Constitutive Release of Purines in Control of Vascular Contractility

Abstract

Most large and medium sized arteries are surrounded by perivascular adipose tissue (PVAT). PVAT was once considered a passive structural component of the vascular wall, not an active regulator of vascular homeostasis. However, PVAT releases several active mediators that have a paracrine effect on vascular tone. Adipocytes express P2X and P2Y receptors, whose activation influences a number of adipocyte functions including the release of adipokines. Adipocytes can also release nucleotides to control adipocyte activity. It is unknown whether this occurs in PVAT and alters the tone of the adjacent vasculature. The constitutive release of purines in blood vessels is relatively unknown. However, it is important to understand vascular control mechanisms to offer new therapeutic approaches for cardiovascular diseases such as hypertension. The main purpose of this study was to determine if nucleotides (ATP/UTP) are released constitutively from PVAT to regulate the vascular tone of porcine mesenteric artery. Further investigation was performed to examine the possibility that P2X, P2Y, and adenosine receptors may contribute to the vasorelaxant effect induced by AR-C118925XX (a P2Y2 receptor antagonist). Additionally, I investigated whether the nucleotides ATP and UTP activate P2Y receptors to release adiponectin and leptin from 3T3-L1 adipocytes. I also examined whether the anti-contractile effect of PVAT in response to ATP and UTP in porcine splenic arteries may be associated with adiponectin release. Isometric tension recordings were performed using mesenteric arteries obtained from large white hybrid pigs with and without PVAT. A direct vasorelaxant effect of suramin (P2 receptor antagonist), AR-C118925XX and MSG228 (P2Y2 receptor antagonists) was found on U46619-preconstricted vascular tone of porcine mesenteric arteries. Apyrase (metabolises nucleotides) abolished the vasorelaxant response of preii contracted mesenteric arteries to AR-C118925XX. Additionally, apyrase caused a dual effect of contraction and relaxation in pre-contracted mesenteric arteries. ARL67156 (ectonucleotidases inhibitor) directly caused an initial small contraction in U46619- pre-constricted mesenteric artery tone. Further experiments demonstrated that the connexin/pannexin channel inhibitors carbenoxolone and probenecid also caused a relaxation of U46619-pre-constricted vascular tone in porcine mesenteric arteries. In this study, evidence ruled out the involvement of PVAT, endothelium and P2X and adenosine receptors in nucleotides release and responses to P2 receptor antagonists, suggesting that nucleotide release and actions involves the vascular smooth muscle. Relaxation of AR-C118925XX was also observed in further vascular beds and species in this study, specifically porcine coronary and splenic arteries, as well as rat mesenteric arteries. Extracellular ATP was measured by luminescence directly from the Krebs solution in porcine mesenteric arteries. It appears that isolated mesenteric arteries are capable of releasing ATP and the presence of PVAT in this experiment increased the amount of ATP released from isolated mesenteric arteries. However, there were no differences in the effects of P2 receptor antagonists in the presence or absence of PVAT on mesenteric artery tone. The present study also identified the role of extracellular nucleotides ATP and UTP in adiponectin release from 3T3-L1 adipocytes via activation of P2Y2 receptors. Adiponectin release was not regulated by constitutive activation of P2Y2 receptors. In isolated arteries, the presence of PVAT in splenic, but not mesenteric, arteries decreased the level of contraction induced by ATP and UTP. The anti-contractile effects of PVAT on porcine splenic arteries in response to ATP and UTP might be mediated by adiponectin and other relaxant factors. Together, these results demonstrated that endogenous ATP/UTP is released constitutively from vascular smooth muscle via connexins and pannexins to act on vasocontractile P2Y2 receptors.