Mechanisms of signal transduction during alpha 2-adrenergic receptor-mediated contraction of vascular smooth muscle

Milestone Papers on Signal Transduction Mechanisms of Hypertension and Its Complications

To celebrate 100 years of American Heart Association-supported cardiovascular disease research, this review article highlights milestone papers that have significantly contributed to the current understanding of the signaling mechanisms driving hypertension and associated cardiovascular disorders. This article also includes a few of the future research directions arising from these critical findings. To accomplish this important mission, 4 principal investigators gathered their efforts to cover distinct yet intricately related areas of signaling mechanisms pertaining to the pathogenesis of hypertension. The renin-angiotensin system, canonical and novel contractile and vasodilatory pathways in the resistance vasculature, vascular smooth muscle regulation by membrane channels, and noncanonical regulation of blood pressure and vascular function will be described and discussed as major subjects.

Signaling pathways in vascular function and hypertension: molecular mechanisms and therapeutic interventions

Hypertension is a global public health issue and the leading cause of premature death in humans. Despite more than a century of research, hypertension remains difficult to cure due to its complex mechanisms involving multiple interactive factors and our limited understanding of it. Hypertension is a condition that is named after its clinical features. Vascular function is a factor that affects blood pressure directly, and it is a main strategy for clinically controlling BP to regulate constriction/relaxation function of blood vessels. Vascular elasticity, caliber, and reactivity are all characteristic indicators reflecting vascular function. Blood vessels are composed of three distinct layers, out of which the endothelial cells in intima and the smooth muscle cells in media are the main performers of vascular function. The alterations in signaling pathways in these cells are the key molecular mechanisms underlying vascular dysfunction and hypertension development. In this manuscript, we will comprehensively review the signaling pathways involved in vascular function regulation and hypertension progression, including calcium pathway, NO-NOsGC-cGMP pathway, various vascular remodeling pathways and some important upstream pathways such as renin-angiotensin-aldosterone system, oxidative stress-related signaling pathway, immunity/inflammation pathway, etc. Meanwhile, we will also summarize the treatment methods of hypertension that targets vascular function regulation and discuss the possibility of these signaling pathways being applied to clinical work.

Functional sympatholysis in mouse skeletal muscle involves sarcoplasmic reticulum swelling in arterial smooth muscle cells

The vasoconstrictive effect of sympathetic activity is attenuated in contracting skeletal muscle (functional sympatholysis), allowing increased blood supply to the working muscle but the underlying mechanisms are incompletely understood. The purpose of this study was to examine α-adrenergic receptor responsiveness in isolated artery segments from non-exercised and exercised mice, using wire myography. Isometric tension recordings performed on femoral artery segments from exercised mice showed decreased α-adrenergic receptor responsiveness compared to non-exercised mice (logEC50 -5.2 ± 0.04 M vs. -5.7 ± 0.08 M, respectively). In contrast, mesenteric artery segments from exercised mice displayed similar α-adrenergic receptor responses compared to non-exercised mice. Responses to the vasoconstrictor serotonin (5-HT) and vasodilator isoprenaline, were similar in femoral artery segments from non-exercised and exercised mice. To study sarcoplasmic reticulum (SR) function, we examined arterial contractions induced by caffeine, which depletes SR Ca2+ and thapsigargin, which inhibits SR Ca2+ -ATPase (SERCA) and SR Ca2+ uptake. Arterial contractions to both caffeine and thapsigargin were increased in femoral artery segment from exercised compared to non-exercised mice. Furthermore, 3D electron microscopy imaging of the arterial wall showed SR volume/length ratio increased 157% in smooth muscle cells of the femoral artery from the exercised mice, whereas there was no difference in SR volume/length ratio in mesenteric artery segments. These results show that in arteries surrounding exercising muscle, the α-adrenergic receptor constrictions are blunted, which can be attributed to swollen smooth muscle cell SR's, likely due to increased Ca2+ content that is possibly reducing free intracellular Ca2+ available for contraction. Overall, this study uncovers a previously unknown mechanism underlying functional sympatholysis.

TRPA1 is essential for the vascular response to environmental cold exposure

The cold-induced vascular response, consisting of vasoconstriction followed by vasodilatation, is critical for protecting the cutaneous tissues against cold injury. Whilst this physiological reflex response is historic knowledge, the mechanisms involved are unclear. Here by using a murine model of local environmental cold exposure, we show that TRPA1 acts as a primary vascular cold sensor, as determined through TRPA1 pharmacological antagonism or gene deletion. The initial cold-induced vasoconstriction is mediated via TRPA1-dependent superoxide production that stimulates α_2C-adrenoceptors and Rho-kinase-mediated MLC phosphorylation, downstream of TRPA1 activation. The subsequent restorative blood flow component is also dependent on TRPA1 activation being mediated by sensory nerve-derived dilator neuropeptides CGRP and substance P, and also nNOS-derived NO. The results allow a new understanding of the importance of TRPA1 in cold exposure and provide impetus for further research into developing therapeutic agents aimed at the local protection of the skin in disease and adverse climates.

