Examination of the role of inhibition of cyclic AMP in alpha 2-adrenoceptor mediated contractions of the porcine isolated palmar lateral vein

Phenylephrine contracts porcine pulmonary veins via alpha(1B)-, alpha(1D)-, and alpha(2)-adrenoceptors

We have recently shown that the postjunctional alpha(2)-adrenoceptor mediating contraction of porcine pulmonary veins is of the alpha(2C)-subtype. We could also demonstrate that alpha(1)-adrenoceptors might contribute to the contraction in that blood vessel. In the present study, we aimed at characterising the alpha(1)-adrenoceptor subtype(s) involved using pharmacological and molecular biological methods. In isolated rings of porcine pulmonary veins the typical alpha(1)-adrenoceptor agonist phenylephrine caused a concentration-dependent contraction that was inhibited by the alpha(1B)-adrenoceptor selective antagonists 1-[4-(4-amino-6,7-dimethoxyquinazolin-2-yl)piperazin-1-yl]-2-[2-(isopropyl)-6-methoxyphenoxy]ethan-1-one (Rec15/2615; pA(2) 8.96+/-0.13) and 4-amino-2-[4-[1-(benzyloxycarbonyl)-2(S)-[[(1,1-dimethylethyl)amino]carbonyl]-piperazinyl]-6,7-dimethoxyquinazoline (L-765,314; pA(2) 7.22+/-0.05), as well as the alpha(1D)-adrenoceptor selective antagonist 8-[2-[4-(2-methoxyphenyl)-1-piperazinyl]ethyl]-8-azaspiro[4.5]decane-7,9-dione (BMY7378; pA(2) 8.29+/-0.15, slope of the Schild plot 0.75+/-0.09, significantly different from unity, P<0.05), but not by the alpha(1A)-adrenoceptor selective antagonists (+/-)-1,3,5-trimethyl-6-[[3-[4-((2,3-dihydro-2-hydroxymethyl)-1,4-benzodioxin-5-yl)-1-piperazinyl]propyl]amino]-2,4(1H,3H)-pyrimidinedione (B8805-033) and N-[2-(2-cyclopropylmethoxyphenoxy)ethyl]-5-chloro-alpha,alpha-dimethyl-1H-indole-3-ethanamine (RS-17053). These findings suggest that phenylephrine activates both alpha(1B)- and alpha(1D)-adrenoceptors. The observation was confirmed by reverse-transcriptase polymerase chain reaction (RT-PCR) in porcine pulmonary veins, where mRNA signals for alpha(1B)- and alpha(1D)-adrenoceptors could be detected. However, the antagonist properties of rauwolscine and yohimbine (non-subtype selective alpha(2)-adrenoceptor antagonists) against phenylephrine showed that this agonist also activates alpha(2)-adrenoceptors in pulmonary veins. This was strengthened in experiments using tissues that were stimulated with forskolin (cell permeable activator of adenylyl cyclase). Phenylephrine mimicked the effect of the selective alpha(2)-adrenoceptor agonist UK14304 by causing an inhibition of forskolin-stimulated cAMP accumulation that was blocked by rauwolscine. It is concluded that, in addition to alpha(1B)- and alpha(1D)-adrenoceptors, phenylephrine can stimulate alpha(2)-adrenoceptors in porcine pulmonary veins.

Activation of protein kinase B/Akt and endothelial nitric oxide synthase mediates agmatine-induced endothelium-dependent relaxation

