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The in vitro effects of nicotinic acid (10-1000 microM), pyridoxine (0.1-500 microM) and pyridoxal-5'-phosphate (0.1-500 microM) and the ex vivo effects of nicotinic acid (2500 mg orally during 12 h) and pyridoxine (600 mg orally daily for seven days) on arachidonic acid metabolism were investigated in calcium ionophore A23187 (calcimycin)-stimulated human whole blood. In vitro nicotinic acid stimulated prostaglandin E2, thromboxane B2 and leukotriene E4 synthesis. Pyridoxine at all concentrations and pyridoxal-5'-phosphate at the highest concentration stimulated prostaglandin E2 and thromboxane B2 production, but had no effect on leukotriene E4 synthesis. Nicotinic acid treatment increased ex vivo prostaglandin E2, thromboxane B2 and leukotriene E4 synthesis to 185%, 165% and 175% of the initial values, respectively. In the pyridoxine-treated subjects, ex vivo prostaglandin E2, thromboxane B2 and leukotriene E4 synthesis was decreased after seven days to 75%, 65% and 45% of the initial values, respectively. In the present study the effects of nicotinic acid on the 5-lipoxygenase pathway in arachidonic acid metabolism were studied for the first time and the drug was found to stimulate this pathway in vitro and ex vivo. In vitro pyridoxine and pyridoxal-5'-phosphate had no effect on the 5-lipoxygenase pathway. The inhibition of leukotriene synthesis by pyridoxine ex vivo might be of therapeutic importance.

This study investigated the effects of smoking cessation with and without nicotine substitution on the excretion of major urinary metabolites of thromboxane A2 and prostacyclin, 11-dehydrothromboxane B2 and 2,3-dinor-6-ketoprostaglandin F1alpha, respectively, as well as on the excretion of leukotriene E4 in man. Urine samples were obtained from 20 healthy non-smoking controls and from 60 healthy smoking volunteers before, and 3, 7 and 14 days after smoking cessation. Fifteen smokers quit smoking without nicotine substitution, 15 used nicotine chewing gum and 30 used nicotine patches as a substitution therapy. Urinary thiocyanate as well as cotinine and trans-3'-hydroxycotinine excretions were used as compliance and nicotine substitution indicators. 11-Dehydrothromboxane B2, 2,3-dinor-6-ketoprostaglandin F1alpha and leukotriene E4 excretion was about two, three and five times higher in smokers than in controls, respectively. Three days after smoking cessation without nicotine substitution, 11-dehydrothromboxane B2 and 2,3-dinor-6-ketoprostaglandin F1alpha levels were lowered to 75% (P<0.01) and 80% (P<0.05) of the initial values, and after 14 days to 50% (P<0.01) and 60% (P<0.05), respectively. In 3 days leukotriene E4 excretion was dropped to 70% of the initial value (P<0.05), but no further decrease was observed during the study. In individuals using nicotine chewing gum or nicotine patches no significant changes were observed in the analytes during the 2-week follow-up. The increased systemic eicosanoid synthesis observed in smokers may be involved in the harmful cardiovascular effects of smoking. The fact that eicosanoid production remains at pre-cessation level in volunteers who quit smoking but use nicotine substitution may be involved in the risk of cardiovascular complications reported during nicotine replacement therapy.

The effects of nicotinic acid (2500 mg orally during 12 hr) and pyridoxine (300 mg orally twice daily for seven days) on the excretion of urinary 2,3-dinor-6-ketoprostaglandin F 1a , 11-dehydrothromboxane B 2 and leukotriene E 4 , the markers of systemic prostacyclin, thromboxane A 2 and cysteinyl leukotriene production, respectively, were investigated in healthy male volunteers (nΩ6-8). Nicotinic acid increased 11-dehydrothromboxane B 2 and leukotriene E 4 excretions to 2.6-and 2.0 times the initial values (PϽ0.05), respectively. In the volunteers treated with pyridoxine, 11-dehydrothromboxane B 2 and leukotriene E 4 excretions were decreased to 70% (PϽ0.05) and 65% (PϽ0.01) of the initial values, respectively, but the excretion of 2,3-dinor-6-ketoprostaglandin F 1a was increased 1.7 times (PϽ0.01). The results suggest that nicotinic acid increases thromboxane and leukotriene synthesis which may not be beneficial for patients with cardiovascular diseases or asthma. In contrast, the increase in prostacyclin production and the inhibition in thromboxane and leukotriene synthesis by pyridoxine might be beneficial in disorders where the production of prostacyclin is decreased and the formation of thromboxane and cysteinyl leukotrienes is enhanced.

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