Blood flow in the skin of mammals changes in response to cold, but the mechanisms driving this response are unclear. Aubdool et al. show that the non-selective cation channel, TRPA1, is a vascular cold sensor and required for the vascular protective response to local cold exposure.

Altered reactivity of tertiary mesenteric arteries following acute myocardial ischemia

BACKGROUND

It is unknown if cardiac ischemia has any deleterious effect on the contractile properties of nonischemic, peripheral vascular beds. Thus, the objective of the present study was to determine whether acute myocardial ischemia results in peripheral vascular dysfunction.

METHODS AND RESULTS

This study characterized force maintenance and the sensitivity to acetylcholine (ACh)-mediated smooth muscle (SM) relaxation of tertiary (3rd) mesenteric arteries from Sprague-Dawley rats following 30 min of myocardial ischemia. Both the phosphorylation of nonmuscle (NM) light chain (LC) and SM-LCs as well as the expression of myosin phosphatase targeting subunit 1 (MYPT1) were also determined. Our data demonstrate that acute myocardial ischemia resulted in vascular dysfunction of 3rd mesenteric vessels, characterized by decreases in force maintenance, ACh- and cGMP-mediated SM relaxation, the phosphorylation of NM-LCs and SM-LCs, and MYPT1 expression. Ischemia was also associated with an increase in protein polyubiquitination, suggesting that during ischemia MYPT1 is targeted for degradation or proteolysis.

CONCLUSION

Acute myocardial ischemia produces peripheral vascular dysfunction; the changes in LC phosphorylation and MYPT1 expression result in a decrease in both tone and the sensitivity to NO-mediated SM relaxation of the peripheral vasculature.

Calcium sensitization involved in dexmedetomidine-induced contraction of isolated rat aorta

Dexmedetomidine, a full agonist of the α2B-adrenoceptor that is mainly involved in vascular smooth muscle contraction, is primarily used for analgesia and sedation in intensive care units. High-dose dexmedetomidine produces hypertension in children and adults. The goal of this in vitro study was to investigate the role of the calcium (Ca2+) sensitization mechanism involving Rho-kinase, protein kinase C (PKC), and phosphoinositide 3-kinase (PI3-K) in mediating contraction of isolated rat aortic smooth muscle in response to dexmedetomidine. The effect of dexmedetomidine on the intracellular Ca2+ level ([Ca2+]i) and tension was measured simultaneously. Dexmedetomidine concentration–response curves were generated in the presence or absence of the following antagonists: rauwolscine, Y 27632, LY 294002, GF 109203X, and verapamil. Dexmedetomidine-induced phosphorylation of PKC and membrane translocation of Rho-kinase were detected with Western blotting. Rauwolscine, Y 27632, GF 109203X, LY 294002, and verapamil attenuated dexmedetomidine-induced contraction. The slope of the [Ca2+]i–tension curve for dexmedetomidine was higher than that for KCl. Dexmedetomidine induced phosphorylation of PKC and membrane translocation of Rho-kinase. These results suggest that dexmedetomidine-induced contraction involves a Ca2+ sensitization mechanism mediated by Rho-kinase, PKC, and PI3-K that is secondary to α2-adrenoceptor stimulation in rat aortic smooth muscle.

Interaction between alpha(1)- and alpha(2)-adrenoreceptors contributes to enhanced constrictor effects of norepinephrine in mesenteric veins compared to arteries

Mesenteric veins are more sensitive than arteries to the constrictor effects of sympathetic nerve stimulation and alpha-adrenoceptor agonists. We tested the hypothesis that alpha(1)- and alpha(2)-adrenoceptors interact to enhance adrenergic reactivity of mesenteric veins. We studied neurogenic and agonist-induced constrictions of mesenteric veins and arteries in vitro. Norepinephrine concentration-response curves were left-shifted in veins compared to arteries. UK 14,304 (0.01-1 microM, alpha(2)-adrenoceptor receptor agonist) did not constrict arteries or veins but enhanced constrictions and Ca(2+) signals mediated by alpha(1)-adrenoceptor stimulation in veins. Yohimbine (alpha(2)-adrenoceptor receptor antagonist) and MK912 (alpha(2C)-adrenoceptor receptor antagonist), but not alpha(2A)- or alpha(2B)-adrenoceptor antagonists, produced rightward shifts in norepinephrine concentration-response curves in veins. Pharmacological studies revealed that alpha(1D)-adrenoceptors mediate venous constrictions. Norepinephrine responses in veins from alpha(2C)-adrenoceptor knock-out (KO) mice were not different from wild type veins. Yohimbine inhibited norepinephrine constrictions in alpha(2C)-adrenoceptor KO veins suggesting that there is upregulation of other alpha(2)-adrenoceptors in alpha(2C)-KO mice. These data indicate that alpha(1D)- and alpha(2C)-adrenoceptors interact in veins but not in arteries. This interaction enhances venous adrenergic reactivity. Mesenteric vein-specific alpha(2)-adrenoceptor linked Ca(2+) and perhaps other signaling pathways account for enhanced venous adrenergic reactivity.

