alpha(2)-adrenoceptor and NPY receptor-mediated contractions of porcine isolated blood vessels: evidence for involvement of the vascular endothelium

Novel vasocontractile role of the P2Y₁₄ receptor: characterization of its signalling in porcine isolated pancreatic arteries

BACKGROUND AND PURPOSE

The P2Y₁₄ receptor is the newest member of the P2Y receptor family; it is G(i/o) protein-coupled and is activated by UDP and selectively by UDP-glucose and MRS2690 (2-thiouridine-5'-diphosphoglucose) (7-10-fold more potent than UDP-glucose). This study investigated whether P2Y₁₄ receptors were functionally expressed in porcine isolated pancreatic arteries.

EXPERIMENTAL APPROACH

Pancreatic arteries were prepared for isometric tension recording and UDP-glucose, UDP and MRS2690 were applied cumulatively after preconstriction with U46619, a TxA₂ mimetic. Levels of phosphorylated myosin light chain 2 (MLC2) were assessed with Western blotting. cAMP concentrations were assessed using a competitive enzyme immunoassay kit.

KEY RESULTS

Concentration-dependent contractions with a rank order of potency of MRS2690 (10-fold) > UDP-glucose ≥ UDP were recorded. These contractions were reduced by PPTN {4-[4-(piperidin-4-yl)phenyl]-7-[4-(trifluoromethyl)phenyl]-2-naphthoic acid}, a selective antagonist of P2Y₁₄ receptors, which did not affect responses to UTP. Contraction to UDP-glucose was not affected by MRS2578, a P2Y₆ receptor selective antagonist. Raising cAMP levels and forskolin, in the presence of U46619, enhanced contractions to UDP-glucose. In addition, UDP-glucose and MRS2690 inhibited forskolin-stimulated cAMP levels. Removal of the endothelium and inhibition of endothelium-derived contractile agents (TxA₂, PGF(2α) and endothelin-1) inhibited contractions to UDP glucose. Y-27632, nifedipine and thapsigargin also reduced contractions to the agonists. UDP-glucose and MRS2690 increased MLC2 phosphorylation, which was blocked by PPTN.

CONCLUSIONS AND IMPLICATIONS

P2Y₁₄ receptors play a novel vasocontractile role in porcine pancreatic arteries, mediating contraction via cAMP-dependent mechanisms, elevation of intracellular Ca²⁺ levels, activation of RhoA/ROCK signalling and MLC2, along with release of TxA₂, PGF(2α) and endothelin-1.

Potentiation by neuropeptide Y of 5HT2A receptor-mediated contraction in porcine coronary artery

Potentiation by neuropeptide Y of serotonin (5-HT)-induced vasoconstriction was investigated in porcine coronary artery. 5-HT caused concentration-dependent contraction through 5-HT2A receptors. Neuropeptide Y (30 nM) significantly increased the 5HT-induced contraction by 16+/-5% in arteries with intact endothelium. Removal of the endothelium abolished the potentiation. A neuropeptide Y1 antagonist, BIBP3226, blocked this neuropeptide Y-induced potentiation. In vessels with intact endothelium, the potentiation by neuropeptide Y was inhibited by in the presence of a cyclo-oxygenase inhibitor, indomethacin (30 microM), but not by the presence of ETA or ETB endothelin receptor antagonists or an NO synthase inhibitor, NG-nitro-L-arginine (L-NNA) (1 mM) at all. A thromboxane A2 (TXA2) synthase inhibitor, ozagrel, and prostanoid TP receptor antagonists, seratrodast and ONO-3708, also inhibited the neuropeptide Y-induced potentiation. In the endothelium-denuded arteries, a prostanoid TP receptor agonist, U-46619 (0.01-0.1 nM), potentiated 5-HT-induced contraction. These results indicate that neuropeptide Y potentiates the 5-HT-induced contraction, due to release of TXA2 from the endothelium via neuropeptide Y1 receptors, in porcine coronary artery.

