Delay of castor oil diarrhoea in rats: a new way to evaluate inhibitors of prostaglandin biosynthesis.

Forty-four non-steroidal anti-inflammatory compounds were tested for possible effects on castor oil-induced diarrhoea in rats. A small but significant delay of intestinal evacuations was found with all compounds. Quantitatively, the oral doses required to delay diarrhoea beyond the first hour after castor oil challenge reflected the acute anti-inflammatory potency of the tested compounds. Qualitatively, the evolution of the effective doses with increasing delay was linear for potent inhibitors of prostaglandin biosynthesis. The evolution for less potent compounds was markedly different and suggested the earlier occurrence of non-specific drug effects. Suprofen, the most potent of the series of compounds, produced the 1 h delay at an oral dose of 1.11 mg kg-1; the ED50 increased linearly to 115 mg kg-1 for a 4 h delay. Compared with other compounds the activity pattern of suprofen was consistent with that of a very potent, short-acting inhibitor of prostaglandin biosynthesis, which maintains its specific action over a wide dose range. It is concluded that delay of castor oil-induced diarrhoea in rats allows a detailed characterization of aspirin-like compounds, and that inhibition of prostaglandin biosynthesis is insufficient to suppress the intestinal effects of the oil.