Nonmuscle myosin, force maintenance, and the tonic contractile phenotype in smooth muscle

Recent studies have demonstrated that nonmuscle (NM) myosin II forms filaments and can generate and maintain force in smooth muscle tissue [Lofgren et al. (2003) J Gen Physiol 121:301-310; Morano et al. (2000) Nat Cell Biol 2:371-375]. To further investigate the mechanical contribution of NM myosin to force maintenance during smooth muscle contraction, we utilized a selective inhibitor of the NM myosin ATPase, blebbistatin [Straight et al. (2003) Science 299:1743-1747]. Force and myosin light chain (MLC(20)) phosphorylation were measured during KCl stimulation of small strips of intact mouse bladder and aorta at 22 degrees C. The bladder strips contracted with a typical phasic force response, characterized by a large, rapid, transient increase in force followed by a decline to a lower, steady-state level. The addition of blebbistatin did not alter the peak force, but decreased force maintenance. KCl depolarization of aortic strips resulted in a tonic contraction; force increased to a sustained steady state. Similar to the bladder tissue, blebbistatin substantially decreased the steady-state force in the aorta. Blebbistatin did not influence the MLC(20) phosphorylation transient in either tissue type. Additionally, blebbistatin did not change the maximum shortening velocity (V (max)) during KCl depolarization of the aorta. Our results also suggest that NMIIA and NMIIB isoforms are differentially expressed. The expression of NMIIA is more prominent in the bladder, while NMIIB expression is predominant in the aorta. These results suggest that NM myosin contributes to the mechanism of force maintenance in smooth muscle, and could suggest that the expression of NMIIB is a factor for determining the tonic contractile phenotype.

Sympathetic neural inhibition of conducted vasodilatation along hamster feed arteries: complementary effects of alpha1- and alpha2-adrenoreceptor activation

Vasodilatation initiated on arterioles of skeletal muscle ascends into the proximal feed arteries through cell-to-cell conduction along the endothelium and into smooth muscle. Whereas perivascular sympathetic nerve activity (SNA) can inhibit conducted vasodilatation and restrict muscle blood flow, the signalling events mediating this interaction are poorly defined. Therefore, using isolated pressurized (75 mmHg) feed arteries (diameter (microm) at rest = 53 +/- 3; maximum = 99 +/- 2; n = 86) of the hamster retractor muscle, we tested the hypothesis that distinct yet complementary signalling pathways underlie the ability of SNA to inhibit conduction. Conducted vasodilatation was initiated using ACh microiontophoresis (1 microA; 250, 500 and 1000 ms) and SNA was initiated using local field stimulation (30-50 V; 1 ms at 2, 8 and 16 Hz). With vasodilatations of 5-20 microM, conduction increased with ACh pulse duration and was inhibited progressively as the frequency of SNA increased. During SNA, conduction was partially restored with inhibition of alpha1- (0.1 microM prazosin) or alpha2- (0.1 microM RX821002) adrenoreceptors and fully restored with both antagonists present. Activating alpha1- (50 nM phenylephrine) or alpha2- (1 microM UK 14,304) adrenoreceptors inhibited conduction partially and their simultaneous activation inhibited conduction cumulatively (P < 0.05). Elevated [K+]o (30 or 40 mM) or phorbol esters (0.5 microM) also inhibited conduction yet similar constriction with l-NNA (50 microM) or Bay K 8644 (10 nM) did not. Thus, the activation of alpha1- and alpha2-adrenoreceptors inhibits conducted vasodilatation through complementary signalling events. With robust coupling along the endothelium, our modelling predicts that the inhibition of conduction by SNA can be explained by reduced electrical coupling through myoendothelial gap junctions or greater current leak across smooth muscle cell membranes.

Tyrosine kinases regulate intracellular calcium during alpha(2)-adrenergic contraction in rat aorta

We have demonstrated enhanced contractile sensitivity to the alpha(2)-adrenoreceptor (alpha(2)-AR) agonist UK-14304 in arteries from rats made hypertensive with chronic nitric oxide synthase (NOS) inhibition (LHR) compared with arteries from normotensive rats (NR); additionally, this contraction requires Ca(2+) entry. We hypothesized that tyrosine kinases augment alpha(2)-AR contraction in LHR arteries by increasing Ca(2+). The tyrosine kinase inhibitor tyrphostin 23 significantly attenuated UK-14304 contraction of denuded thoracic aortic rings from NR and LHR. However, tyrphostin 23 did not alter UK-14304 contraction in ionomycin-permeabilized aorta, which indicates that tyrosine kinases regulate intracellular Ca(2+) concentration. The Src family inhibitor PP1 and the epidermal growth factor receptor kinase inhibitor AG-1478 did not alter alpha(2)-AR contraction, whereas the mitogen-activated protein kinase extracellular signal-regulated kinase kinase inhibitor PD-98059 attenuated the contraction. Contraction to CaCl(2) in ionomycin-permeabilized LHR rings was greater than in NR rings. UK-14304 augmented CaCl(2) contraction in ionomycin-permeabilized rings from both groups but to a greater extent in LHR aorta. Together, these data suggest that alpha(2)-AR stimulates contraction via two pathways. One, which is enhanced with NOS inhibition hypertension, activates Ca(2+) sensitivity and is independent of tyrosine kinases. The other is tyrosine kinase dependent and regulates intracellular Ca(2+) concentration.

