Presence of immunoreactive platelet-activating factor in peritoneal exudate induced by zymosan in rats

The presence of the platelet-activating factor (PAF) in peritoneal exudate, induced by zymosan, was investigated in rats. Intraperitoneal injection of zymosan induced an extravasation of plasma proteins and an accumulation of polymorphonuclear leukocytes in the peritoneal cavity. Zymosan induced a release of immunoreactive PAF in peritoneal exudate which reached maximum levels at 30 min and then declined slowly. Immunoreactive PAF was evenly distributed in the fluid and cellular fractions of the exudate. A PAF-acetyltransferase activity was found as well in the exudate as in plasma. Histamine levels, measured by bioassay, increased after injection of zymosan, reached a peak at 15 min and declined to a normal level within 4 hr. Protein extravasation was inhibited by WEB 2086, a PAF antagonist, during the 6 hours of the study, whereas the accumulation of leukocytes was suppressed at 2 hour by WEB 2086 and SM 12502, two PAF antagonists. PAF is present in the peritoneal exudate induced by zymosan in rats, and plays a role in the development of the inflammatory response. Its main origin might be resident macrophages and polymorphonuclear leukocytes.