Prostaglandins A1,A2 and 19-hydroxy A1; their actions on smooth muscle and their inactivation on passage through the pulmonary and hepatic portal vascular beds.

1. Prostaglandins A(1), A(2) and 19-hydroxy A(1) have qualitatively similar actions to prostaglandin E(1) on smooth muscle.2. The prostaglandins A have little activity on gastrointestinal, respiratory and reproductive smooth muscle but are potent depressors of the systemic arterial blood pressure of the dog, cat and rabbit.3. Our experiments support the view that the depressor action of the prostaglandins E and A is due to a direct dilator action on many peripheral vascular beds and not due to changes in nervous tone to these beds.4. A single passage through the pulmonary circulation of the cat or dog causes substantial loss of the vasodilator activity of prostaglandin E(1) but little if any loss of the vasodilator activity of the prostaglandins A.5. A single passage through the hepatic portal circulation of the cat causes substantial loss of the vasodilator activity of prostaglandins E(1), A(1) and A(2).6. The cat blood pressure and rat fundal strip would be a suitable combination for the parallel biological assay of the prostaglandins A(1) and A(2).