Acute and chronic NOS inhibition enhances alpha(2)- adrenoreceptor-stimulated RhoA and Rho kinase in rat aorta

We demonstrated that arteries from rats made hypertensive with chronic nitric oxide (NO) synthase (NOS) inhibition (N(omega)-nitro-L-arginine in drinking water, LHR) have enhanced contractile sensitivity to alpha(2)-adrenergic receptors (alpha(2)-AR) agonist UK-14304 compared with arteries from normotensive rats (NR). NO may regulate vascular tone in part through suppression of RhoA and Rho kinase (ROK). We hypothesized that enhanced RhoA and ROK activity augments alpha(2)-AR contraction in LHR aortic rings. Y-27632 eliminated UK-14304 contraction in LHR and NR aortic rings. The order of increasing sensitivity to Y-27632 was the following: endothelium-intact NR, LHR, and endothelium-denuded NR. UK-14304 stimulated RhoA translocation to the membrane fraction in LHR and denuded NR but not in intact NR aorta. Basally, more RhoA was present in the membrane fraction in denuded NR than in intact NR or LHR aorta. Relaxation to S-nitroso-N-acetyl-penicillamine and Y-27632 in denuded ionomycin-permeabilized rings was greater in NR than in LHR. Together these studies indicate alpha(2)-AR contraction depends on ROK activity more in NR than LHR aorta. Additionally, endogenous NO may regulate RhoA activation, whereas chronic NOS inhibition appears to cause RhoA desensitization.

Capacitative Ca(2+) entry in agonist-induced pulmonary vasoconstriction

Agonist-induced increases in cytosolic Ca(2+) concentration ([Ca(2+)](cyt)) in pulmonary artery (PA) smooth muscle cells (SMCs) consist of a transient Ca(2+) release from intracellular stores followed by a sustained Ca(2+) influx. Depletion of intracellular Ca(2+) stores triggers capacitative Ca(2+) entry (CCE), which contributes to the sustained increase in [Ca(2+)](cyt) and the refilling of Ca(2+) into the stores. In isolated PAs superfused with Ca(2+)-free solution, phenylephrine induced a transient contraction, apparently by a rise in [Ca(2+)](cyt) due to Ca(2+) release from the intracellular stores. The transient contraction lasted for 3-4 min until the Ca(2+) store was depleted. Restoration of extracellular Ca(2+) in the presence of phentolamine produced a contraction potentially due to a rise in [Ca(2+)](cyt) via CCE. The store-operated Ca(2+) channel blocker Ni(2+) reduced the store depletion-activated Ca(2+) currents, decreased CCE, and inhibited the CCE-mediated contraction. In single PASMCs, we identified, using RT-PCR, five transient receptor potential gene transcripts. These results suggest that CCE, potentially through transient receptor potential-encoded Ca(2+) channels, plays an important role in agonist-mediated PA contraction.

Agonist trafficking of G(i/o)-mediated alpha(2A)-adrenoceptor responses in HEL 92.1.7 cells

1. The ability of 19 agonists to elevate Ca(2+) and inhibit forskolin-induced cyclic AMP elevation through alpha(2A)-adrenoceptors in HEL 92.1.7 cells was investigated. Ligands of catecholamine-like- (five), imidazoline- (nine) and non-catecholamine-non-imidazoline-type (five) were included. 2. The relative maximum responses were similar in both assays. Five ligands were full or nearly full agonists, six produced 20 - 70% of the response to a full agonist and the remaining eight gave lower responses (< 20%) so that their potencies were difficult to evaluate. 3. Marked differences in the potencies of the agonists with respect to the two measured responses were seen. The catecholamines were several times less potent in decreasing cyclic AMP than in increasing Ca(2+), whereas the other, both imidazoline and ox-/thiazoloazepine ligands, were several times more potent with respect to the former than the latter response. For instance, UK14,304 was more potent than adrenaline with respect to the cyclic AMP response but less potent than adrenaline with respect to the Ca(2+) response. 4. All the responses were sensitive to pertussis toxin-pretreatment. Also the possible role of PLA(2), beta-adrenoceptors or ligand transport or metabolism as a source of error could be excluded. The results suggest that the active receptor states produced by catecholamines and the other agonists are markedly different and therefore have different abilities to activate different signalling pathways.