The ability of agmatine, formed from L-arginine by the enzyme arginine decarboxylase (ADC), to modulate vasomotor function in rat aorta was investigated in the present study. Agmatine-mediated modulation of vasomotor tone was studied in organ chambers, protein expression quantified by Western blot analysis and cyclic guanosine 5'-monophosphate (cGMP) levels measured by radioimmunoassay. Agmatine (10(-10) to 10(-3) M) produced concentration-dependent relaxations (82+/-5%) in phenylephrine-contracted endothelium intact rat aorta. Relaxations to agmatine were diminished on denudation of endothelium and nitric oxide synthase (NOS) inhibition by L-Nomega-nitro arginine or soluble guanylate cyclase inhibition by 1H-[1,2,4]oxadiazolo[4,3-a]quinoxalin-1-one (P<0.001) abolished agmatine-mediated relaxations, while relaxations were insensitive to inducible NOS inhibition by 1400W. Agmatine-treated aorta demonstrated increased protein expression of phosphorylated S473-Akt and phosphorylated S1177-endothelial nitric oxide synthase (eNOS), and elevated the levels of cyclic GMP (P<0.01). Agmatine-mediated potentiation of relaxations and elevation of cGMP levels was sensitive to phosphatidylinositol 3'-kinase inhibitor, wortmannin. Relaxations to agmatine were also affected by pre-treatment with tetraethylammonium (P<0.01) or apamin (P<0.05), and were not affected by charybdotoxin. Relaxations to agmatine were partially affected by pre-treatment of aortic rings with barium chloride (P<0.05), and glybenclamide (P<0.05). Results obtained suggest that agmatine activates protein kinase B/Akt to phosphorylate eNOS and elevate cyclic GMP levels to produce vasodilatation of aorta. Agmatine-mediated relaxations in rat aorta seems to be mediated mainly by endothelial NO-mediated activation of small conductance Ca2+-activated K+ channels, and partly by ATP-sensitive and inward rectifying K+ channels.

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.

Alpha 2 adrenoceptor-mediated vasoconstriction in porcine palmar lateral vein: role of phosphatidylinositol 3-kinase and EGF receptor transactivation

1 Alpha(2) adrenoceptors cause vasoconstriction in the porcine palmar lateral vein through a mechanism involving the ERK signal transduction cascade, calcium influx, and a Src tyrosine kinase. The aim of the present study was to determine if phosphatidylinositol 3-kinase (PI 3-kinase) and/or epidermal growth factor (EGF) receptor transactivation are also involved. 2 alpha(2) Adrenoceptor-mediated vasoconstriction and ERK2 activation in the porcine palmar lateral vein was inhibited in the presence of either the PI 3-kinase inhibitor LY294002, or the EGF receptor tyrosine kinase inhibitor AG1478 suggesting the involvement of both PI 3-kinase and EGF receptor transactivation. 3 Akt phosphorylation was increased in segments of porcine palmar lateral vein contracted with UK14304 indicating an increase in Akt activation. This is a further indication that PI 3-kinase is involved in alpha(2) adrenoceptor-mediated vasoconstriction. Akt activation was inhibited by the Src tyrosine kinase inhibitor PP2, and removal of extracellular calcium. 4 UK14304 (10 microM) stimulated an increase in intracellular calcium in segments of palmar lateral vein. This was inhibited by removal of extracellular calcium, but not by nifedipine suggesting the rise in calcium is due to influx of calcium through non-L type calcium channels. The increase in calcium was also inhibited by LY294002 indicating that PI 3-kinase is upstream of calcium influx. 5 These data indicate that alpha(2) adrenoceptor-mediated vasoconstriction in the porcine palmar lateral vein is dependent upon stimulation of PI 3-kinase, leading to an influx of calcium. This results in activation of the EGF receptor tyrosine kinase, and finally activation of ERK-MAP kinase.

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.

Characterization of human recombinant alpha(2A)-adrenoceptors expressed in Chinese hamster lung cells using extracellular acidification rate changes

