Alpha2-adrenoceptor-mediated contractions of the porcine isolated ear artery: evidence for a cyclic AMP-dependent and a cyclic AMP-independent mechanism

An α2-Adrenergic Agonist, Brimonidine, Beneficially Affects the TGF-β2-Treated Cellular Properties in an In Vitro Culture Model

We report herein on the effects of brimonidine (BRI), an α2-adrenergic agonist, on two-dimensional (2D) and three-dimensional (3D) cell-cultured TGF-β2-untreated and -treated human trabecular meshwork (HTM) cells. In the presence of TGF-β2 (5 ng/mL), (1) the effects of BRI on (1) the 2D HTM monolayers’ barrier function were investigated as estimated using trans-endothelial electrical resistance (TEER) measurement and FITC dextran permeability; (2) real-time analyses of cellular metabolism using a Seahorse Bioanalyzer; (3) the largeness and hardness of 3D spheroids; and (4) the expression of genes that encode extracellular matrix (ECM) proteins, including collagens (COL) 1, 4, and 6; fibronectin (FN) and α-smooth muscle actin (α-SMA); ECM modulators, including a tissue inhibitor of matrix proteinase (TIMP) 1–4; matrix metalloproteinase (MMP) 2, 9, and 14; and several endoplasmic reticulum (ER) stress-related genes, including the X-box-binding protein 1 (XBP1), the spliced XBP1 (sXBP1), glucose-regulated protein (GRP)78, GRP94, and CCAAT-enhancer-binding protein homologous protein (CHOP). BRI markedly inhibited the TGF-β2-induced increase in the values of TEER of the 2D cell monolayer and the hardness of the 3D spheroids, although it had no effect on their sizes. BRI also cancelled the TGF-β2-induced reduction in mitochondrial maximal respiration but had no effect on the glycolytic capacity. In addition, the gene expression of these molecules was quite different between the 2D and 3D cultures of HTM cells. The present observations found in this study indicate that BRI may beneficially affect TGF-β2-induced changes in both cultures, 2D and 3D, of HTM cells, although their structural and functional properties that were altered varied significantly between both cultures of HTM cells.

UDP-sugars activate P2Y14 receptors to mediate vasoconstriction of the porcine coronary artery

Aims

UDP-sugars can act as extracellular signalling molecules, but relatively little is known about their cardiovascular actions. The P2Y₁₄ receptor is a G_i/o-coupled receptor which is activated by UDP-glucose and related sugar nucleotides. In this study we sought to investigate whether P2Y₁₄ receptors are functionally expressed in the porcine coronary artery using a selective P2Y₁₄ receptor agonist, MRS2690, and a novel selective P2Y₁₄ receptor antagonist, PPTN (4,7-disubstituted naphthoic acid derivative).

Methods and results

Isometric tension recordings were used to evaluate the effects of UDP-sugars in porcine isolated coronary artery segments. The effects of the P2 receptor antagonists suramin and PPADS, the P2Y₁₄ receptor antagonist PPTN, and the P2Y₆ receptor antagonist MRS2578, were investigated. Measurement of vasodilator-stimulated phosphoprotein (VASP) phosphorylation using flow cytometry was used to assess changes in cAMP levels. UDP-glucose, UDP-glucuronic acid UDP-N-acetylglucosamine (P2Y₁₄ receptor agonists), elicited concentration-dependent contractions of the porcine coronary artery. MRS2690 was a more potent vasoconstrictor than the UDP-sugars. Concentration dependent contractile responses to MRS2690 and UDP-sugars were enhanced in the presence of forskolin (activator of cAMP), where the level of basal tone was maintained by addition of U46619, a thromboxane A₂ mimetic. Contractile responses to MRS2690 were blocked by PPTN, but not by MRS2578. Contractile responses to UDP-glucose were also attenuated by PPTN and suramin, but not by MRS2578. Forskolin-induced VASP-phosphorylation was reduced in porcine coronary arteries exposed to UDP-glucose and MRS2690, consistent with P2Y₁₄ receptor coupling to G_i/o proteins and inhibition of adenylyl cyclase activity.

Conclusions

Our data support a role of UDP-sugars as extracellular signalling molecules and show for the first time that they mediate contraction of porcine coronary arteries via P2Y₁₄ receptors.