Pertussis toxin-sensitive G(i)-proteins and intracellular calcium sensitivity of vasoconstriction in the intact rat tail artery

1. We studied the involvement of pertussis toxin (PTX)-sensitive G-proteins in the sensitivity of arterial constriction to intracellular calcium ([Ca(2+)](i)) mobilization. 2. Vasoconstriction was measured in vitro in perfused, de-endothelialized rat tail arteries loaded with the calcium-sensitive dye, fura-2 and treated or not with PTX (30 - 1000 ng ml(-1)). Arteries were stimulated with noradrenaline (NA, 0.1 - 100 microM) or KCl (15 - 120 mM). 3. KCl elicited a smaller vasoconstrictor response (E(max)=94+/-8 mmHg) than NA (E(max)=198+/-9 mmHg) although [Ca(2+)](i) mobilization was similar (E(max)=123+/-8 and 135+/-7 nM for KCl and NA, respectively). PTX (1000 ng ml(-1)) had no effect on [Ca(2+)](i) mobilization but lowered NA- (but not KCl-) induced vasoconstriction (E(max)=118+/-7 mmHg). 4. G(i/o)-proteins were revealed by immunoblotting with anti-G(i alpha) and anti-G(o alpha) antibodies in membranes prepared from de-endothelialized tail arteries. [alpha(32)P]-ADP-ribosylation of G-proteins by PTX (1000 ng ml(-1)) was demonstrated in the intact rat tail artery (pixels in the absence of PTX: 3150, presence: 25053). 5. In conclusion, we suggest that smooth muscle cells possess a PTX-sensitive G(i)-protein-mediated intracellular pathway which amplifies [Ca(2+)](i) sensitivity of contraction in the presence of agonists such as NA.

In vivo analysis of adrenergic and serotoninergic constrictions of the rabbit saphenous vein

We aimed to develop a model to study in vivo the rabbit saphenous vein pharmacology and to investigate constrictions mediated by adrenoceptor and 5-HT receptor subtypes. We used the technique of high precision ultrasonic echo-tracking for direct measurement of saphenous vein diameters in pentobarbital anesthetized rabbits. Saphenous vein constrictions induced in rabbits by the alpha(1)-adrenoceptor agonist L-phenylephrine and the 5-HT(1B) receptor agonist sumatriptan were comparable with those induced in dogs but those induced by the 5-HT(1B) and 5-HT(7) receptor agonist 5-carboxamidotryptamine failed to appear in dogs. Dose-related constrictions of rabbit veins were obtained with L-phenylephrine and the alpha(2)-adrenoceptor agonist dexmedetomidine. Frequency-related constrictions of rabbit veins induced by nerve stimulation were partially inhibited by an alpha(1)-adrenoceptor or a postsynaptic alpha(2)-adrenoceptor antagonist (prazosin and SKF 104,078) but not affected by the pre- and post-synaptic alpha(2)-adrenoceptor antagonists BRL 44408 or rauwolscine. Constrictions of rabbit veins to sumatriptan and 5-CT were inhibited by GR 127935 and those induced by quipazine, a 5-HT(2) receptor agonist were prevented by ritanserin. The initial constrictions induced by 5-CT were followed by dilatations which were inhibited by the 5-HT(7) receptor antagonist mesulergine. These data indicate that rabbit saphenous veins, in vivo and at rest, respond to activation of 5-HT(1B) and 5-HT(2) receptors, alpha(1)- and alpha(2)-adrenoceptors and nerve stimulation; the dilator effect mediated by 5-HT(7) receptor activation was also detected. The data validate a new animal model to study superficial vein reactivity and its pharmacological sensitivity.

cGMP-mediated phosphorylation of heat shock protein 20 may cause smooth muscle relaxation without myosin light chain dephosphorylation in swine carotid artery

Nitrovasodilators such as nitroglycerine, via production of nitric oxide and an increase in [cGMP], can induce arterial smooth muscle relaxation without proportional reduction in myosin light chain (MLC) phosphorylation or myoplasmic [Ca2+]. These findings suggest that regulatory systems, other than MLC phosphorylation and Ca2+, partially mediate nitroglycerine-induced relaxation. In swine carotid artery, we found that a membrane-permeant cGMP analogue induced relaxation without MLC dephosphorylation, suggesting that cGMP mediated the relaxation. Nitroglycerine-induced relaxation was associated with a reduction in O2 consumption, suggesting that the interaction between phosphorylated myosin and the thin filament was inhibited. Nitroglycerine-induced relaxation was associated with a 10-fold increase in the phosphorylation of a protein on Ser16. We identified this protein as heat shock protein 20 (HSP20), a member of a family of proteins known to bind to thin filaments. When homogenates of nitroglycerine-relaxed tissues were centrifuged at 6000 g, phosphorylated HSP20 preferentially sedimented in the pellet, suggesting that phosphorylation of HSP20 may increase its affinity for the thin filament. We noted that a domain of HSP20 is partially homologous to the 'minimum inhibitory sequence' of skeletal troponin I. The peptide HSP20110-121, which contains this domain, bound to actin-containing filaments only in the presence of tropomyosin, a characteristic of troponin I. High concentrations of HSP20110-121 abolished Ca2+-activated force in skinned swine carotid artery. HSP20110-121 also partially decreased actin-activated myosin S1 ATPase activity. These data suggest that cGMP-mediated phosphorylation of HSP20 on Ser16 may have a role in smooth muscle relaxation without MLC dephosphorylation. HSP20 contains an actin-binding sequence at amino acid residues 110-121 that inhibited force production in skinned carotid artery. We hypothesize that phosphorylation of HSP20 regulates force independent of MLC phosphorylation via binding of HSP20 to thin filaments and inhibition of cross-bridge cycling.