1. Human alpha(2A)-adrenoceptors heterologously expressed in Chinese hamster lung (CHL) fibroblasts have been characterized pharmacologically using a cytosensor microphysiometer to measure ligand-induced extracellular acidification rate changes. 2. In untransfected CHL cells, noradrenaline had no effect at concentrations up to 100 microM. In alpha(2A)-adrenoceptor transfected cells the rank order of agonist potency was A-54741 (mean pEC(50)=8.96)>dexmedetomidine (8.88)>UK-14304 (8.42)>B-HT 920 (7.05)>noradrenaline (6.92). A-54741, UK-14304 and noradrenaline had the same maximum response while dexmedetomidine and B-HT 920 behaved as partial agonists. 3. The selective alpha(2)-adrenoceptor ligand rauwolscine antagonized acidification rate changes with an affinity independent of the agonist used; the affinity (mean pK(B)) against noradrenaline was 8.43. 4. The selective alpha(1)-adrenoceptor ligands prazosin and doxazosin (each 3 microM) had no effect on noradrenaline responses. 5. Acidification rate changes induced by each agonist were abolished by pre-treatment of cells with pertussis toxin. 6. These data suggest that agonist-induced acidification rate responses in CHL cells transfected with the human alpha(2A)-adrenoceptor are mediated exclusively by the recombinant protein, via pertussis toxin sensitive G(i/o) proteins.

The mechanisms of alpha(2)-adrenoceptor agonist-induced contraction in longitudinal muscle of the porcine uterus

The aim of the present study was to clarify the cellular mechanisms underlying the alpha(2)-adrenoceptor-mediated contraction of porcine myometrium (nonvascular smooth muscle). Acetylcholine (3 nM-1 microM), clonidine (1 nM-10 microM) and 5-bromo-N-[2-imidazolin-2-yl]-6-quinoxalinamine (UK14304) (1 nM-10 microM) in Krebs solution caused a concentration-dependent contraction in the longitudinal muscles of the porcine uterus with similar EC(50) values and maximum responses. A lowered external Ca(2+) concentration and verapamil (10 nM-10 microM) decreased the contractile response to clonidine and UK14304 more markedly than the response to acetylcholine. However, in Kumagai solution, neither clonidine nor UK14304 caused contractile responses, but acetylcholine remained effective. The effects of alpha(2)-adrenoceptor agonists on intracellular Ca(2+) concentration ([Ca(2+)](i)) and smooth muscle force were measured simultaneously using fura-PE3-loaded muscle preparations. Clonidine and UK14304 caused increases in [Ca(2+)](i) and force of the longitudinal muscle. The increases in [Ca(2+)](i) and muscle force were markedly inhibited by verapamil and in Ca(2+)-free solution (EGTA, 1 mM). In the absence of external Ca(2+), clonidine caused only a small increase in [Ca(2+)](i) in Ca(2+)-loaded preparations compared with those increases caused by carbachol, histamine, and oxytocin. Ca(2+) (2.5 mM) caused increases in [Ca(2+)](i) and force of the longitudinal muscles in a Ca(2+)-free high K(+) solution. Clonidine concentration dependently potentiated the Ca(2+)-induced contraction without significantly changing the increase in [Ca(2+)](i), and this potentiation was inhibited by yohimbine. These results suggested that clonidine increases the Ca(2+) sensitivity of the contractile elements through activation of alpha(2)-adrenoceptors. During the development of the contractile response to clonidine (1 microM, 0-5 min), tissue cyclic AMP levels did not change significantly. In vitro treatment with pertussis toxin (1 microg/ml for 2 h) significantly decreased the contraction induced by clonidine without affecting the responses to carbachol and high K(+). The present results indicate that in porcine myometrium, alpha(2)-adrenoceptor stimulation caused contraction of the longitudinal muscles by mechanisms largely dependent on the influx of extracellular Ca(2+), probably through voltage-dependent Ca(2+) channels (VDCCs), and that the potentiation of the Ca(2+) sensitivity of the contractile elements is another mechanism of the contractile responses. These actions involve a pertussis-toxin-sensitive G protein (probably G(i) type) in the signal transduction pathway.