Evidence for a cyclic AMP-dependent pathway in angiotensin AT1-receptor activation of human omental arteries

Enhanced responses to vasoconstriction induced by neuropeptide Y and α2-adrenoceptor agonists have been seen following pharmacological activation of the adenylyl cyclase (AC) system. Since preliminary studies revealed only minor responses to angiotensin II (Ang II) in human omental arteries, we have investigated whether enhanced activity of AC may unravel further functional Ang II receptors. Human omental arteries were obtained in conjunction with elective gut surgery. After dissection of the vessel, the endothelium was removed by 10 sec of Triton X-100 treatment. Ring segments (1—2 mm long) were mounted on a myograph and studied. Ang II produced small contractions, 27±5% relative to the response elicited by 60 mM K+. However, enhanced Ang II (105±10%, p<0.001) responses were seen during AC activation by forskolin (0.1—1 µM). This enhanced contractile response to Ang II was not inhibited by the angiotensin II type 2 (AT2-receptor antagonist PD 123319 (0.1 µM), but was blocked in an insurmountable way by the angiotensin II type 1 (AT1)-receptor antagonist candesartan (1 nM) and in a surmountable manner by losartan (0.1 µM) and irbesartan (0.1 µM). Pertussis toxin (a Gi-protein blocker) and the protein kinase C inhibitor, RO31—8220 (0.01, 0.1 and 1 µM), markedly reduced this response, while the protein kinase A inhibitor, H89 (1, 10 µM), had no effect. RT-PCR provided evidence for the presence of mRNA for both AT1- and AT2-receptors. The results suggest that both a cAMP-dependent and a cAMP-independent mechanism are involved in the contractile responses to Ang II in human omental arteries and that both responses are mediated via the AT1-receptor.

Progesterone decreases the relaxing effect of the beta3-adrenergic receptor agonist BRL 37344 in the pregnant rat myometrium

Although the published results regarding the function of the beta(3)-adrenergic receptors (beta(3)-ARs) in the regulation of smooth muscle activity are very promising, the question of the mechanism of beta(3)-ARs' action in the pregnant myometrium cannot be fully answered by human investigations. To assess whether it possesses an essential role in the regulation of uterine contractility in pregnant rats, as in humans, we performed functional, western blotting and molecular biology experiments on the late-pregnant rat myometrium. The influence of progesterone on the function of the beta(3)-ARs was also investigated. We demonstrated the presence and the functional activity of the beta(3)-ARs in the late-pregnant rat myometrium. The maximum dose-dependent uterus-relaxing effect of the selective beta(3)-agonist BRL 37344 was recorded at the end of pregnancy in rats, similarly as in humans. The extent of its relaxing action was regarded as moderate. The expression of beta(3)-AR protein and mRNA remained unchanged during the investigated period. The administration of progesterone had no effect on the beta(3)-AR mRNA and protein expression or the maximum relaxation effect of BRL 37344, but shifted the dose-response curve to the right and decreased the synthesis of the second messenger, cAMP. It can be concluded that the beta(3)-ARs play an additional role in the regulation of the contractile activity of the pregnant rat uterus. The inhibitory effect of progesterone on the functional activity of the beta(3)-ARs may have important consequences in the case of human application if this effect is also demonstrated in pregnant human myometrial tissue.

The roles of alpha2-adrenoceptor subtypes in the control of cervical resistance in the late-pregnant rat

