Ricinoleic acid, the active ingredient of castor oil, increases nitric oxide synthase activity in the rat ileum and colon

The effect of ricinoleic acid on nitric oxide synthase (NOS) activity has been investigated in the rat. NOS, determined by the oxidation of oxyhemoglobin to methemoglobin or by the formation of citrulline, was detected in homogenates of both ileal and colonic tissue. Ricinoleic acid (10(-5)-10(-3) M) incubated with intestinal tissue for 15 min, increased NOS activity (P < 0.05-0.01); however the effect was more evident in the ileum than in the colon. A pretreatment with N-G-nitro-L-arginine methyl ester (L-NAME, 6.25-25 mg kg(-1) i.p., 15 min before the removal of tissue) inhibited the increase in NOS activity (P < 0.05). These findings confirm an involvement of NO in the laxative effect of ricinoleic acid (castor oil).