Molecular and pharmacological evidence for MT1 melatonin receptor subtype in the tail artery of juvenile Wistar rats

1. In this study reverse transcriptase-polymerase chain reaction (RT-PCR) has been used to identify mt1 and MT2 receptor mRNA expression in the rat tail artery. The contributions of both receptors to the functional response to melatonin were examined with the putative selective MT2 receptor antagonists, 4-phenyl-2-propionamidotetraline (4-P-PDOT) and 2-benzyl-N-pentanoyltryptamine. In addition, the action of melatonin on the second messenger cyclic AMP was investigated. 2. Using RT-PCR, mt1 receptor mRNA was detected in the tail artery from seven rats. In contrast MT2 receptor mRNA was not detected even after nested PCR. 3. At low concentrations of the MT2 selective ligands, neither 10 nM 4-P-PDOT (pEC50=8.70+/-0.31 (control) vs 8.73+/-0.16, n=6) nor 60 nM 2-benzyl-NV-pentanoyltryptamine (pEC50= 8.53+/-0.20 (control) vs 8.83+/-0.38, n = 6) significantly altered the potency of melatonin in the rat tail artery. 4. At concentrations non-selective for mt1 and MT2 receptors. 4-P-PDOT (3 microM) and 2-benzyl-N-pentanoyltryptamine (5 microM) caused a significant rightward displacement of the vasoconstrictor effect of melatonin. In the case of 4-P-PDOT, the estimated pKB (6.17+/-0.16, n=8) is similar to the binding affinity for mt1 receptor. 5. Pre-incubation with 1 microM melatonin did not affect the conversion of [3H]-adenine to [3H]-cyclic AMP under basal condition (0.95+/-0.19% conversion (control) vs 0.92+/-0.19%, n=4) or following exposure to 30 microM forskolin (5.20+/-1.30% conversion (control) vs 5.35+/-0.90%, n=4). 6. Based on the above findings, we conclude that melatonin receptor on the tail artery belongs to the MT1 receptor subtype, and that this receptor is probably independent of the adenylyl cyclase pathway.

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.

Mechanisms of melatonin-induced vasoconstriction in the rat tail artery: a paradigm of weak vasoconstriction

1. Vasoconstrictor effects of melatonin were examined in isolated rat tail arteries mounted either in an isometric myograph or as cannulated pressurized segments. Melatonin failed by itself to mediate observable responses but preactivation of the arteries with vasopressin (AVP) reliably uncovered vasoconstriction responses to melatonin with maxima about 50% of maximum contraction. Further experiments were conducted with AVP preactivation to 5-10% of the maximum contraction. 2. Responses to melatonin consisted of steady contractions with superimposed oscillations which were large and irregular in isometric but small in isobaric preparations. Nifedipine (0.3 microM) reduced the responses and abolished the oscillations. Charybdotoxin (30 nM) increased the magnitude of the oscillations with no change in the maximum response. 3. Forskolin (0.6 microM) pretreatment increased the responses to melatonin compared to control and sodium nitroprusside (1 microM) treated tissues. The AVP concentration required for preactivation was 10 fold higher than control in both the forskolin and nitroprusside treated groups. 4. In isometrically-mounted arteries treated with nifedipine, melatonin receptor agonists had the potency order 2-iodomelatonin > melatonin > S20098 > GR196429, and the MT2-selective antagonist luzindole antagonized the effects of melatonin with a low pK(B) of 6.1+/-0.1. 5. It is concluded that melatonin elicits contraction of the rat tail artery via an mt1 or mt1-like receptor that couples via inhibition of adenylate cyclase and opening of L-type calcium channels. Calcium channels and charybdotoxin-sensitive K channels may be recruited into the responses via myogenic activation rather than being coupled directly to the melatonin receptors. 6. It is proposed that the requirement of preactivation for overt vasoconstrictor responses to melatonin results from the low effector reserve of the melatonin receptors together with the tail artery having threshold inertia. Potentiative interactions between melatonin and other vasoconstrictor stimuli probably also result from the threshold inertia. A simple model is presented and a general framework for consideration of interactions between weak vasoconstrictor agonists and other vasoconstrictor stimuli is discussed.

