The Effect Of Nicotine On Insulin Sensitivity In Rats

OBJECTIVE:The present study was to investigate the effect and mechanism of nicotine on rat insulin sensitivity.METHODS:Firstly, we study the effect of nicotine on rat insulin sensitivity in vivo. At the age of 10-11 weeks, the male Sprague-Dawley rats were randomly divided into two groups: saline group and nicotine-treated group. Saline group was subcutaneously injected with saline for 6 weeks. Nicotine-treated group was subcutaneously injected with nicotine (3mg/kg/d) for 6 weeks. Body weight was recorded during the treatment period. In the end, the two groups were fasted over night. Blood samples were obtained for detection of serum biochemical indicators including triglyceride (TG), high density lipoprotein (HDL), low density lipoprotein (LDL), glucose, insulin, total cholesterol (Total Cho) and then we calculated the value of insulin/glucose, HOMAβcell function index (HOMA B), HOMA insulin resistance index (HOMA-IR). Then we carried out insulin tolerance test (ITT) and oral glucose tolerance test (OGTT) on the two groups. To investigate the mechanism of the nicotine's effect in rats, we determined PPAR-γprotein expression from inguinal subcutaneous adipose tissue, Mesenteric adipose tissue and vascular smooth muscle between nicotine-treated and saline group by western-blot analysis.Secondly, we use isolated organ bath system to study the effect of nicotine on insulin sensitivity. Animal models were made as mentioned. After six weeks nicotine treatment, rats were sacrificed and the thoracic aortas were taken out. The endothelium and adventitia fat of aorta were removed and aorta was cut into 5-mm-wide rings. Changes in isometric tension were recorded using a computerized system. We study the 10-9 M-10-5 M insulin induced aorta relaxation in two groups. Thirdly, we study the effect of nicotine on insulin sensitivity of vascular smooth muscle cells (VSMCs). We estabilished two insulin-resistance models based on VSMCs. After insulin resistance was induced, VSMC were cultured with nicotine (6×10-6 M) for 48 or 24 hours, then we detected the insulin-stimulating glucose uptake to evaluate the insulin sensitivity effect of nicotine. The insulin-stimulating glucose uptake function of VSMC can be calculated by primitive glucose concentration minus the residual glucose concentration after the stimulation.RESULTS:After 3 weeks treatment, HDL, TG, Insulin/Glucose and HOMA-B were lower in the nicotine-treated group than those in the saline group (P<0.05), and the TG is significantly lower in the nicotine group than that in the saline group (P<0.01). After 6 weeks treatment, the HOMA-IR is lower in the nicotine-treated group than that in the saline group (P<0.05), and TG,insulin and Insulin/Glucose is significantly lower in the nicotine group than that in the saline group (P<0.01).In insulin tolerance test after 3 weeks nicotine treatment, basal plasma glucose levels showed no difference between the two groups. Plasma glucose levels at 30 and 45 min after insulin administration were lower in the nicotine group than those in the saline group. In oral glucose tolerance test after 3 weeks nicotine treatment, HOMA-B value of nicotine group was lower than that in the saline group. In oral glucose tolerance test after 6 weeks nicotine treatment, the value of HOMA-B andΔI30/ΔPG30 in nicotine group were lower than those in the saline group.PPAR-γprotein expression in subcutaneous adipose tissue, Mesenteric adipose tissue and vascular smooth muscle of nicotine-treated rat didn't show statisticly difference from that of saline group.Isolated aorta function test showed that insulin induced more relaxation in nicotine group compared with saline group when insulin concentration accumulated to 10-5M (P<0.05).Both dexamethasone (1μM, 48h) and high concentration insulin (500nM, 24h) culturing can induce insulin resistance in VSMC. Cultured with nicotine (6×10-6M) for 48h, glucose uptake function of VSMC which have been made insulin resistance was enhanced.CONCLUSIONS:1. We verified the nicotine's sensitization effect by conducting insulin tolerance test, oral glucose tolerance test, isolated aorta tension determination and calculating the insulin resistance index (HOMA-IR),βcell function index(HOMA-B), and insulin/glucose. Nicotine can improve the insulin sensitivity in rat but simultaneously deteriorateβcell function.2. Nicotine improves insulin sensitivity in rats not through the PPARγpathway, but partly by enhancing the glucose uptake in vascular smooth muscle cell.