Antagonism of some smooth muscle actions of prostaglandins by polyphloretin phosphate.

1. The antagonism of the smooth muscle stimulating actions of PGF(2a) and PGE(2) by polyphloretin phosphate (PPP) was studied on several isolated smooth muscle preparations and on the blood pressure of the anaesthetized rabbit.2. PPP (2.5-20 mug/ml) reversibly inhibited contractions of the jird colon produced by PGE(2) or PGF(2a); PGF(2a) was more readily antagonized than PGE(2).3. PPP (2.5-30 mug/ml) reversibly antagonized contractions produced by PGE(2) and PGF(2a) on the isolated rabbit jejunum and uterus. In these preparations PPP antagonized PGE(2) as readily as PGF(2a).4. It is concluded that PPP is a selective antagonist to the prostaglandins on these tissues, for contractions produced by other agonists, such as acetylcholine, angiotensin, 5-hydroxytryptamine and bradykinin were not reduced by concentrations of PPP which markedly antagonized responses to the prostaglandins.5. Intravenous injections of PPP (25-200 mg/kg) resulted in a variable antagonism to the fall in blood pressure produced by intravenous injections of PGF(2a) in the anaesthetized rabbit; vasodepressor responses produced by PGE(2) and acetylcholine were not antagonized.6. The mechanism of this antagonism by PPP is not clear and must await further investigation.