| Objective:To investigate the role of melatonin in the improvement of mesenteric arterial function induced by aerobic exercise in spontaneously hypertensive rats (SHR) and to explore the possible peripheral vascular mechanisms.Methods:A total of 42 twelve-week-old male rats were used in the experiments. A group of normotensive WKY rats (n=8) were used as controls and the SHRs were separated into five groups:SHR controls (SHR-SED, n=8), SHR with sterile saline (SHR-SED+vehicle, n=6), SHR with melatonin receptor blocker luzindole (SHR-SED+luz, n=6), SHR exercise training (SHR-EX, n=8), and SHR exercise training with luzindole (SHR-EX+luz, n=6). Luzindole were injected 1mg/kg/day intraperitoneally. Rats in the aerobic exercise group were subjected with 8 week-treadmill exercise,0°slope,18-20 m/min,60 min/d,5 d/wk. Blood pressure and heart rate were measured in awake animals before the beginning of the exercise training protocol and at the end of the protocol using tail-cuff method. Serum melatonin levels were examined by ELISA with commercially available kits. The mechanical properties of small mesenteric arteries were studied with myograph. Western blotting and immunofluorescence colocalization were performed to examine the protein expression and distribution of MT1/MT2 and eNOS.Results:(1) After exercise training for 8 weeks, systolic blood pressure (SBP), diastolic blood pressure (DBP), mean arterial pressure (MAP) and heart rate (HR) were significantly lower in SHR-EX group than those in SHR-SED group. SBP and HR in SHR-EX+luz group was greater compared to SHR-EX group. However, there were no significant differences of SBP and HR between SHR-SED group and SHR-SED+luz group. (2) The melatonin serum levels in WKY group and SHR-SED group reached peak levels at 21:00-22:00 pm and were significantly higher than those at 12:00-13:00 pm and 8:00-9:00 am. However, in SHR-EX group, it reached the peak level at 8:00-9:00 am. In these three time periods of day, melatonin serum concentrations in WKY group were always significantly higher than those in SHR-SED group, while at 8:00-9:00 am they were significantly higher in SHR-EX group than those in WKY and SHR-SED groups. (3) ①Melatonin (MT,10-6～10-3 M) caused a concentration-dependent relaxation in small mesenteric arterial beds precontracted by NE (105 M), with pIC50 value WKY> SHR-EX> SHR-SED. In the presence of L-NAME (eNOS inhibitor,10-4 M), the pIC50 of the concentration-response curve for melatonin-induced relaxation was lower than in the absence of L-NAME in both SHR and WKY rats. ②The endothelium-dependent relaxation induced by ACh in NE-contracted small mesenteric arteries was reduced in SHR-SED compared with WKY and exercise training improved this relaxation. MT (10-4 M) pre-treatment potentiated the vasorelaxation in response to ACh (10-9～10-5 M), but not the nitric oxide donor, sodium nitroprusside (SNP,10-9-10-5 M).③The vasorelaxation by melatonin was blocked by the melatonergic receptor antagonist, luzindole (2×10-6M). (4) Immunohistochemistry and western blotting showed that MT1/MT2 and eNOS expression in SHR-SED was significantly decreased compared with WKY and exercise training attenuated this reduction. (5) Immunohistochemistry showed that eNOS was localized primarily in the intimal endothelium layer of the mesenteric artery, whereas MT2 receptor appeared more evenly distributed throughout the vessel wall (media, intima, adventitia). Immunohistochemical analysis revealed that MT2 receptors colocalized with eNOS in the endothelial cell layer.Conclusions:Aerobic exercise can reduce basal blood pressure and enhance mesenteric artery vasodilation, in which melatonin plays an important role. Melatonin possibly acts on the vascular endothelium and/or melatonin receptors to increase mesenteric arterial relaxation. |
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