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3 On other respiratory tissues known to contain mixtures of the 'X1,2,3'-and 'j'-receptors (guinea-pig trachea and lung strip, cat lung strip and human bronchial muscle), TR4979 consistently acted as a potent relaxant whereas PGE2 and to a lesser extent PGE1 had significant contractant activities. 4 Human pregnant uterus, guinea-pig and rat pseudo-pregnant uteri, rat colon and fundic strips and chick ileum are known to contain one or more of the three subclasses of the 'X'-receptor. TR4979 (l09-10-5M) was inactive on all these tissues whereas all of the reference prostanoids were contractants of varying potencies.5 PGEI and histamine-induced contractions of the guinea-pig isolated ileum were both noncompetitively antagonized by increasing concentrations of TR4979 suggesting that '*'-receptors also exist on this tissue. 6 TR4979 is a highly selective agonist of prostanoid '4' (relaxant/inhibitory)-receptors which at present have been demonstrated to exist mainly in the lung. This prostaglandin analogue is a useful new selective pharmacological tool for revealing as yet unidentified prostanoid '4'-receptors and actions in a wide range of non-respiratory tissues/organs such as the guinea-pig ileum.

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Mammalian serum and plasma contain an endogenous inhibitor of prostaglandin synthetase (EIPS). Human plasma fractions rich in EIPS show anti-inflammatory activity in vivo. In rats, glucocorticoids raise EIPS activity of plasma and serum. These findings suggest the existence of a natural mechanism of controlling prostaglandin synthesis, possibly related to corticosteroid action.