The roles of the alpha(2)-adrenoceptor subtypes in the regulation of cervical resistance have previously not been investigated. The aim of the present study was to identify these receptors in the late-pregnant cervix and determine their functions in vitro in the rat. The expressions of the alpha(2)-adrenoceptor subtypes were determined by means of RT-PCR and Western blotting techniques. The changes in cervical resistance due to subtype-selective antagonists were investigated in stretching tests. The cyclic AMP immunoassay technique was used to detect the level of cyclic AMP following stimulation of the alpha(2)-adrenoceptors with or without pertussis toxin. On pregnancy days 18, 20, 21 and 22, the RT-PCR and Western blotting studies revealed the expressions of all three alpha(2)-adrenoceptor subtype mRNAs and proteins. On days 18 and 20, noradrenaline increased and decreased the resistance, respectively. Its effect was blocked by each of the antagonists used, except ARC 239 on both days. On day 21, noradrenaline again increased the resistance, this effect being maintained only in the presence of spiroxatrine. Noradrenaline was ineffective on day 22. These results were supported by the changes in cyclic AMP levels. Pertussis toxin pretreatment eliminated the changes in the cyclic AMP level on days 18 and 21. We presume that the alpha(2A)- and alpha(2C)-adrenoceptors play predominant roles in the regulation of cervical resistance on days 18-21. Depending on the day of pregnancy, stimulation of these alpha(2)-adrenoceptors could even result in opposite effects. This fluctuation can be explained by the changes in the G(i)/G(s)-coupling of the alpha(2A)- and alpha(2C)-adrenoceptors.

Vasopressors for cardiopulmonary resuscitation. Does pharmacological evidence support clinical practice?

Adrenaline (epinephrine) has been used for cardiopulmonary resuscitation (CPR) since 1896. The rationale behind its use is thought to be its alpha-adrenoceptor-mediated peripheral vasoconstriction, causing residual blood flow to be diverted to coronary and cerebral circulations. This protects these tissues from ischaemic damage and increases the likelihood of restoration of spontaneous circulation. Clinical trials have not demonstrated any benefit of adrenaline over placebo as an agent for resuscitation. Adrenaline has deleterious effects in the setting of resuscitation, predictable from its promiscuous pharmacological profile. This article discusses the relevant pharmacology of adrenaline in the context of CPR. Experimental and clinical evidences for the use of adrenaline and alternative vasopressor agents in resuscitation are given, and the properties of an ideal vasopressor are discussed.

Role of cytosolic phospholipase A2 in the enhancement of α2-adrenoceptor-mediated vasoconstriction by the thromboxane-mimetic U46619 in the porcine isolated ear artery: Comparison with vasopressin-enhanced responses

Pre-contraction with the thromboxane-mimetic U46619 enhances the subsequent alpha(2)-adrenoceptor-mediated vasoconstriction in the porcine ear artery through an enhanced activation of ERK-MAP kinase. In this study we determined the role of cPLA(2) in this enhanced response, and determined whether vasopressin is also able to enhance alpha(2)-adrenoceptor-mediated vasoconstriction through the same pathway. The cPLA(2) inhibitors AACOCF3 (50 microM) and MAFP (50 microM) both inhibited the U46619-enhanced alpha(2)-adrenoceptor response, but had no effect on the direct alpha(2)-adrenoceptor response. AACOCF3 also inhibited the enhanced ERK activation associated with the enhanced alpha(2)-adrenoceptor-mediated vasoconstriction. Pre-contraction with arachidonic acid mimicked the effect of U46619 by enhancing the contractile response to the alpha(2)-adrenoceptor agonist UK14304 (1 microM) and enhancing the alpha(2)-adrenoceptor-mediated ERK activation. Pre-contraction with vasopressin also enhanced the contractile response to UK14304, but neither PD98059 (50 microM) nor AACOCF3 (50 microM) had any effect this vasopressin-enhanced response, indicating that neither the ERK pathway, nor cPLA(2) are involved in vasopressin-enhanced responses. The alpha(2)-adrenceptor-stimulated activation of ERK was also unaffected by pre-contraction with vasopressin. On the other hand, inhibition of PKCzeta inhibited the enhanced alpha(2)-adrenoceptor contraction after pre-contraction with both U46619 and vasopressin. This study demonstrates that alpha(2)-adrenoceptor-mediated vasoconstriction can be enhanced through two different pathways-one dependent upon the enhanced activation of ERK-MAP kinase through activation of cPLA(2), and the other through a different, ERK/cPLA(2)-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.