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.

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.

Alpha2B-adrenoceptors couple to Ca2+ increase in both endogenous and recombinant expression systems

The ability of cloned human alpha2B-adrenoceptors heterologously expressed in Sf9 cells and endogenous alpha2B-adrenoceptors in NG 108-15 neuroblastoma x glioma cells to couple to increase of intracellular Ca2+ was studied. Ca2+ increases in NG 108-15 cells were detectable but slight, whereas those in alpha2B-adrenoceptor-expressing Sf9 cells were greater. In the latter, the maximum Ca2+ increase correlated positively, and the EC50-value of noradrenaline negatively, with the receptor expression density. The order of potency of the agonists was D-medetomidine ([D]-4-[5]-[1-(2,3-dimethylphenyl)ethyl]-1H-imidazole) > noradrenaline approximately = clonidine > oxymetazoline, with clonidine and UK14,304 (5-bromo-N-[4,5-dihydro-1H-imidazole-2-yl]-6-quinoxalinamine) being weak partial agonists. In Sf9 cells Ca2+ increases consisted of concomitant mobilization from an intracellular store and influx of extracellular Ca2+. In these cells alpha2B-adrenoceptor stimulation also increased the inositol 1,4,5-trisphosphate mass. We conclude that alpha2B-adrenoceptors can couple to intracellular Ca2+ increases which may involve prior activation of phospholipase C.

Functional cross-talk between endothelial muscarinic and alpha2-adrenergic receptors in rabbit cerebral arteries

Interactions between two classes of receptors have been observed in several cell lines and preparations. The aim of this work was to assess the impact of simultaneous stimulation of endothelial muscarinic and alpha2-adrenergic receptors (alpha2-AR) on vascular reactivity. Rabbit middle cerebral arteries were isolated and changes in isometric tension were recorded in the presence of indomethacin. Inhibition of nitric oxide (NO) synthase with Nomega-nitro-L-arginine (L-NOARG, 100 micromol l(-1)) revealed alpha-AR-dependent contractions. Pre-addition of acetylcholine (ACH, 1 micromol l(-1)) augmented oxymetazoline (OXY, 10 micromol l(-1), alpha2-AR agonist)-, but decreased phenylephrine (PE, 10 micromol(-1), alpha1-AR agonist)-induced contraction (P<0.05). The effects of ACH were endothelium-dependent. Vessels were precontracted with 40 mmol l(-1) KCl-physiological salt solution (PSS) in the absence of L-NOARG, or PE or OXY in the presence of L-NOARG. In the presence of high external K+ or PE, ACH induced a potent relaxation (P<0.05). In the presence of OXY, however, ACH mediated contraction (P<0.05). After pertussis toxin (PTX, inactivator of Galpha(i/o) proteins) pre-treatment, alpha2-AR-dependent contractions were abolished. Forty mmol l(-1) KCl-PSS induced contraction was not altered by PTX whereas ACH-induced relaxation was augmented (P<0.05). To investigate if endothelin-1 (ET-1) intervened in the endothelium-dependent contractile response to ACH in the presence of OXY-dependent tone, vessels were incubated in the presence of BQ123 (1 micromol l(-1)), an ETA receptor antagonist. OXY-mediated tone was not affected by BQ123; however, ACH-induced contraction was reversed to a relaxation (P<0.05). These data indicate that activation of endothelial alpha2-AR triggers an endothelium-dependent, ET-1 mediated, contraction to ACH. This suggests that activation of alpha2-AR affects muscarinic receptor/G protein coupling leading to an opposite biological effect.

