The effects of sennosides on colonic motility were investigated in eight conscious dogs chronically fitted with two strain gauge transducers in the proximal colon, an intracolonic silicone catheter and a polyethylene catheter implanted in a branch of the right colonic artery. Oral sennosides (30 mg/kg) inhibited colonic motility for 12 to 18 h after a three to six hours delay, and associated with giant contractions and diarrhoea. The minimal oral dose of sennosides to produce such changes varied from 5 to 15 mg/kg. Intracolonic sennosides at the minimal effective dose and at 30 mg/kg reproduced the effects of oral sennosides, but with a shorter latency (0.5-1.5 h). Intracolonic PGE2 (100 micrograms/kg) in viscous gel medium or intra-arterial PGE2 (10 micrograms/h) inhibited colonic motility and induced giant contractions often associated with defecation. The colonic motor changes induced by intracolonic sennosides at the minimal effective dose, but not those induced by intracolonic PGE2, were blocked by intra-arterial indomethacin (10 micrograms/h) or piroxicam (5 micrograms/h). These results suggest that colonic motor actions of sennosides are mediated through a local prostaglandins synthesis, as they were blocked by cyclooxygenase inhibitor and reproduced by PGE2.
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