Studies on prostaglandin antagonists.

1. Three prostaglandin antagonists have been examined for their ability to block PGE(2) and PGF(2alpha) on human, guinea-pig and isolated rat gastrointestinal muscle.2. 7-oxa-13-Prostynoic acid was either a non-selective antagonist, or was ineffective on the tissues studied; it had marked spasmogenic activity on the rat fundus.3. 1-Acetyl-2-(8-chloro-10,11-dihydrodibenz (b,f)(1,4) oxazepine-10-carbonyl) hydrazine selectively antagonized the excitatory effects of PGE(2) and PGF(2alpha) in guinea-pig and rat tissues, but not in human muscle.4. Polyphloretin phosphate selectively antagonized the excitatory effects of prostaglandins in both human and guinea-pig muscle preparations, but it caused stimulation of the rat fundus.5. All the antagonists lowered the tone in many tissues. They also reduced contractions caused by potassium.6. None of the compounds blocked the inhibitory effect of PGE(2) on intestinal circular muscle.7. The implication of these results on the nature of prostaglandin receptors, and the value of each compound as a prostaglandin antagonist are discussed.