Induction mechanism of small intestinal lesions caused by intravenous injection of endotoxin in rats

The pathogenesis of intestinal damage caused by bolus intravenous injection of endotoxin (ETX; 3 mg/kg) was investigated. Administration of ETX to rats induced reddish discoloration suggestive of bleeding, increased hemoglobin amounts, and leakage of plasma protein in the intestine. However, light microscopic examination of the intestine demonstrated blood congestion of the microvessels. Plasma accumulation was partially inhibited by combined pretreatment with a histamine H₁ antagonist and a serotonin (5-HT) antagonist. Neither a 5-lipoxygenase inhibitor, a soybean trypsin inhibitor, nor atropine was observed to inhibit plasma accumulation. Both the intestinal leakage of plasma and the accumulation of hemoglobin were completely inhibited by indomethacin, a selective thromboxane A synthetase inhibitor (OKY 1581), and a stable PGI₂ analogue (beraprost). Intravital microscopic observation of the microvessels of the small intestinal villi demonstrated microthrombus formation within several minutes after the injection of ETX, and pretreatment with OKY 1581 attenuated the formation of microthrombus. Platelet counts decreased significantly 10 min after ETX administration, and the decrease was not inhibited by pretreatment with either OKY 1581 or beraprost. Prothrombin time (PT) and activated partial thromboplastin time (APTT) were not prolonged. These observations thus suggest that microcirculatory disturbances by platelet thrombus, which are mediated by thromboxane A₂ at least in part, play an important role in ETX-induced intestinal damage.