Clonidine induces rat aorta relaxation by nitric oxide-dependent and -independent mechanisms

The alpha1- and alpha2-adrenoceptors coexist in vascular smooth muscle cells producing vascular contraction and relaxation. This study was designed to investigate which is the mechanism activated by clonidine in the rat aorta, and the endothelial factors possibly involved in the relaxation induced by clonidine. The alpha2-adrenoceptors agonist clonidine relaxed rat aortas pre-contracted with phenylephrine, with or without endothelium. In non-contracted denuded arteries, clonidine produced contractions instead of relaxation. In intact endothelium aortic rings, clonidine induced greater relaxation than in denuded aortic rings. In aortas with intact endothelium, the NO-synthase inhibitor L-NAME (10 micromol/L) and the NO-scavenger hemoglobin (10 micromol/L) reduced the relaxation to clonidine. On the other hand, indomethacin (10 micromol/L) failed to alter the relaxation induced by clonidine. These results suggest the participation of NO, but not prostacyclin in clonidine-induced relaxation. In aortic rings pre-contracted with KCl (60 mmol/L) the relaxation induced by clonidine was abolished; however, the K+ channel blockers glibenclamide (K(ATP)), tetraethylamonium (K(Ca)), and the combination of apamin and charybdotoxin (K(Ca)) did not change the relaxation induced by clonidine. The relaxation induced by clonidine on PGF2alpha-contracted arteries was not affected by prazosin. However, in the absence of prazosin, clonidine had an additional contractile effect in PGF2alpha-contracted arteries. In conclusion, our results show that in rat aorta clonidine can activate alpha2-adrenoceptors in the smooth muscle cells and alpha2-adrenoceptors in the endothelial cells that activates NO production, but not prostacyclin and/or EDHF. In the absence of phenylephrine and prazosin, clonidine can also activate alpha1-adrenoceptors and rat aorta contraction.

The Role of Rho Kinase and Extracellular Regulated Kinase-Mitogen-Activated Protein Kinase in α2-Adrenoceptor-Mediated Vasoconstriction in the Porcine Palmar Lateral Vein

Alpha2-adrenoceptor-mediated vasoconstriction in the porcine palmar lateral vein is dependent upon activation of the extracellular signal-regulated kinase-mitogen-activated protein (ERK-MAP) kinase signal transduction pathway. Recent studies have shown that alpha2-adrenoceptor-mediated vasoconstriction in the rat aorta is also dependent upon activation of Rho kinase. The aim of this study was to determine whether Rho kinase and ERK-MAP kinase are part of the same signaling pathway. The Rho kinase inhibitor Y27632 (trans-4-[(1R)-1-aminoethyl]-N-4-pyridinylcyclohexanecarboxamide dihydrochloride) (10 microM) almost completely inhibited the contractile response to the alpha2-adrenoceptor agonist UK14304 (5-bromo-6-[2-imidazolin-2-ylamine]-quinoxaline bitartrate) in segments of porcine palmar lateral vein [maximum response 2.9 +/- 2.3% of 60 mM KCl response (mean +/- S.E.M.) in the presence of Y27632, compared with 64.9 +/- 7.1% in control tissues, n = 4]. However, Y27632 had no effect on alpha2-adrenoceptor-mediated ERK activation, as measured by Western blotting. Alpha2-adrenoceptor-mediated vasoconstriction was associated with an increase in phosphorylation of the myosin phosphatase-targeting subunit (MYPT) at Thr696 (the Rho kinase phosphorylation site). This phosphorylation was inhibited by 10 microM Y27632. In contrast, inhibition of ERK activation with the MAP kinase kinase inhibitor PD98059 (2-amino-3-methoxyflavone) (50 microM) had no effect on MYPT phosphorylation. Both Y27632 and PD98059 inhibited myosin light chain phosphorylation. These data indicate that alpha2-adrenoceptor-mediated vasoconstriction in the porcine palmar lateral vein is dependent upon both Rho kinase and ERK activation, although these are separate pathways. Rho kinase causes vasoconstriction through inhibition of myosin phosphatase and an increase in myosin light chain phosphorylation, whereas ERK causes vasoconstriction through a myosin phosphatase-independent pathway.