Studies on the influence of insulin on cyclic adenosine monophosphate in human vascular smooth muscle cells: dependence on cyclic guanosine monophosphate and modulation of catecholamine effects

Insulin increases both cyclic guanosine monophosphate (cGMP) and cyclic adenosine monophosphate (cAMP) in human vascular smooth muscle cells (hVSMC) and attenuates noradrenaline-induced vasoconstriction. In the present study, we aimed at investigation in hVSMC: 1) the interrelationships between insulin-induced increases of cGMP and cAMP; 2) the insulin effect on the catecholamine modulation of cAMP. Catecholamines cause both vasoconstriction and vasodilation. Vasoconstriction is attributable to the reduced synthesis of cAMP in hVSMC through alpha 2-adrenoceptors and to direct effects on calcium fluxes through alpha 1-adrenoceptors; vasodilation is attributable to the increased synthesis of cAMP through beta-adrenoceptors. In the present study, we determined the influence of insulin on cAMP in hVSMC incubated with or without: a) the inhibitor of guanylate cyclase methylene blue or the inhibitor of nitric oxide synthase NG-monomethyl-L-arginine (L-NMMA); b) the beta-adrenergic agonists isoproterenol and salbutamol; c) the physiological catecholamines noradrenaline and adrenaline; d) noradrenaline+the beta-adrenergic antagonist propranolol or the alpha 2-adrenergic antagonist yohimbine; e) noradrenaline+methylene blue of L-NMMA. We demonstrated that: 1) the inhibition of the insulin-induced cGMP synthesis blunts the insulin-induced increase of cAMP; 2) insulin induces a significant increase of cAMP also in the presence of isoproterenol, salbutamol, noradrenaline and adrenaline: the combined effects of insulin and catecholamines were additive in some, but not in all the experiments; 3) insulin enhances the cAMP concentrations induced by noradrenaline also in the presence of alpha 2- or beta-adrenergic antagonists; 4) in the presence of methylene blue or L-NMMA insulin does not modify the noradrenaline effects on cAMP.

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.

Involvement of protein kinase C activation in alpha 2-adrenoceptor-mediated contractions of rabbit saphenous vein

The role of protein kinase C alpha 2-adrenoceptor-induced contractions of rabbit saphenous vein was investigated. Contractions induced by the alpha 2-adrenoceptor-selective agonist 5-bromo-6-[2-imidazolin-2-ylamino]-quinoline (UK14304) were inhibited by prior treatment with pertussis toxin and by Ca2+ removal, confirming a Gi/Go-dependent coupling pathway which was highly dependent upon Ca2+ influx. Protein kinase C inhibitors calphostin-C and staurosporine each caused a non-competitive inhibition of UK14304 response. Down-regulation of protein kinase C by pretreatment with tetradecanoylphorbol acetate reduced UK14304 response by almost 90% with no effect on contractions induced by elevated KCl. The ineffectiveness of L-type Ca2+ channel blockers and the absence of stimulated 45Ca2+ uptake or efflux by UK14304 indicated that phospholipid-derived products were most likely responsible for protein kinase C activation. alpha 2-Adrenoceptor stimulation failed to increase [3H]myoinositol phosphate formation, but caused a significant increase in the formation of both [32P]phosphatidic acid and diacylglycerol, indicating the possible activation of phospholipase D activity. These results suggest that protein kinase C is important for the vasoconstriction induced by alpha 2-adrenoceptors and that diacylglycerol derived from receptor-initiated phospholipase D activity may provide protein kinase C stimulation.

Alpha 2A-adrenoceptors mediate activation of non-selective cation channels via Gi-proteins in human erythroleukaemia (HEL) cells. No evidence for a functional role of imidazoline receptors in modulating calcium

Human erythroleukaemia (HEL) cells were investigated to characterize their alpha 2-adrenoceptor and imidazoline receptor sites. Membranes from HEL cells bound [3H]2-(2-methoxy-1, 4-benzodioxan-2yl)-2-imidazoline ([3H]RX821002) in a saturable and specific manner with a KD of 0.64 +/- 0.07 nM and a Bmax of 126 +/- 4 fmol/mg protein. [3H]RX821002 was displaced from HEL membranes by adrenergic drugs with the order of potency being yohimbine approximately oxymetazoline >> prazosin = 2-[2-[4-(o-methoxyphenyl)piperazin-1-yl]ethyl]-4,4-dimethyl- 1,3(2H,4H)-isochinolindione HCl (ARC 239), consistent with this site being an alpha 2A-adrenoceptor. HEL membranes also bound [3H]idazoxan in the presence of adrenaline to block alpha 2-adrenoceptors. This binding was saturable and specific with a KD of 3.5 +/- 1.0 nM and a Bmax of 31 +/- 6 fmol/mg protein. Adrenergic drugs from both the phenylethylamine and imidazoline classes increased high-affinity GTPase activity, an index of activation of regulatory heterotrimeric guanine-nucleotide binding proteins (G-proteins), and produced increases in cytosolic free calcium concentration ([Ca2+]i). The effects of these agonists in both systems were abolished by pertussis toxin pretreatment, and oxymetazoline and clonidine were antagonists. The potency of adrenergic drugs to inhibit 5-bromo-6-(2-imidazolin-2-ylamino)-quinoxaline (UK 14304)-induced increases in [Ca2+]i was yohimbine approximately oxymetazoline >> ARC 239, consistent with the binding data and an action at alpha 2A-adrenoceptors. No evidence was found for a role of imidazoline receptors in stimulating G-proteins or modulating [Ca2+]i. The adrenergic agonist-induced increases in [Ca2+]i were due to both release of Ca2+ from intracellular stores and entry of extracellular Ca2+. Ca2+ entry was blocked by 1-(beta-[3-(4-methoxyphenyl)propoxy]-4-methoxyphenylethyl)-1H- imidazole hydrochloride (SKF 96365), but not by nitrendipine. Adrenaline also stimulated Mn2+ entry in HEL cells. Taken together, these results suggest that HEL cells have alpha 2A-adrenoceptors that activate non-selective cation channels via pertussis toxin-sensitive G-proteins, i.e. Gi-proteins.

