Influence of nicotine and cotinine on retinal phospholipase A(2) and its significance to macular function

The macula is a constituent of the sensory retina that is necessary for sha rp contrast and color vision. A significant relationship has been found bet ween tobacco smoking and age-related macular degeneration. Opsin, rhodopsin and phospholipase A(2) (PLA(2)) are located in excitable membranes of reti na which are enriched with polyunsaturated fatty acids (PUFA). A question m ay arise as to whether nicotine and its major metabolite cotinine influence PLA(2) so that arachidonic acid (AA) and proinflammatory prostaglandins (P G) are produced in the retina. Therefore, the effects of nicotine and cotin ine on the retinal. PLA(2) were studied. PLA(2) activity of rat retinal sonicates was assayed using 1-palmitoyl-2[1- C-14]arachidonyl-Phosphatidylethanolamine (PE, 2.2 nmol) as a substrate in Tris buffer (10 mM, pH 7.4) at 37 degrees C with and without nicotine or co tinine in the assay medium. These studies gave the following results: (I) R at retinal PLA(2) activity was 4.2 +/- 0.8 pmol PE hydrolyzed/100 ng protei n/hr. (2) Nicotine in low concentrations (1-150 nM) activated PLA(2) (EC50 5nM). (3) Cotinine also activated PLA(2) (EC50 300 nM). (4) Only high conce ntrations of nicotine (> 1.0 mu M) and cotinine (> 25 mu M) exhibit inhibit ion of PLA(2). (5) All three known PLA(2) inhibitors, mepacrine, 4-bromophe nacyl bromide and bromoacetylcholine bromide (IC50: 0.5mM, 88 mu M, 30mM, r espectively) inhibited retinal PLA(2) activity. These observations suggest that polyunsaturated fatty acids are cleaved, and arachidonic acid, the pre cursor for prostaglandins and related pro-inflammatory mediators, is libera ted by nicotine and cotinine. Oxidative stress (reduced levels of antioxida nts), vascular insufficiency, as well as activation of PLA(2) by nicotine a nd cotinine may contribute for retinal degeneration in smokers during aging .