The present study was designed to test whether oral challenge with an established periodontal pathogen amends vascular responsiveness in a murine model of spontaneous atherosclerosis. Surprisingly, our data show that oral P gingivalis infection not only increases the vascular contractile response to phenylephrine in atherosclerotic mice but also in systemically healthy mice. The ApoE model of atherosclerosis used in this study is well established [27, 28]. In agreement with previous studies we detected high levels of plasma cholesterol in ApoE mice fed a standard chow diet [28–31]. We also noted that the systemic inflammation observed in this study is consistent with the fact that atherosclerosis is a systemic inflammatory disease . An important finding of this study is that oral P gingivalis inoculation did not change the systemic inflammatory status in atherosclerosis, a result consistent with that described by Miyamoto et al. . In healthy animals it was not detected systemic inflammatory response induced by oral P gingivalis infection, although some authors have reported an increase in systemic inflammatory markers such as C-reactive protein, interleukin 6 and neutrophils in subjects with periodontitis [6–8]. Therefore, more studies are necessary to elucidate this controversial data. One consideration and limitation is that in the present study we investigated the effects of only one periodontal pathogen, while human periodontitis encloses several microorganisms.
The oral P gingivalis infection did not influence the serum cholesterol levels. This result is in agreement with those found in mice infected with P gingivalis and humans  with periodontitis. Although literature reports an increase in the area of atherosclerotic lesion caused by P gingivalis in animal models [4, 5, 33, 34], in this study we did not find changes in the atherosclerotic lesion area. We attribute this difference to different methodologies used in the studies. Li et al.  reported increase in the area of injury only after 14 intravenously weekly inoculations of P gingivalis (107 CFU) in atherosclerotic mice. Animals inoculated for 10 weeks did not show any change in the area of plaque . In our study we performed only three inoculations, and to mimic normal conditions for which periodontal pathogens could reach the circulatory system, the inoculation of P gingivalis was oral topical, and not intravenously as described by those authors. Lalla et al.  also observed increased aortic lesions in ApoE mice after 15 inoculations (1012 CFU) with P gingivalis. Again, the number of inoculations in that study was high and was performed by oral gavage and anal topical application; the latter mode used to establish a cycle of oral reinfection, because mice use to be coprophagic. In the present study, however, the extent of the injury may not have changed because we used smaller burden of pathogens (109 CFU), the inoculations were less sparse and smaller in number, and because we used only oral topical inoculation, that is more compatible with periodontitis in humans. Despite of a great difficulty in establishing a model of periodontitis similar to periodontal disease in humans, P gingivalis inoculation reproduces the periodontal tissue destruction found in humans, making this model well accepted to study periodontitis.
Endothelial dysfunction has been considered one of the early steps in atherosclerosis . Although endothelial dysfunction has been frequently considered when an impaired endothelium-dependent vasodilation is observed, the localized modulation of vascular endothelium to a nonadaptive functional state can be termed as endothelial dysfunction . In ApoE mice, endothelial dysfunction, taken as an impaired endothelium-dependent dilation, is controversial. Endothelial dysfunction can be detected or not in ApoE mice depending on the type of diet, age, gender, and type of vessel [36–45]. Recently it was shown in the mesenteric vascular bed from male ApoE mice vascular dysfunction, characterized by increased pressor responsiveness to norepinephrine, despite of normal endothelium-dependent and -independent relaxations . Similarly, we observed a hyperreactive response to phenylephrine in atherosclerotic animals without changes in endothelium-dependent and -independent vasodilations to acetylcholine and sodium nitroprusside, respectively. At present, the mechanism by which hypercholesterolemia alters vascular responsiveness in mesenteric arteriolar bed is unknown and further studies will try to elucidate this issue.
Interestingly, the hyperreactivity to the α-adrenoceptor agonist in ApoE mice infected with P gingivalis was exacerbated when compared with noninfected ApoE animals. In the systemically healthy mice, the response to phenylephrine was also increased, but if we compare its maximal responses, the hyperreactivity was more pronounced in ApoE mice. The mechanism by which oral P gingivalis infection interferes with the reactivity to phenylephrine is unknown. However, based on the finding of an increased production of endothelin in crevicular fluid in subjects with periodontitis  and that the actions of endothelin include cell proliferation, migration and contraction , we speculate that one of the possible mechanisms by which periodontitis leads to exacerbated pressor response to α-adrenoceptor agonists could be the increase of systemic levels of endothelin.