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

1. We have examined the effect of elevation of cellular adenosine 3':5'-cyclic monophosphate (cyclic AMP) on alpha 1- and alpha 2-adrenoceptor-mediated contraction of the isolated palmar lateral vein of the pig. Cellular cyclic AMP was increased by either inhibition of phosphodiesterase by rolipram, or direct activation of adenylyl cyclase by forskolin. 2. Noradrenaline (1 nM-10 microM) caused concentration-dependent contractions of the porcine isolated palmar lateral vein (pD2 7.32 +/- 0.07, n = 10). The selective alpha 1-adrenoceptor antagonist, prazosin (0.1 microM) and the selective alpha 2-adrenoceptor antagonist, rauwolscine (1 microM) caused a 10 fold rightward displacement of the concentration-response curve and a combination of the two antagonists caused a 200 fold rightward displacement of the concentration-response curve. The selective alpha 2-adrenoceptor agonist, UK-14304, also produced concentration-dependent contractions of the palmar lateral vein (pD2 7.70 +/- 0.15, n = 5), but the maximum response was 55.5 +/- 7.6% (n = 5) of that produced by noradrenaline. Prazosin (0.1 microM) failed to affect responses to UK-14304 but rauwolscine, 1 microM, caused a 200 fold rightward displacement. The estimated pKB value for rauwolscine (8.28 +/- 0.19, n = 10) is consistent with inhibition of alpha 2-adrenoceptors. Thus, the porcine isolated palmar lateral vein has a population of alpha 1- and alpha 2-adrenoceptors capable of producing a contraction. 3. Rolipram, 10 micro M, and forskolin, 1 micro M, caused a 2-3 fold rightward displacement of the noradrenaline concentration-response curve (CRC), but 1,9-dideoxyforskolin, 1 micro M, a forskolin analogue which does not activate adenylyl cyclase, failed to produce a significant inhibition of noradrenaline induced contractions. The combination of forskolin (1 micro M) and rolipram (10 micro M) were additive, producing a 20 fold rightward displacement of the noradrenaline CRC.4. Responses to noradrenaline were similarly affected by a combination of rolipram (10 micro M) and prazosin (0.1 micro M) (isolation of alpha 2-adrenoceptors) and the combination of rolipram (10 micro M) and rauwolscine(1 micro M) (isolation of alpha l-adrenoceptors), resulting in a 100 fold rightward displacement of the noradrenaline CRC. Although forskolin inhibited both alpha l- and alpha 2-adrenoceptor-mediated contractions,the effects produced were not similar. In particular, noradrenaline, 0.3-3 micro M, produced a significant contraction in the presence of forskolin (1 micro M) and prazosin (0.1 micro M) (an alpha 2-adrenoceptor-mediated response) but not in the presence of forskolin (1 micro M) and rauwolscine (1 micro M) (an alpha l-adrenoceptor mediated response).5. Five minute exposure to either rolipram (10 micro M) or forskolin (1 micro M) elevated [3H]-cyclic AMP of the porcine isolated palmar lateral vein by approximately 70% and 150-200%, respectively. Neither noradrenaline (1 nM- 100 micro M) nor UK-14304 (1 nM- 100 micro M) affected basal levels of [3H]-cyclic AMP,but both produced a concentration-dependent inhibition of forskolin-stimulated [3H]-cyclic AMP accumulation with a pKi of 7.43 +/- 0.1 (n = 3) and 7.97 +/- 0.18 (n = 3), respectively. The effect of noradrenaline against forskolin-stimulated [3H]-cyclic AMP accumulation was reversed by rauwolscine(1 micro M) but not by prazosin (0.1 micro M). In contrast, alpha 2-adrenoceptor activation did not affect rolipram induced elevation of [3H]-cyclic AMP.6. These findings indicate that M2-adrenoceptor contractions of the porcine isolated palmar lateral vein are not produced by reduction in cellular cyclic AMP per se. It is proposed that this response involves a novel signal transduction mechanism. However, when cellular cyclic AMP has been elevated by agents that stimulate adenylyl cyclase, rather than through inhibition of phosphodiesterase, the ability of alpha 2-adrenoceptors to inhibit cyclic AMP formation may be of functional importance in vascular smooth muscle.