THE INTRAPERITONEAL EFFECTS OF 5-HYDROXYTRYPTAMINE ON HEMODYNAMICS, GASTRIC-MUCOSAL BLOOD-FLOW AND PLATELET-AGGREGATION

The effects of 5-hydroxytryptamine (5-HT) on gastric mucosal blood flow and lesion formation have been established. However, the mechanisms accounting for the reduction of gastric mucosal blood flow have not been defined. The current study aimed to test the hypothesis that decrease of gastric mucosal blood flow in rats is the result of changes of systemic blood pressure and/or platelet aggregation. 5-HT (given i.p. 5 or 10 mg/kg) time and dose dependently reduced gastric mucosal blood flow and systemic arterial blood pressure; it also potentiated ethanol-induced mucosal damage. Methysergide (a 5-HT2-receptor blocker) pretreatment alleviated the decrease of gastric mucosal blood flow and lesion formation but not the systemic blood pressure. Also in the 5-HT-treated animals, the mucosal oxygen (O-2) and haemoglobin levels as well as the systemic blood CO2 were reduced, but the blood O-2 was increased. The latter mio parameters correlated with an elevation of the respiratory rate. The blood platelet count was not affected by 5-HT pretreatment. Adenosine diphosphate (ADP) dose dependently induced a similar degree of platelet aggregation in platelet rich plasma fractions in the saline and 5-HT-treated rats in vitro. 5-HT in the concentrations of 1 or 10 mu M, promoted the platelet aggregation produced by ADP. However, this action was attenuated in the 5-HT-pretreated rats, indicating that tachyphylaxis of 5-HT action on platelet aggregation could occur. It is concluded that the depression of gastric mucosal blood flow by 5-HT is caused by the decrease of systemic blood pressure and gastric vascular constriction but not by the induction of platelet aggregation in vivo.