Subtypes of functional alpha1- and alpha2-adrenoceptors

In this review, subtypes of functional alpha1- and alpha2-adrenoceptors are discussed. These are cell membrane receptors, belonging to the seven transmembrane spanning G-protein-linked family of receptors, which respond to the physiological agonists noradrenaline and adrenaline. Alpha1-adrenoceptors can be divided into alpha1A-, alpha1B- and alpha1D-adrenoceptors, all of which mediate contractile responses involving Gq/11 and inositol phosphate turnover. A 4th alpha1-adrenoceptor, the alpha1L-, has been postulated to mediate contractions in some tissues, but its relationship to cloned receptors remains to be established. Alpha2-adrenoceptors can be divided into alpha2A-, alpha2B- and alpha2C-adrenoceptors, all of which mediate contractile responses. Prejunctional inhibitory alpha2-adrenoceptors are predominantly of the alpha2A-adrenoceptor subtype (the alpha2D-adrenoceptor is a species orthologue), although alpha2C-adrenoceptors may also occur prejunctionally. Although alpha2-adrenoceptors are linked to inhibition of adenylate cyclase, this may not be the primary signal in causing smooth muscle contraction; likewise, prejunctional inhibitory actions probably involve restriction of Ca2+ entry or opening of K+ channels. Receptor knock-out mice are beginning to refine our knowledge of the functions of alpha-adrenoceptor subtypes.

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

1. The aim of this study was to determine the conditions under which the alpha2-adrenoceptor agonist UK14304 produces vasoconstriction in the porcine isolated ear artery. 2. UK14304 (0.3 microM) produced a small contraction of porcine isolated ear arteries which was 7.8+/-3.3% of the response to 60 mM KCl. Similar sized contractions were obtained after precontraction with either 30 nM angiotensin II, or 0.1 microM U46619 (8.2+/-1.8% and 10.2+/-2.6% of 60 mM KCl response, respectively). However, an enhanced alpha2-adrenoceptor response was uncovered if the tissue was precontracted with U46619, and relaxed back to baseline with 1-2 microM forskolin before the addition of UK14304 (46.9+/-9.6% of 60 mM KCl response). 3. The enhanced responses to UK14304 in the presence of U46619 and forskolin were not inhibited by the alpha1-adrenoceptor antagonist prazosin (0.1 microM), but were inhibited by the alpha2-adrenoceptor antagonist rauwolscine (1 microM), indicating that the enhanced responses were mediated via postjunctional alpha2-adrenoceptors. 4. In the presence of 0.1 microM U46619 and 1 mM isobutylmethylxanthine (IBMX), 1 microM forskolin produced an increase in [3H]-cyclic AMP levels in porcine isolated ear arteries. Addition of 0.3 microM UK14304 prevented this increase. 5. The enhanced UK14304 response was dependent upon the agent used to relax the tissue. After relaxation of ear arteries precontracted with 10 nM U46619 and relaxed with forskolin the UK14304 response was 46.9+/-9.6% of the 60 mM KCl response, and after relaxation with sodium nitroprusside (SNP) the response was 24.8+3.3%. However, after relaxation of the tissue with levcromakalim the UK14304 response was only 8.2+/-1.7%, which was not different from the control response in the same tissues (12.2+/-5.6%). An enhanced contraction was also obtained after relaxation of the tissue with the cyclic AMP analogue dibutyryl cyclic AMP (23.2+/-1.3%) indicating that at least part of the enhanced response to UK14304 is independent of the ability of the agonist to inhibit cyclic AMP production. 6. Relaxation of U46619 contracted ear arteries with SNP could be inhibited by the NO-sensitive guanylyl-cyclase inhibitor 1H-[1,2,4] oxadiazolo[4,3-a]quinoxalin-1-one (ODQ) indicating that production of cyclic GMP is necessary for the relaxant effect of SNP. However, ODQ had no effect on the relaxation of tissue by forskolin, suggesting that this compound does not act via production of cyclic GMP. Biochemical studies showed that while forskolin increases the levels of cyclic AMP in the tissues, SNP had no effect on the levels of this cyclic nucleotide. 7. In conclusion, enhanced contractions to the alpha2-adrenoceptor agonist UK14304 can be uncovered in porcine isolated ear arteries by precontracting the tissue with U46619, followed by relaxation back to baseline with forskolin, SNP or dibutyryl cyclic AMP before addition of UK14304. There was a greater contractile response to UK14304 after relaxation with forskolin than with SNP or dibutyryl cyclic AMP, suggesting that cyclic AMP-dependent and- independent mechanisms are involved in the enhancement of the UK14304 response.

