Nicotine, as a constituent of cigarette smoke, has been identified as a major contributor to vascular disease. It has previously been reported that nicotine and cotinine exposure caused an impairment of endothelial-dependent vasorelaxation in porcine carotid and coronary arteries, where significantly, cotinine, the major metabolite of nicotine, was shown to possess the greater effect (Conklin et al., 2001). In the present study we have examined the effects of prolonged nicotine and cotinine exposure on endothelial-dependent relaxation in mesenteric resistance-like arteries.

Male Wistar rats (240-260g) were sacrificed by CO₂ asphyxiation. Mesenteric arteries were dissected and treated with 6-hydroxydopamine (2 mM) and capsaicin (0.1 mM) for 30 minutes in order to remove nerve influences. Following this, some vessels were incubated in DMEM containing nicotine (10^-7M), cotinine
(10^-7M) or vehicle for a period of 2 or 7 days. Some arteries were tested on the day of isolation (day 0), with and without nicotine present. 2 mm segments of artery were mounted on a wire myograph under normalised tension in oxygenated (95% O₂/5% CO₂ Krebs’ buffer maintained at 37°C. Maximum contraction to KCl (120 mM) was initially determined and sub maximal tone subsequently induced using phenylephrine (0.1-10 μM) in the presence of nifedipine (0.3 μM). Endothelial-dependent responses to acetylcholine (ACh) were used as a measure of endothelial function. Maximal responses to ACh (E_max) are expressed as percent relaxation of active tone (mean ± SEM) and differences in pEC₅₀ and E_max determined by ANOVA followed by Dunnett’s post test.

Table 1 pEC₅₀ and E_max values for ACh-induced vasodilation in mesenteric arteries pre-treated with nicotine (10^-7M), cotinine (10^-7M) or vehicle.

* P<0.05, ** P<0.01, denotes difference between day 0 control and treated vessels .

Incubation of vessels in culture led to a time-dependent reduction in endothelial-dependent relaxation, an effect which reached significance at the 7 day time point with respect to E_max. Interestingly, however, E_max in vessels treated with nicotine or cotinine did not differ significantly from control vessels at either time point assessed (Table 1). These data suggest that at concentrations noted in plasma following smoking, exposure to nicotine or cotinine over a time period of up to seven days does not reduce the endothelial-dependent relaxatory response to ACh in resistance-like arteries. Indeed, both drugs appear to protect against the decline in this function over time in culture.