Influence of Nicotine and Cotinine on Retinal Phospholipase A2 and its Significance to Macular Function

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Abstract
The macula is a constituent of the sensory retina that is necessary for sharp contrast and color vision. A significant relationship has been found between tobacco smoking and age-related macular degeneration. Opsin, rhodopsin and phospholipase A2 (PLA2) are located in excitable membranes of retina which are enriched with polyunsaturated fatty acids (PUFA). A question may arise as to whether nicotine and its major metabolite cotinine influence PLA2 so that arachidonic acid (AA) and proinflammatory prostaglandins (PG) are produced in the retina. Therefore, the effects of nicotine and cotinine on the retinal PLA2 were studied. PLA2 activity of rat retinal sonicates was assayed using 1-palmitoyl-2[1-14C]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 cotinine in the assay medium. These studies gave the following results: (1) Rat retinal PLA2 activity was 4.2+/-0.8 pmol PE hydrolyzed/100 ng protein/hr. (2) Nicotine in low concentrations (1-150 nM) activated PLA2 (EC50 5 nM). (3) Cotinine also activated PLA2 (EC50 300 nM). (4) Only high concentrations of nicotine (> 1.0 microM) and cotinine (> 25 microM) exhibit inhibition of PLA2. (5) All three known PLA2 inhibitors, mepacrine, 4-bromophenacyl bromide and bromoacetylcholine bromide (IC50: 0.5mM, 88 microM, 30 mM, respectively) inhibited retinal PLA2 activity. These observations suggest that polyunsaturated fatty acids are cleaved, and arachidonic acid, the precursor for prostaglandins and related pro-inflammatory mediators, is liberated by nicotine and cotinine. Oxidative stress (reduced levels of antioxidants), vascular insufficiency, as well as activation of PLA2 by nicotine and cotinine may contribute for retinal degeneration in smokers during aging.