Analysis of the factor(s) involved in pathogenesis of zymosan-induced inflammation in rats

The role of mast cell degranulation in increased vascular permeability in zymosan-air-pouch inflammation, an experimental model of inflammation induced by zymosan in rats, was investigated. The complement in the inflammatory pouch fluid was exhausted, and mast cells in the pouch wall subcutaneous tissues were degranulated. The histamine level in the pouch fluid was elevated immediately after application of zymosan in the preformed air-pouch and then quickly declined. Plasma exudation into the pouch fluid changed in close parallel with the change of histamine level. Application of compound 48/80 in the air-pouch also brought about liberation of histamine from mast cells, accompanied with elevation of vascular permeability similar to that observed in the zymosan-air-pouch inflammation. However, the amount of the plasma exudation in the zymosan-air-pouch inflammation was about twice as high as that induced by compound 48/80, though the quantity of histamine liberated in the two cases was almost equal. Rats depleted of histamine and serotonin were incapable of responding to compound 48/80, but zymosan still induced increased vascular permeability. A combination treatment with pyrilamine and methysergide did not abolish plasma exudation caused by zymosan, but brought about complete blockade of the vascular permeability response to compound 48/80. These results suggest that some mechanisms independent of degranulation of mast cells are responsible in part for the initial sudden elevation of vascular permeability in zymosan-induced inflammation.