Enhancement of alpha2-adrenoceptor-mediated vasoconstriction by the thromboxane-mimetic U46619 in the porcine isolated ear artery: role of the ERK-MAP kinase signal transduction cascade

1. Alpha(2)-adrenoceptor-mediated contractions in porcine blood vessels can be enhanced in the presence of the thromboxane-mimetic U46619, and forskolin. The aim of this study was to determine the role of U46619 in the enhanced contractions, and to determine whether signalling through the ERK-MAP kinase pathway is involved. 2. Responses to the alpha(2)-adrenoceptor agonist UK14304 (1 micro M) were increased from 22+/-3% of the response to 60 mM KCl to 68+/-12% (n=8, mean+/-s.e.m.) in the presence of a low concentration of U46619 (< 20% of the 60 mM KCl response). 3. Both the direct and the U46619-enhanced UK14304 responses were inhibited by 50 microM PD98059, an inhibitor of the ERK-MAP kinase pathway. UK14304-induced contractions were associated with an increase in ERK2 phosphorylation, indicating an increased activity. In the presence of U46619, there was an enhanced phosphorylation of ERK2. U46619 on its own had no effect on ERK phosphorylation. 4. Both the direct and enhanced UK14304 contractions were inhibited in the absence of extracellular calcium. These conditions also prevented the increase in ERK2 phosphorylation. This indicates a role for calcium influx in the enhanced contractions. 5. In conclusion, this study demonstrates that precontraction with the thromboxane-mimetic U46619 enhances alpha(2)-adrenoceptor-mediated vasoconstriction through the enhancement of the ERK-MAP kinase pathway, and influx of extracellular calcium.

Vasopressor response to angiotensin II infusion in patients with chronic heart failure receiving beta-blockers

BACKGROUND

A synergistic interaction between the angiotensin II (Ang II) type 1 receptor and alpha1-adrenergic receptors has been described. We hypothesized that the nonselective beta-antagonist carvedilol, through its alpha1-adrenergic blocking properties, may modulate vascular reactivity to Ang II in patients with chronic heart failure (CHF). Accordingly, we compared the vasopressor response to infused Ang II in patients treated with carvedilol and metoprolol, a selective beta-antagonist.

METHODS AND RESULTS

All subjects were treated with carvedilol or metoprolol for at least 3 months. ACE inhibitor therapy was standardized to enalapril 40 mg/d or the maximally tolerated dose. Exogenous Ang II was administered as sequential intravenous bolus injections (2.5 to 30 ng/kg) titrated to a rise in radial artery systolic pressure of > or =20 mm Hg. The dose of Ang II required to elicit a change of 20 mm Hg in radial artery systolic pressure (PD20) defined the vasopressor response to Ang II. Twenty subjects with CHF (mean left ventricular ejection fraction 28+/-9%, New York Heart Association class II [n=13] and III [n=7]) were studied. There was no correlation between plasma Ang II levels and PD20. However, the PD20 was significantly higher in patients treated with carvedilol than in those treated with metoprolol (20 [range 2.5 to 30] versus 5 [range 2.5 to 10] ng/kg, P=0.019).

CONCLUSIONS

The vasopressor response to Ang II infusion in patients treated with carvedilol was significantly lower than in patients treated with metoprolol. Whether this is due to the alpha1-adrenergic blocking or other ancillary properties of carvedilol warrants further investigation.

Endothelial adrenoceptors

alpha2 -Adrenergic agonists cause endothelium-dependent relaxation in a number of isolated blood vessels. This effect is explained by the activation of endothelial alpha 2 -adrenoceptors linked to nitric oxide synthase by G i -coupling proteins. The endothelial response to alpha 2 -adrenergic agonists is blunted considerably after regeneration of the endothelium and in atherosclerotic arteries. The relaxation of isolated arteries caused by beta-adrenergic agonists is reduced by removal of the endothelium and, in most cases, by inhibitors of the l -arginine nitric oxide pathway. Likewise, in the intact animal and in the human forearm the vasodilatation to beta 2 -adrenergic agonists is blunted by inhibitors of nitric oxide synthase. Whether these findings reflect the presence of functional beta-adrenoceptors on the endothelium remains controversial. Several beta-adrenergic blockers cause endothelium-dependent relaxation in vitro or augment the production of nitric oxide in vivo. However, these responses cannot be attributed to interactions with endothelial beta-adrenoceptors.