Comparison of endothelium-related responses to nucleotides of dog and monkey cerebral arteries

How selective antagonists and genetic modification have helped characterise the expression and functions of vascular P2Y receptors

Vascular P2Y receptors mediate many effects, but the role of individual subtypes is often unclear. Here we discuss how subtype-selective antagonists and receptor knockout/knockdown have helped identify these roles in numerous species and vessels. P2Y1 receptor-mediated vasoconstriction and endothelium-dependent vasodilation have been characterised using the selective antagonists, MRS2179 and MRS2216, whilst AR-C118925XX, a P2Y2 receptor antagonist, reduced endothelium-dependent relaxation, and signalling evoked by UTP or fluid shear stress. P2Y2 receptor knockdown reduced endothelial signalling and endothelial P2Y2 receptor knockout produced hypertensive mice and abolished vasodilation elicited by an increase in flow. UTP-evoked vasoconstriction was also blocked by AR-C118925XX, but the effects of P2Y2 receptor knockout were complex. No P2Y4 receptor antagonists are available and P2Y4 knockout did not affect the vascular actions of UTP and UDP. The P2Y6 receptor antagonist, MRS2578, identified endothelial P2Y6 receptors mediating vasodilation, but receptor knockout had complex effects. MRS2578 also inhibited, and P2Y6 knockout abolished, contractions evoked by UDP. P2Y6 receptors contribute to the myogenic tone induced by a stepped increase in vascular perfusion pressure and possibly to the development of atherosclerosis. The P2Y11 receptor antagonists, NF157 and NF340, inhibited ATP-evoked signalling in human endothelial cells. Vasoconstriction mediated by P2Y12/P2Y13 and P2Y14 receptors was characterised using the antagonists, cangrelor, ticagrelor, AR-C67085 and MRS2211 or PPTN respectively. This has yet to be backed up by receptor knockout experiments. Thus, subtype-selective antagonists and receptor knockout/knockdown have helped identify which P2Y subtypes are functionally expressed in vascular smooth muscle and endothelial cells and the effects that they mediate.

Protecting against vascular disease in brain

Endothelial cells exert an enormous influence on blood vessels throughout the circulation, but their impact is particularly pronounced in the brain. New concepts have emerged recently regarding the role of this cell type and mechanisms that contribute to endothelial dysfunction and vascular disease. Activation of the renin-angiotensin system plays a prominent role in producing these abnormalities. Both oxidative stress and local inflammation are key mechanisms that underlie vascular disease of diverse etiology. Endogenous mechanisms of vascular protection are also present, including antioxidants, anti-inflammatory molecules, and peroxisome proliferator-activated receptor-γ. Despite their clear importance, studies of mechanisms that underlie cerebrovascular disease continue to lag behind studies of vascular biology in general. Identification of endogenous molecules and pathways that protect the vasculature may result in targeted approaches to prevent or slow the progression of vascular disease that causes stroke and contributes to the vascular component of dementia and Alzheimer's disease.

Mechanism underlying endothelium-dependent relaxation by 2-methylthio-ADP in monkey cerebral artery

We recently reported endothelium-dependent relaxation caused by nucleotides in the non-human primate cerebral artery. Here, we investigated the endothelium-dependent, nitric oxide- and prostanoid-independent relaxation induced by 2-methylthio-ADP (2MeSADP) in monkey cerebral artery. Mechanical responses of isolated monkey cerebral arteries to the agents were isometrically recorded. In endothelium-intact arterial strips treated with indomethacin plus N(G)-nitro-L-arginine and partially contracted with prostaglandin F(2α), 2MeSADP (1 nM - 10 µM) induced concentration-dependent relaxation that was abolished by removal of endothelium but was not influenced by either carboxy PTIO or 18α-glycyrrhetinic acid. The 2MeSADP-induced relaxation was inhibited by MRS2179 and U73122. The relaxation was markedly suppressed by exposure of the strips to high K(+) media, but was not affected by glibenclamide. Combination of charybdotoxin plus apamin markedly suppressed the relaxation, whereas iberiotoxin partially attenuated it. Relaxation induced by 2MeSADP was inhibited by arachidonyl trifluoromethyl ketone, ketoconazole, and 14,15-epoxyeicosa-5(Z)-enoic acid. The inhibitors that affected the 2MeSADP-induced relaxation did not influence relaxation caused by sodium nitroprusside or forskolin. These findings indicate that 2MeSADP elicits 'endothelium-derived hyperpolarizing factor (EDHF)-type' relaxation via stimulation of endothelial P2Y(1) receptors in monkey cerebral artery. Furthermore, phospholipase A(2), cytochrome P450-derived epoxyeicosatrienoic acids and Ca(2+)-activated K(+) channels appear to be involved in the relaxation.