Role of the extracellular signal-regulated kinase (Erk) signal transduction cascade in alpha(2) adrenoceptor-mediated vasoconstriction in porcine palmar lateral vein

The mechanism of alpha(2) adrenoceptor-mediated vasoconstriction is unknown, but may involve activation of voltage-sensitive calcium channels, and/or a protein tyrosine kinase. Recently the extracellular signal-regulated kinase (Erk) cascade, often an event downstream of tyrosine kinase activation, has been shown to mediate vasoconstriction to a variety of agents. The aim of this present study was to determine the involvement of the Erk signal transduction cascade in alpha(2) adrenoceptor-mediated vasoconstriction, and to confirm the involvement of activation of voltage-sensitive calcium channels, and protein tyrosine kinase. Contractions to the alpha(2) adrenoceptor agonist UK14304 in the porcine palmar lateral vein in vitro were reduced 70 - 80% by the MEK inhibitors PD98059 (10 - 50 microM) and U0126 (10 - 50 microM), indicating the involvement of the Erk signal transduction cascade. Immunoblots also demonstrated an increase in the phosphorylated (activated) form of Erk in palmar lateral vein segments after contraction with UK14304, which was inhibited by PD98059 and U0126. The calcium channel blockers nifedipine and verapamil, or removal of extracellular calcium inhibited UK14304-induced contractions and phosphorylation of Erk, demonstrating the importance of an influx of extracellular calcium. UK14304-induced contractions were inhibited by PP2 (1 - 10 microM), a selective inhibitor of Src tyrosine kinases, but not by PP3, an inactive analogue. PP2 also prevented the phosphorylation of Erk by UK14304. These data demonstrate that alpha(2) adrenoceptor-mediated vasoconstriction in the porcine palmar lateral vein is dependent upon activation of the Erk signal transduction cascade, which is downstream of an influx of extracellular calcium, and activation of Src tyrosine kinases.

Evidence for a non-adrenoceptor, imidazoline-mediated contractile response to oxymetazoline in the porcine isolated rectal artery

Imidazoline derivatives are known to elicit responses through both alpha(2)-adrenoceptor and non-adrenoceptor, imidazoline sites, though as yet there are no examples of the latter on vascular smooth muscle. In the presence of 0.3 microM prazosin, neither UK-14304 (0.01 - 3 microM) nor oxymetazoline (0.01 - 30 microM) caused a significant contraction of the porcine isolated rectal artery, a preparation with a low density of alpha(2)-adrenoceptors. In the presence of a combination of U46619 and forskolin, however, both agonists produced concentration-dependent contractions. Pretreatment with phenoxybenzamine (3 microM) abolished responses to UK-14304, but left those elicited by oxymetazoline largely unaffected. The putative I(3) imidazoline antagonist 2-(2,3 dihydro-2-benzofuranyl)-2-imidazole (KU-14R, 10 microM) caused a 6 fold rightward displacement of the phenoxybenzamine-insensitive concentration - response curve to oxymetazoline. Our data indicates that non-adrenoceptor, imidazoline sites, pharmacologically similar to the I(3) imidazoline site on islet cells, mediate vasoconstriction in the porcine isolated rectal artery.

Neuropeptide Y regulates intracellular calcium through different signalling pathways linked to a Y(1)-receptor in rat mesenteric small arteries

Simultaneous measurements of intracellular calcium concentration ([Ca(2+)](i)) and tension were performed to clarify whether the mechanisms which cause the neuropeptide Y (NPY)-elicited contraction and potentiation of noradrenaline contractions, and the NPY inhibition of forskolin responses are linked to a single or different NPY receptor(s) in rat mesenteric small arteries. In resting arteries, NPY moderately elevated [Ca(2+)](i) and tension. These effects were antagonized by the selective Y(1) receptor antagonist, (R)-N(2)-(diphenacetyl)-N-[(4-hydroxyphenyl)methyl]-D-argininea mide (BIBP 3226) (apparent pK(B) values of 8.54+/-0.25 and 8.27+/-0.17, respectively). NPY (0.1 microM) caused a near 3 fold increase in sensitivity to noradrenaline but did not significantly modify the tension-[Ca(2+)](i) relationship for this agonist. BIBP 3226 competitively antagonized the contractile response to NPY in arteries submaximally preconstricted with noradrenaline (pA(2) 7.87+/-0.20). In arteries activated by vasopressin, the adenylyl cyclase activator forskolin (3 microM) induced a maximum relaxation and a return of [Ca(2+)](i) to resting levels. NPY completely inhibited these effects. The contractile responses to NPY in arteries maximally relaxed with either sodium nitroprusside (SNP) or nifedipine were not significantly higher than those evoked by the peptide at resting tension, in contrast to the contractions to NPY in forskolin-relaxed arteries. BIBP 3226 competitively antagonized the contraction to NPY in forskolin-relaxed arteries with a pA(2) of 7.92+/-0.29. Electrical field stimulation (EFS) at 8-32 Hz caused large contractions in arteries relaxed with either forskolin or noradrenaline in the presence of phentolamine. These responses to EFS were inhibited by BIBP 3226. Similar EFS in resting, non-activated arteries did not produce any response. The present results suggest that different intracellular pathways are linked to a single NPY Y(1) receptor in intact rat mesenteric small arteries, and provide little support for involvement of other postjunctional NPY receptors in the contractile responses to NPY. Neurally released NPY also seems to act through Y(1) receptors, and may serve primarily as an inhibitor of vasodilatation.