Quantitative changes in pharmacodynamic parameters of noradrenaline in different rat aorta preparations: influence of endogenous EDRF

1. The aim of the present study was to assess the role of endothelial cells in the modulation of vasocontractile responses to noradrenaline in rat isolated aorta when cut as standard helical strips or as ring segments. 2. Noradrenaline-potency in helical strip preparations evaluated as -logEC50 was greater than that obtained in endothelium-intact ring preparations (9.45 +/- 0.28 versus 8.69 +/- 0.09, respectively) (P < 0.05). The maximum contractile response of helical strips was significantly higher than the response of ring preparations (P < 0.05). 3. Subsequent experiments were performed on helical strips and ring preparations where the endothelium was removed by rubbing the luminal surface of the aorta with filter paper. Removal of the endothelium potentiated the noradrenaline-induced contraction in ring preparations, but not in the helical strips. 4. The nitric oxide synthase inhibitors L-NAME (3 x 10(-5)-3 x 10(-4) M) or L-NNA (1 x 10(4)-3 x 10(-4) M) which were added to the tissue bath potentiated the noradrenaline-induced contraction in the endothelium-intact ring preparations, although only L-NNA induced a statistically significant potentiation. Both L-NAME and L-NNA had no effect on the noradrenaline-contraction induced in rings without endothelium, or in helical strips with or without endothelium. 5. Vascular acetylcholine-induced relaxation is dependent on endothelium derived relaxing factor (nitric oxide). Acetylcholine (10(-9)-10(-6) M) induced a concentration-dependent relaxation in noradrenaline preconstricted intact rings. The relaxant response was strongly reduced by L-NAME (3 x 10(-5)-1 x 10(-4) M). The relaxant response to acetylcholine was very weak in ring and helical strip preparations without endothelium, but also, surprisingly, in unrubbed standard helical strips. 6. The present results suggest that the endothelium of standard helical strip preparations may be greatly damaged, a view confirmed by morphological studies. The structural and functional damage of the endothelium induced very important changes in pharmacodynamic parameters such as in the potency and the maximal responses of vascular preparations to noradrenaline. Therefore, caution must be observed when the potency and intrinsic activity of agonists evaluated on different preparations are compared, even if these come from the same vascular segment.

Norepinephrine attenuates serotonin inhibition of pial venous tone

Serotonin (5-hydroxytryptamine, 5-HT) has been shown to inhibit the rhythmic constrictions, accompanied by an increase in cAMP synthesis, in porcine pial veins. Since porcine pial veins contain predominant postsynaptic alpha 2-adrenoceptors which are coupled to Gi-protein, the possibility that the inhibitory effect of 5-HT is antagonized by norepinephrine was examined pharmacologically, using tissue bath techniques. The results indicated that norepinephrine (0.1-1 microM) attenuated 5-HT-induced inhibition of rhythmic constriction. This effect of norepinephrine was mimicked by clonidine (an alpha 2-adrenoceptor agonist), but not by methoxamine (an alpha 1-adrenoceptor agonist). Furthermore, the effect of norepinephrine was prevented by yohimbine (an alpha 2-adrenoceptor antagonist) and pertussis toxin, but was not prevented by prazosin (an alpha 1-adrenoceptor antagonist). In parallel studies, the basal concentration of cAMP and that induced by 5-HT in the pial veins were inhibited by norepinephrine (0.3 microM). These results are consistent with the previous findings that 5-HT-induced inhibition of rhythmic constriction in the porcine pial veins is associated with an increase in vascular cAMP synthesis and suggest that norepinephrine attenuates 5-HT-induced inhibition of rhythmic constriction in part by negatively coupling to adenylate cyclase via alpha 2-adrenoceptors.

