br Materials and methods br Results br

Materials and methods
Results
Discussion In the present study, we aimed to elucidate the contribution of adenosine receptors to vascular tone in mice with T1D. We showed that diabetes resulted in decreased A1 adenosine receptor-mediated contraction in the aorta, which was accompanied by increased aortic contraction to PE. However, A1 adenosine receptor-mediated responses in mesenteric arteries were not different between vehicle- and STZ-treated mice, suggesting a differential vascular bed effect of diabetes on adenosine receptor function. Interestingly, A1adenosine receptor expression in the mk-801 was not different between the two groups, suggesting a possibly decreased sensitivity of A1 adenosine receptor to its agonist in the aortas of diabetic mice. Our results are in concordance with earlier studies showing that, in STZ-treated diabetic rats, there was a decrease in A1 adenosine receptor sensitivity to adenosine and impaired Gi coupling, with no change in A1 adenosine receptor expression in adipocytes (Barrington et al., 1996, Green and Johnson, 1991). Furthermore, T1D had a tissue-specific effect on A1 adenosine receptor sensitivity (Barrington et al., 1996), which may be the case in the present study as we did not see a difference in response to A1 adenosine receptor activation in mesenteric arteries, contrary to aortas. The disconnect between the expression and activity/signaling of adenosine receptors is not novel. In fact, our laboratory has previously shown that, while mesenteric arteries express A2A adenosine receptor, this adenosine receptor subtype does not contribute to the regulation of their vascular tone (Teng et al., 2013). Also, another study showed that, while the coronary arteries express A2B adenosine receptor, this receptor did not play an active role in the endogenous regulation of coronary blood flow (Berwick et al., 2010). Interestingly, the same group demonstrated that obesity and metabolic syndrome augmented the contribution of A2B adenosine receptor to adenosine-induced dilation, which was surprisingly associated with a decrease in coronary A2B adenosine receptor protein expression (Bender et al., 2009). Together, these data, with other studies, suggest that diseases such as diabetes may alter receptor sensitivity and/or signaling independent of receptor expression levels. Unfortunately, little is known about the posttranslational modifications of adenosine receptors during both healthy and disease conditions, as is how these modifications (such as O-linked N-acetylglucosamineprotein modification in diabetes) may affect the receptor's intracellular signaling pathways and activity (Palmer and Stiles, 1999). As expected, STZ treatment resulted in an increase in blood glucose levels, accompanied by a significant decrease in body weight. These effects have been well documented by our laboratory and many others (Grden et al., 2005, Grden et al., 2007, Labazi et al., 2016a, Pawelczyk et al., 2005). While we did not observe differences in endothelium-mediated vasorelaxation in aortas and mesenteric arteries from vehicle- and STZ-treated groups, contractile responses to the adrenergic receptor agonist PE was significantly higher in aortas from diabetic mice. These data are in concordance with previous studies (Rehman et al., 2014, Xie et al., 2010). The observed decreased contraction to the A1 adenosine receptor agonist may also be a compensatory mechanism to counteract increased aortic contractility to other agonists, such as PE, in diabetes to regulate vascular tone, at least at this early stage of T1D. The newly considered notion that adenosine levels and adenosine receptor expression are dramatically changed in pathological conditions allowed for a new proposed role for the adenosine receptors. However, our understanding of the contribution of adenosine receptors to the physiology and pathophysiology of the cardiovascular system is still at an early stage, and the findings in the field seemed contradictory. For instance, increased adenosine levels and activation of A2B adenosine receptor signaling was shown to be protective during diabetic nephropathy (Tak et al., 2014). However, increased adenosine and expression of A2B adenosine receptor was shown to contribute to hypertension in chronic kidney disease (Zhang et al., 2013). This contradictory role of elevated adenosine levels and A2B adenosine receptor signaling can be explained by the fact that A2B adenosine receptor signaling can be tissue specific (endothelial cells vs whole kidney) or disease specific (diabetes vs hypertension). In addition, adenosine receptor signaling pathways are still under investigation.