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

1. Enhanced contractions to the alpha(2)-adrenoceptor agonist UK14304 and neuropeptide Y (NPY) in the porcine ear artery can be uncovered by pharmacological manipulation. The aim of this study was to determine whether similar pharmacological manipulation can uncover enhanced contractions in the porcine splenic artery, and to determine whether the endothelium modulates these responses. 2. UK14304 (0.3 microM) and NPY (0.1 microM) produced small contractions of the porcine splenic artery. After pre-contraction of the tissue with U46619, followed by relaxation with forskolin, the responses to both UK14304 and NPY were enhanced. Enhanced contractions to both UK14304 and NPY were also obtained after relaxation with SNP. These results demonstrate that, as in the porcine ear artery, alpha(2)-adrenoceptors and NPY receptors are able to produce enhanced contractile responses through both adenylyl cyclase-dependent and -independent signal transduction pathways. 3. Removal of the endothelium had no significant effect on responses to UK14304 either alone or in the presence of U46619 and forskolin in the porcine splenic artery. On the other hand, responses to UK14304 after relaxation with SNP were reduced after endothelium-denudation in both the porcine splenic artery and ear artery. Similar results were obtained with NPY in the porcine ear artery. 4. In conclusion, enhanced contractile responses to UK14304 and NPY in the porcine splenic artery can be uncovered using methods similar to those employed in the porcine ear artery. Under certain conditions the responses to both agents are modulated by the endothelium. These data highlight further the similarities in the signal transduction pathways used by both alpha(2)-adrenoceptors and NPY receptors to induce vasoconstriction.

A study of NPY-mediated contractions of the porcine isolated ear artery

Enhanced contractions of the porcine isolated ear artery by the alpha2-adrenoceptor agonist UK14304 are uncovered by pharmacological manipulation. As both neuropeptide Y (NPY) receptors and alpha2-adrenoceptors are negatively-coupled to adenylyl cyclase in this tissue, we determined whether NPY is also able to produce an enhanced contraction in the same tissue, under the same conditions. NPY (0.1 microM) produced a small contraction of porcine isolated ear arteries which was 5.1+/-0.8% of the response to 60 mM KCl (n = 14). An enhanced NPY response was uncovered if the tissue was pre-contracted with 0.1 microM U46619, and relaxed back to baseline with 1-2 microM forskolin before the addition of NPY (49.8+/-5.3%, n = 14). Forskolin (1 microM) stimulated cyclic AMP accumulation in porcine ear artery segments in the presence of 0.1 microM U46619 and 1 mM isobutylmethylxanthine (IBMX), NPY (0.1 microM) inhibited this response by 40%, but had no effect on basal levels of cyclic AMP. An enhanced response to 0.1 microM NPY was also obtained after pre-contraction with 0.1 microM U46619 and relaxation with either SNP (28.9+/-5.7%, n = 14), or dibutyryl cyclic AMP (21.2+/-4.6%, n = 14). This indicates that at least part of the enhanced response to NPY is independent of the agonist's ability to inhibit adenylyl cyclase. In conclusion, an enhanced contraction to NPY in the porcine isolated ear artery can be obtained by prior pharmacological manipulation. The enhanced responses are mediated through adenylyl cyclase-dependent and independent pathways similar to those reported for alpha2-adrenoceptors in this preparation.