Ans. is 'a' i.e.. Water retention o Prostaglandins and thromboxanes are synthesised from arachidonic acid cobtained from membrane phospholipids due to action of phospholipase A2, rate limiting enzyme. A summary of the actions of major prostaglandins, prostacyclin and thromboxane Organ Prostaglandin E2 (PGE2a) Prostaglandin F2a (PGF2a) Prostacyclin (PGI2) Thromboxane A2 (TXA2) 1. Blood vessels Vasodilatation. X BP Vasodilatation, larger veins constrict, little effect on BP Vasodilatation (marked and widespread), || BP Vasoconstriction 2. Heart Waek inotropic, reflex cardiac stimulation Weak inotropic -- -- 3. Platelets Variable effect -- Antiaggregatory Aggregation and release reaction 4. Uterus Contraction (in vivo), relaxes nongravid human uterus in vitro, so fining of cervix contraction (in vivo and in vitro, softening of cervix 5. Bronchi Dilatation. Inhibit histamine release Constriction Dilatation (mild), inhibit histamine release Constriction 6. Stomach | acid secietion, | mucus production -- | acid secretion (weak), mucosal vasodilatation -- 7. Intestine Contracts longitudinal & relaxes circular muscles, | peristalsis, | Cl & water secretion Spasmogenic, | fluid &. Electrolyte secretion (weak) Weak spasmogenic, inhibit toxin-induced fluid secretion Weak spasmogenic 8. Kidney Natriuresis, | Cl reabsorption, inhibit ADH action, vasodilatation, renin release -- Natriuresis, vasodilatation, renin release Vasoconstrict ion 9. CNS Pyrogenic, variety of effects on i.v. inj.