The mechanism of action of alpha 2-adrenoceptors in human isolated subcutaneous resistance arteries

1. The effect of noradrenaline and the selective alpha 2-adrenoceptor agonist, azepexole, on tone and intracellular Ca2+ ([Ca2+]i) was examined in human isolated subcutaneous resistance arteries. Isolated arteries were mounted on an isometric myograph and loaded with the Ca2+ indicator, fura-2, for simultaneous measurement of force and [Ca2+]i. 2. High potassium solution (KPSS), noradrenaline and azepexole increased [Ca2+]i and contracted subcutaneous arteries in physiological saline. When extracellular Ca2+ was removed and the calcium chelator, BAPTA, added to the physiological saline (PSSo), responses to noradrenaline were transient and reduced, and responses to azepexole were markedly inhibited. 3. Ryanodine, an agent which interferes with Ca2+ release from intracellular stores, had little effect on contractile responses to KPSS, noradrenaline or azepexole in physiological saline. The response to caffeine in physiological saline was inhibited by ryanodine. In PSSo, ryanodine partially inhibited contractile responses to noradrenaline and azepexole, and completely abolished the response to caffeine. 4. Noradrenaline and azepexole both significantly increased maximum force achieved by cumulative addition of Ca2+ to a Ca(2+)-free depolarizing solution and shifted the calculated relationship between [Ca2+]i and force to the left, suggesting these agents increase the sensitivity of the contractile apparatus to [Ca2+]i. 5. (-)-202 791, a dihydropyridine antagonist of voltage-operated calcium channels partially inhibited both the contractile response and the rise in [Ca2+]i induced by azepexole. Pre-treatment of arteries with pertussis toxin inhibited responses to azepexole, but had no significant effect on tone induced by KPSS or noradrenaline. ETYA, an inhibitor of phospholipase A2, lipoxygenase and cyclo-oxygenase, had no effect on azepexole-induced contraction in the presence of N omega nitro-L-arginine methyl ester.6. Azepexole, a selective alpha2-adrenoceptor agonist, contracts human subcutaneous resistance arteries by a mechanism largely dependent on the influx of extracellular Ca2", probably through voltage-operated calcium channels. This action involves a pertussis toxin-sensitive G protein, possibly Gi.

Alpha 2-adrenoceptor mediated inhibition of forskolin-stimulated cyclic AMP accumulation in isolated porcine palmar lateral veins

The aim of this study was to use a 3H-adenine pre-labelling technique to characterise the effect of alpha 2-adrenoceptor activation on forskolin-stimulated cyclic AMP accumulation in the isolated porcine palmar lateral vein. Forskolin (10(-7)-10(-4) M) stimulated 3H-cyclic AMP accumulation in the isolated porcine palmar lateral vein in a biphasic and concentration-dependent manner. In the absence of the cyclic AMP-selective phosphodiesterase inhibitor rolipram, forskolin stimulated 3H-cyclic AMP accumulation approximately 7-8 fold. The response reached a peak after 5 min. In the presence of rolipram (10(-5) M), basal 3H-cyclic AMP levels were approximately 70% higher than in its absence (basal: 1823 +/- 57 dpm; rolipram: 3088 +/- 229, n = 3) and forskolin (3 x 10(-5) M) stimulated 3H-cyclic AMP accumulation approximately 8 fold. The latter response reached a plateau 10 min after the addition of forskolin. In all subsequent studies, the tissues were incubated with forskolin (3 x 10(-5) M) for 5 min in the absence of rolipram. Noradrenaline (NA; 10(-9)-10(-4) M) and UK14304 (10(-9)-10(-4) M) inhibited forskolin-stimulated 3H-cyclic AMP accumulation in a concentration-dependent manner with mean pIC50 values of 7.61 +/- 0.37 (n = 4) and 7.76 +/- 0.23 (n = 5), respectively. With either NA or UK14304, the maximal inhibition of the forskolin response obtained was approximately 75%. Neither NA (10(-4) M) nor UK14304 (10(-4) M) altered basal 3H-cyclic AMP levels.(ABSTRACT TRUNCATED AT 250 WORDS)