Gut-blood vessel interactions: short-chain fatty acids and protease-activated receptors
Creators: Poitras, Erika 
Publisher: name: University of Alberta Library
Year: 2022

<p>Background: Gut microbiota and diet, specifically fiber rich diets, have been associated with blood pressure regulation and cardiovascular health. Gut microbiota metabolize fiber into short-chain fatty acids (SCFAs) of which, acetate is the most abundant, followed by propionate and butyrate. It has been reported that these SCFAs cause a reduction in blood pressure in vivo and that this may be due to their direct vasorelaxation effect on blood vessels. My goal is to further understand gut to blood vessel interactions, specifically by investigating the effect of these SCFAs on blood vessels and the mechanism through which they may mediate their effects. In my research I aimed to address the hypothesis that SCFAs have direct vasodilator actions on resistance arteries mediated through endothelium-dependent mechanisms. In addition to this, I also aimed to investigate blood vessel to gut interactions with focus on regulation of gut motility. Gut motility is influenced by many factors including hormones, neural inputs, and pacemaker interstitial cells of Cajal (ICC). These pacemaker ICC express a receptor tyrosine-protein kinase, known as c-kit, that allows them to differentiate and develop and can be used as a cell surface marker for ICC. Previous work in our lab has shown that activation of protease-activated receptor 2 (PAR-2) by serine protease, trypsin, inhibits ICC initiated spontaneous contractile activity of colonic smooth muscle. Serine proteases that can activate PAR-2 such as trypsin, factor VIIa, and factor Xa are transported in the blood. My goal with this section of my research was to further elucidate blood vessel to gut interactions, focusing on how PAR-2 activation may influence ICC activity in the rat mid-colon. I aimed to address the hypothesis that PAR-2 protein is located on c-kit+ ICC and influences gut motility through direct action on ICC in the rat mid-colon.<br>
Methods: To address the first hypothesis, investigation of the direct effects of SCFAs on vascular tone was done by constructing cumulative concentration-response curves to SCFAs in isolated mesenteric arteries mounted in a wire myograph and pre-stimulated with phenylephrine (PE). To determine the effects of SCFAs within the mesenteric vascular bed, nerve-evoked vasoconstriction was used to construct frequency-response curves to perivascular nerve stimulation in a perfused mesenteric bed in the presence and absence of SCFAs. Pharmacological inhibitors were used to investigate the mechanism of action of SCFAs. In order to address my second hypothesis, sections of rat mid-colon were collected and prepared for immunofluorescence staining using antibodies to PAR-2, α smooth muscle actin, and c-kit prior to imaging.<br>
Results: In isolated arteries, sodium acetate (0.3 - 100 μM), sodium propionate (3 - 100 μM) and sodium butyrate (0.3 – 100 μM) caused reductions in PE-evoked tone. These effects were differentially inhibited by combinations of the pharmacological inhibitors of nitic oxide synthase and endothelial Ca2+-activated K+ channels. In the perfused mesenteric bed, nerve-evoked vasoconstriction was not altered by SCFAs at μM concentrations. However, at a concentration of 30 mM, sodium acetate and sodium propionate did attenuate responses. The PAR-2 immunofluorescence imaging presented staining of PAR-2, α smooth muscle actin and c-kit in the intestinal wall but no apparent overlapping of PAR-2 and c-kit staining was present in the intestinal wall of the rat mid-colon.<br>
Conclusion: These data indicate that SCFAs may cause direct vasorelaxation in mesenteric resistance arteries via activation of endothelium-dependent pathways. SCFAs did not have a significant effect on the perfused mesenteric vascular bed at μM concentrations but did have a significant effect at mM concentrations. Immunofluorescence images of the rat mid-colon indicate that PAR-2 is present in the intestinal wall, but it is not located on ICC. This suggests that it is improbable that PAR-2 activation inhibits ICC-mediated spontaneous smooth muscle contractile activity through direct action on ICC. Future studies are required to determine how PAR-2 activation may influence ICC activity. Overall, this work presents examples of the complex interaction between the gastrointestinal tract and the cardiovascular system. However, further research is required to better explain how SCFAs may be influencing blood pressure regulation and how PAR-2 activation may influence gastrointestinal motility. </p>