Potentiation by phenolphthalein of the responses of guinea-pig ileum and rat stomach strip to PGE2 and other agonists

Comparative assessment of phenolphthalein and phenolphthalein glucuronide: is phenolphthalein glucuronide a better laxative?

BACKGROUND

Phenolphthalein is widely used as a safe and effective laxative. After oral administration, phenolphthalein is absorbed in the small bowel and is conjugated in the liver to phenolphthalein glucuronide which passes into the colon where it is deconjugated and the active compound, phenolphthalein, is released. Since phenolphthalein glucuronide does not undergo enterohepatic circulation it should theoretically have a more rapid onset of action and a lower threshold dose for laxation. The present study was designed to examine this issue.

METHODS

Ten normal healthy subjects volunteered for the study. All subjects were administered placebo, phenolphthalein (at doses of 15, 30, 45, 60, 75 and 90 mg) or phenolphthalein glucuronide (at equivalent doses of 24, 48, 72, 96, 120 and 144 mg) in a random order. Stool weight, the frequency and consistency of stools, and the development of symptoms were recorded at 12-h intervals for 84 h.

RESULTS

There was a significant increase in the mean stool weight obtained within the first 24 h of administration of a 30 mg dose of phenolphthalein and its glucuronide equivalent compared to the values obtained with placebo. A further increase in the dose did not improve the therapeutic response. There was no difference between phenolphthalein and phenolphthalein glucuronide with respect to the rapidity of action, the threshold dose, effectiveness of laxation, or the frequency of adverse effects.

CONCLUSIONS

The therapeutic response and side effect profile of the different doses favoured 30 mg phenolphthalein as the optimum laxative dose. Although theoretically superior, phenolphthalein glucuronide was not found to be a more effective laxative compared to phenolphthalein in normal subjects.

Platelet activating factor and the responses of rat isolated stomach strip to prostaglandin E2

The effect of platelet activating factor (PAF) on contractions evoked by acetylcholine, 5-hydroxytryptamine (5-HT) and prostaglandin E2 (PGE2) was studied in-vitro on rat stomach strip. Addition of PAF to the organ bath increased PGE- but not 5-HT- or acetylcholine-evoked responses. The effect of PAF was unaffected by atropine, methysergide or indomethacin, but prevented by a specific PAF receptor antagonist BN 52021. The data support a specific interaction between PAF and PGE2 on rat stomach strip.