Maternal smoking during pregnancy causes reduction of fetal breathing movements, an effect attributed to nicotine in fetal blood. Nicotine is metabolized to cotinine which has a long plasma half-life and exhibits slow clearance across membrane barriers. It is also known to activate placental phospholipase-A₂-like enzymes, resulting in formation of prostaglandins. Therefore, we studied transport of nicotine in isolated perfused cotyledon of normal human term placenta. The placental cotyledon was perfused with aerated (21% O₂, 5% CO₂) Krebs-Ringer bicarbonate buffer (pH 7.4, 37°C) containing 2% albumin on both maternal (230 ml, 15 ml/min, 35 mm Hg) and fetal (93 ml, 1.75 ml/min, 70 mm Hg) sides in a closed recirculating system. Nicotine (2 mg) was added to the maternal perfusate; perfusate samples (1 ml) were collected from both sides at regular intervals and analyzed for nicotine and cotinine by high-pressure liquid chromatography. This study gave the following results: (1) In about 60–80 min, 18.6% of the nicotine added to the maternal perfusate was transferred to the fetal perfusate, and the maternal/fetal concentration ratio reached 1.0. These results show rapid placental transfer of nicotine, consistent with its high lipid solubility. (2) Less than 1% is metabolized to cotinine in placenta. The ratio of cotinine concentrations in maternal and fetal perfusates reached 1.0 in about 40 min. These studies were also verified using ¹⁴C-nicotine. (3) Maximal reduction in fetal breathing movements occurs at about 30 min, and recovery occurs at 90 min after tobacco smoking by the mother. These observations agree with the rate of placental transfer of nicotine. (4) When nicotine was added on the fetal side, part of it was metabolized to cotinine. However, the maximal concentration of cotinine was twice higher on fetal than on maternal side. These observations suggest that accumulation of cotinine on fetal side may activate prostaglandin formation and trigger spontaneous abortions in pregnant smokers.

Tobacco smoking by pregnant women increases the frequency of spontaneous abortions and preterm births. Human labor is associated with enhanced intrauterine phospholipid metabolism and production of prostaglandin E₂ (PGE₂) which induces labor, initiates uterine contractions and maintains the homeostasis of placental blood flow. Therefore, we studied: (a) the influence of nicotine and cotinine on the effects of PGE₂ on placental vasculature in perfused human placental cotyledon, and (b) the activation of placental phospholipase A₂ (PLA₂) by nicotine and cotinine using 1-palmitoyl-2-[1-¹⁴C]arachidonyl-phosphatidylethanolamine (PE, 2.2 nmol) as substrate. These studies revealed that: (1) increasing concentrations of PGE₂ (10– 150 ng/ml) increased umbilical perfusion pressure by 170 ± 10% (n = 6) of control (100%). Cotinine (2 µg/ml) enhanced this effect at all concentrations of PGE₂. Nicotine (2 µg/ml) prevented the effect of PGE₂; (2) both cotinine (EC₅₀ 470–500 fmol/l) and nicotine (EC₅₀ 18–32 pmol/l) activated PLA₂ in human placental tissues. These observations indicated that cotinine was more potent than in nicotine activating PLA₂ and potentiating the vasoconstrictive effects of PGE₂ on fetal placental circulation. Nicotine activates nicotinic receptors and releases placental acetylcholine, a vasodilator of placental arteries. Acetylcholine stimulates muscarinic receptors of endothelial cells resulting in the release of endothelium-derived relaxing factor (EDRF), and possibly nitric oxide. Therefore, nicotine prevents or abolishes the vasoconstrictive effects of PGE₂ through the release of EDRF. Cotinine is inactive at nicotinic and muscarinic receptors. Therefore, accumulation of cotinine, the major metabolite of nicotine, in fetal circulation may contribute to production of PGE₂ and induction of preterm labor and spontaneous abortions.

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.

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