Wragg, Edward
(2023)
Cardiovascular responses induced by selective adenosine A2A and A2B receptor agonists.
PhD thesis, University of Nottingham.
Abstract
Adenosine is an important modulator of the cardiovascular system, acting at four GPCRs, known as the adenosine receptors. A crucial role of two of those receptors, adenosine A2A and A2B receptors, is the induction of vasodilation in resistance arteries that comprise vascular beds across the body, leading to an increase in the ease with which blood flows, known as vascular conductance, in these beds. As a result of these properties, both A2 receptors have been suggested to have therapeutic potential in treating hypertension and other diseases associated with reductions in blood flow to various regions of the body. However, adenosine receptor expression across the different vasculature beds that make up the circulatory system is neither universal nor ubiquitous, and neither are the physiological changes induced by activation of these receptors; different regions of the body have different sensitivities and responses to adenosine through differences in adenosine receptor expression and function across the body. To gain a greater understanding of how A2A and A2B receptors influence the cardiovascular system, this project aimed to investigate the physiological consequences and potential uses of adenosine A2A and A2B receptor activation on the regional flow of blood in the renal, mesenteric and hindquarter vascular circulations, in conscious, freely moving rats, using Doppler flowmetry.
First, functional responses in conscious rats to adenosine, the A2A receptor agonist CGS 21680, and the A2B receptor agonist BAY 60-6583 were investigated in the presence or absence of adenosine A2A and A2B receptor antagonists, SCH 58261 and PSB 1115, respectively. Adenosine was shown to cause increases in the vascular conductance of all three vascular beds studied. It was discovered that A2A receptor activation increased vascular conductance in the hindquarters vascular beds, but had limited effects on vascular conductance in the renal and mesenteric vascular beds. The opposite was the case with the A2B receptor, which caused increases in the vascular conductance in the renal and mesenteric vascular beds, but not the hindquarters. Both A2A and A2B receptor activation induced tachycardia, but only agonism of the A2A receptor caused a decrease in mean arterial blood pressure.
Second, experiments were undertaken to ascertain if the physiological responses observed due to CGS 21680 (A2A receptor) and BAY 60-6583 (A2B receptor) treatment were in part caused by a secondary reflex-driven increase in sympathetic nervous system activity resulting in catecholamine stimulation of the heart and vasculature, causing the activation of adrenergic β1 and β2 receptors, respectively. Haemodynamic experiments involving the pretreatment of the β1 selective antagonist CGP 20712A, the β2 selective antagonist ICI 118,551 and the non-selective β antagonist propranolol showed that the increase in heart rate caused by both A2 receptor agonists was primarily caused by secondary activation of β1 receptors in the heart. Additionally, it was demonstrated that the vasodilatory effects of A2A receptor activation on the hindquarters were not caused by, or enhanced by secondary stimulation of β receptors. Of interest, β2 receptor antagonism appeared to enhance the haemodynamic response to A2B receptor activation, including increases in heart rate and increases in regional vascular conductances.
Third, in vivo experiments were undertaken with sunitinib, a chemotherapeutic receptor tyrosine kinases inhibitor (RTKI) known to inhibit vascular endothelial growth factor receptor 2 (VEGFR2) signalling, which over a three-day protocol caused hypertension associated with reductions in vascular conductance in the vascular beds that make up the renal, mesenteric and hindquarters. Experiments demonstrated that the hypertension and reductions in vascular conductance could be temporarily reversed by CGS 21680 administration via A2A receptor activation.
Finally, to explore the link between VEGFR2 and adenosine A2 receptor signalling, explorative in vitro studies were conducted. Cyclic adenosine monophosphate (cAMP) accumulation assays found that vascular endothelial growth factor A165a (VEGF-A165a) acting at VEGFR2 enhanced an A2A receptor-induced cAMP response, with this enhancement able to be blocked by sunitinib. Additionally, bioluminescence resonance energy transfer (BRET) based proximity assays were undertaken to explore receptor-receptor colocalisation; no evidence was found that VEGFR2 and A2A or A2B receptors colocalise on the plasma membrane.
Results from this thesis suggest that A2A receptor activation could help treat hypertension associated with vasoconstriction, such as RTKI-induced hypertension. Furthermore, the ability of A2B receptor activation to selectively cause vasodilations in the renal and mesenteric vascular beds demonstrate that the selective targeting of A2B receptors could be beneficial in conditions associated with deleterious vasoconstrictions in the renal and mesenteric vasculatures, such as acute kidney injury or mesenteric ischemia.
Actions (Archive Staff Only)
 |
Edit View |
Tools
Tools
