[Involvement of nitric oxide, which was synthesized by constitutive nitric oxide synthase, and prostaglandin on diarrhea induced by castor oil in rats].

To clarify the mechanism of castor oil-induced diarrhea, this study was performed by using rats in relation to nitric oxide (NO) and prostaglandin (PG). Castor oil induced diarrhea in all rats within 3 hr in the control group. The pretreatment of NG-nitro-L-arginine methyl ester prevented the diarrhea, and this effect was attenuated by the combined treatment of L-arginine. Amino-guanidine inhibiting inducible NO synthase had no effect on the diarrhea, but dexamethasone, an inhibitor of both inducible NO synthase and phospholipase A2 that synthesizes PGs, significantly prevented it. Indomethacin, an inhibitor of cyclooxygenase that synthesizes PGs, also significantly prevented diarrhea. Therefore, the mechanism of the preventive effect by dexamethazone on diarrhea was suggested to be the inhibition of PGs generation. From the above results, it became clear that PG and NO, especially that synthesized by constitutive NO synthase, are involved in the mechanism of diarrhea induction by castor oil in rats.