Studies on the incorporation of [1-14C]arachidonic acid into glycerolipids and its conversion into prostaglandins by rabbit iris. Effects of anti-inflammatory drugs and phospholipase A2 inhibitors.

ABSTRACT

The effects of the anti-inflammatory drugs, indomethacin and aspirin, and the phospholipase A2 inhibitors, p-bromophenacyl bromide and mepacrine, on the in vitro metabolism of [1-14C]arachidonic acid by rabbit iris smooth muscle and iris microsomes were investigated. The incorporation of arachidonate into glycerolipids and its conversion into prostaglandins were rapid and time-dependent. About 65% of the total radioactivity was recovered in triacylglycerol, followed by that in phosphatidylcholine (20%), diacylglycerol (6%), phosphatidylethanolamine (5%) and phosphatidylinositol (3%), respectively. Time-course studies on arachidonate release from glycerolipids of prelabelled tissue showed that triacylglycerol, phosphatidylcholine, phosphatidylethanolamine and phosphatidylinositol are the major source for arachidonate in prostaglandin synthesis in this tissue. Arachidonate release from glycerolipids was not blocked by indomethacin and the effects of the phospholipase A2 inhibitors were nonspecific. p-Bromophenacyl bromide inhibited the labelling of glycerolipids in a dose-dependent manner. Mepacrine stimulated the labelling of phosphatidic acid, phosphatidylinositol and diacylglycerol, and inhibited that of phosphatidylcholine, phosphatidylethanolamine and triacylglycerol. At concentrations under 0.25 mM it stimulated prostaglandin synthesis in microsomes and at concentrations over 0.25 mM it inhibited their synthesis in both muscle and microsomes. Indomethacin and aspirin moderately increased the labelling of glycerolipids; however, both drugs inhibited prostaglandin synthesis by iris and iris microsomes in a dose-dependent manner. Possible explanations for mechanisms underlying these effects were presented. It is concluded that the phospholipase A2 inhibitors and the anti-inflammatory drugs exert profound effects on the incorporation of [1-14C]arachidonate into glycerolipids of the rabbit iris and on its conversion into prostaglandins by both iris and iris microsomes.