of venules and capillaries around the injection site. This initial labelling disappears within 30 minutes and is succeeded by a second phase of leakage which begins at about 1 1/2 hours and lasts for several hours. The second phase is confined to venules, and is both more intense and more extensive than the initial response. It is accompanied by obvious oedema of the injected tissues and by massive emigration of leucocytes. Vasodilatation is prominent at all stages of the reaction.
Electron microscope examination shows that leakage in both phases of the response occurs via gaps in vascular endothelium, which appears undamaged at all stages examined after carrageenan injection.
Injection of carrageenan into the foot pad causes biphasic vascular labelling of similar time-course and intensity to that seen in the cremaster, but the precise pattern of labelled vessels could not be determined. Carrageenan has no effect on the permeability of vessels in the mucosa of the small bowel.
It is suggested that swelling in the first hour after carrageenan injection is due to vasodilatation and consequent transudation of fluid. In the later stages of the reaction, increased vascular permeability appears to play the major role in local oedema formation. This increased permeability appears to be due to release of endogenous chemical mediators, but the active agents have not been identified.
(C) 1973 Royal College of Pathologists of Australasia