Aalpha,beta-methylene ATP enhances P2Y4 contraction of rabbit basilar artery

Investigation of the functional expression of purine and pyrimidine receptors in porcine isolated pancreatic arteries

Receptors for purines and pyrimidines are expressed throughout the cardiovascular system. This study investigated their functional expression in porcine isolated pancreatic arteries. Pancreatic arteries (endothelium intact or denuded) were prepared for isometric tension recording and preconstricted with U46619, a thromboxane A(2) mimetic; adenosine-5'-diphosphate (ADP), uridine-5'-triphosphate (UTP) and MRS2768, a selective P2Y(2) agonist, were applied cumulatively, while adenosine-5'-triphosphate (ATP) and αβ-methylene-ATP (αβ-meATP) response curves were generated from single concentrations per tissue segment. Antagonists/enzyme inhibitors were applied prior to U46619 addition. ATP, αβ-meATP, UTP and MRS2768 induced vasoconstriction, with a potency order of αβ-meATP > MRS2768 > ATP ≥ UTP. Contractions to ATP and αβ-meATP were blocked by NF449, a selective P2X(1) receptor antagonist. The contraction induced by ATP, but not UTP, was followed by vasorelaxation. Endothelium removal and DUP 697, a cyclooxygenase-2 inhibitor, had no significant effect on contraction to ATP but attenuated that to UTP, indicating actions at distinct receptors. MRS2578, a selective P2Y(6) receptor antagonist, had no effect on contractions to UTP. ADP induced endothelium-dependent vasorelaxation which was inhibited by MRS2179, a selective P2Y(1) receptor antagonist, or SCH58261, a selective adenosine A(2A) receptor antagonist. The contractions to ATP and αβ-meATP were attributed to actions at P2X(1) receptors on the vascular smooth muscle, whereas it was shown for the first time that UTP induced an endothelium-dependent vasoconstriction which may involve P2Y(2) and/or P2Y(4) receptors. The relaxation induced by ADP is mediated by P2Y(1) and A(2A) adenosine receptors. Porcine pancreatic arteries appear to lack vasorelaxant P2Y(2) and P2Y(4) receptors.

Metabotropic Ca(2+) channel-induced Ca(2+) release and ATP-dependent facilitation of arterial myocyte contraction

Voltage-gated Ca(2+) channels in arterial myocytes can mediate Ca(2+) release from the sarcoplasmic reticulum and, thus, induce contraction without the need of extracellular Ca(2+) influx. This metabotropic action of Ca(2+) channels (denoted as calcium-channel-induced calcium release or CCICR) involves activation of G proteins and the phospholipase C-inositol 1,4,5-trisphosphate pathway. Here, we show a form of vascular tone regulation by extracellular ATP that depends on the modulation of CCICR. In isolated arterial myocytes, ATP produced facilitation of Ca(2+)-channel activation and, subsequently, a strong potentiation of CCICR. The facilitation of L-type channel still occurred after full blockade of purinergic receptors and inhibition of G proteins with GDPbetaS, thus suggesting that ATP directly interacts with Ca(2+) channels. The effects of ATP appear to be highly selective, because they were not mimicked by other nucleotides (ADP or UTP) or vasoactive agents, such as norepinephrine, acetylcholine, or endothelin-1. We have also shown that CCICR can trigger arterial cerebral vasoconstriction in the absence of extracellular calcium and that this phenomenon is greatly facilitated by extracellular ATP. Although, at low concentrations, ATP does not induce arterial contraction per se, this agent markedly potentiates contractility of partially depolarized or primed arteries. Hence, the metabotropic action of L-type Ca(2+) channels could have a high impact on vascular pathophysiology, because, even in the absence of Ca(2+) channel opening, it might mediate elevations of cytosolic Ca(2+) and contraction in partially depolarized vascular smooth muscle cells exposed to small concentrations of agonists.

Noradrenaline and extracellular nucleotide cotransmission involves activation of vasoconstrictive P2X(1,3)- and P2Y6-like receptors in mouse perfused kidney

Nucleotides like ATP and UTP act as potent extracellular signalling molecules. Released from sympathetic nerve endings as cotransmitters of noradrenaline or paracrine from nonexcitatory cells, they activate specific receptors (ion-gated P2X(1-7) and G-protein-coupled P2Y(1,2,4,6,11-15)). Which of these subtypes, however, are able to modulate vasoconstriction in the kidney is unclear. Wild-type- and P2Y4-receptor-deficient mice kidneys were isolated and perfused with Krebs-Henseleit solution. Pressor responses to renal nerve stimulations (RNS) and added drugs were recorded. Release of endogenous noradrenaline was measured by HPLC. RNS (1-15 Hz) induced a frequency-dependent increase in the perfusion pressor (14.2+/-5.1-67.3+/-6.9 mmHg) and noradrenaline release (1.4+/-0.3-24.2+/-3.4 ng g(-1) kidney). Pressor responses to RNS were not (1-2 Hz) or only partially (5-15 Hz) blocked by the alpha-adrenoceptor antagonist phentolamine (1 microM). Combination of phentolamine and the P2-receptor blocker PPADS (5 microM) prevented RNS-induced pressor responses. The P2X(1,3)-receptor selective antagonist NF279 (10 microM) reduced RNS-induced pressor responses in a frequency-dependent manner. Perfusion of ATP, ADP, UTP, UDP and alpha,beta-meATP concentration dependently increased perfusion pressor with the following rank order of potency alpha,beta-meATP>ADP approximately ATP approximately UDP > or = UTP. NF279 (10 microM) reduced alpha,beta-meATP- (0.1 microM) (21.7+/-3.9% of control) but not UTP- (0.3 microM) (102.6+/-15.3% of control) induced pressor responses. No differences in nucleotide-induced effects were detected among wild-type and P2Y4-receptor knockout mice. Continuous perfusion of alpha,beta-meATP (0.01 microM) potentiated UTP-, UDP- and ATP-gamma S-induced pressor responses. Neuronally and paracrine-released nucleotides evoked renal vasoconstriction by activation of P2X(1,3)- and P2Y6-like receptors in mice. Pretreatment with the P2X(1,3)-receptor agonist alpha,beta-meATP potentiated P2Y6-like receptor-mediated vasoconstrictions.