Acute dilation to alpha(2)-adrenoceptor antagonists uncovers dual constriction and dilation mediated by arterial alpha(2)-adrenoceptors

BACKGROUND AND PURPOSE

In mouse tail arteries, selective alpha(2)-adrenoceptor antagonism with rauwolscine caused powerful dilation during constriction to the alpha(1)-adrenoceptor agonist phenylephrine. This study therefore assessed phenylephrine's selectivity at vascular alpha-adrenoceptors and the mechanism(s) underlying dilation to rauwolscine.

EXPERIMENTAL APPROACH

Mouse isolated tail arteries were assessed using a pressure myograph.

KEY RESULTS

The alpha(2)-adrenoceptor agonist UK14,304 caused low-maximum constriction that was inhibited by rauwolscine (3 x 10(-8) M) but not by the selective alpha(1)-adrenoceptor antagonist prazosin (10(-7) M). Concentration-effect curves to phenylephrine, cirazoline or noradrenaline were unaffected by rauwolscine but were inhibited by prazosin, which was more effective at high compared with low levels of constriction. In the presence of prazosin, rauwolscine inhibited the curves and was more effective at low compared with high levels of constriction. Although rauwolscine alone did not affect concentration-effect curves to phenylephrine, noradrenaline or cirazoline, it caused marked transient dilation when administered during constriction to these agonists. Dilation was mimicked by another alpha(2)-adrenoceptor antagonist (RX821002, 3 x 10(-8) M), was dependent on agonist selectivity, and did not occur during adrenoceptor-independent constriction (U46619). During constriction to UK14,304 plus U46619, rauwolscine or rapid removal of UK14,304 caused transient dilation that virtually abolished the combined constriction. Endothelial denudation reduced these dilator responses.

CONCLUSIONS AND IMPLICATIONS

Inhibition of alpha(2)-adrenoceptors caused transient dilation that was substantially greater than the contribution of alpha(2)-adrenoceptors to the constriction. This reflects a slowly reversing alpha(2)-adrenoceptor-mediated endothelium-dependent dilation and provides a rapid, sensitive test of alpha(2)-adrenoceptor activity. This approach also clearly emphasizes the poor selectivity of phenylephrine at vascular alpha-adrenoceptors.

In vivo endothelial denudation disrupts smooth muscle caveolae and differentially impairs agonist-induced constriction in small arteries

Experiments were performed to determine the effects of endothelial denudation in vivo on vasoconstrictor responses of mouse tail artery segments in vitro. A sterile wire (70 microm diameter) was inserted into tail arteries of anesthetized mice to mechanically denude the endothelium, and the animals were allowed to recover for 48 hours. The function of pressurized tail artery segments was then studied in vitro. Intimal injury markedly reduced endothelium-dependent relaxation to acetylcholine. Constriction evoked by the selective alpha1-adrenoceptor (alpha1-AR) agonist, phenylephrine, was not affected by in vivo endothelial denudation, indicating that the contractile function of vascular smooth muscle cells (VSMCs) was not impaired. However, constriction to the selective alpha2-AR agonist UK14304 or to endothelin-1 was significantly inhibited. Confocal microscopy of intact tail arteries localized caveolin-1 to punctuate structures, arranged in rows on or close to the surface of VSMCs. After in vivo endothelial denudation, this pattern was disrupted and caveolin-1 was localized to intracellular sites. When VSMC caveolae were disrupted in control arteries using the cholesterol acceptor methyl-beta-cyclodextrin, there was a similar impairment in constriction to endothelin-1 or alpha2-AR stimulation, but not alpha1-AR activation. These results suggest that intimal injury to small cutaneous arteries disrupts VSMC surface caveolae and selectively impairs constriction to stimuli that are dependent on these structures for signaling.

Estrogen increases smooth muscle expression of α2C-adrenoceptors and cold-induced constriction of cutaneous arteries

Raynaud's phenomenon, which is characterized by intense cold-induced constriction of cutaneous arteries, is more common in women compared with men. Cold-induced constriction is mediated in part by enhanced activity of α2C-adrenoceptors (α2C-ARs) located on vascular smooth muscle cells (VSMs). Experiments were therefore performed to determine whether 17β-estradiol regulates α2C-AR expression and function in cutaneous VSMs. 17β-Estradiol (0.01–10 nmol/l) increased expression of the α2C-AR protein and the activity of the α2C-AR gene promoter in human cultured dermal VSMs, which was assessed following transient transfection of the cells with a promoter-reporter construct. The effect of 17β-estradiol was associated with increased accumulation of cAMP and activation of the cAMP-responsive Rap2 GTP-binding protein. Transient transfection of VSMs with a dominant-negative mutant of Rap2 inhibited the 17β-estradiol-induced activation of the α2C-AR gene promoter, whereas a constitutively active mutant of Rap2 increased α2C-AR promoter activity. The effects of 17β-estradiol were inhibited by the estrogen receptor (ER) antagonist, ICI-182780 (1 μmol/l), and were mimicked by a cell-impermeable form of the hormone (estrogen:BSA) or by the selective ER-α receptor agonist 4,4′,4‴-(4-propyl-[1H]-pyrazole-1,3,5-triyl)tris-phenol (PPT; 10 nmol/l) or the selective ER-β receptor agonist 2,3-bis(4-hydroxyphenyl)-propionitrile (DPN; 10 nmol/l). Therefore, 17β-estradiol increased expression of α2C-ARs by interacting with cell surface receptors to cause a cAMP/Rap2-dependent increase in α2C-AR transcription. In mouse tail arteries, 17β-estradiol (10 nmol/l) increased α2C-AR expression and selectively increased the cold-induced amplification of α2-AR constriction, which is mediated by α2C-ARs. An estrogen-dependent increase in expression of cold-sensitive α2C-ARs may contribute to the increased activity of cold-induced vasoconstriction under estrogen-replete conditions.

Acute vibration increases alpha2C-adrenergic smooth muscle constriction and alters thermosensitivity of cutaneous arteries

The vascular symptoms of hand-arm vibration syndrome, including cold-induced vasospasm, are in part mediated by increased sensitivity of cutaneous arteries to sympathetic stimulation. The goal of the present study was to use a rat tail model to analyze the effects of vibration on vascular function and alpha-adrenoceptor (AR) responsiveness. Rats were exposed to a single period of vibration (4 h, 125 Hz, constant acceleration 49 m/s2 root mean square). The physical or biodynamic response of the tail demonstrated increased transmissibility or resonance at this frequency, similar to that observed during vibration of human fingers. Morphological analysis demonstrated that vibration did not appear to cause structural injury to vascular cells. In vitro analysis of vascular function demonstrated that constriction to the alpha1-AR agonist phenylephrine was similar in vibrated and control arteries. In contrast, constriction to the alpha2-AR agonist UK14304 was increased in vibrated compared with control arteries, both in endothelium-containing or endothelium-denuded arteries. The alpha2C-AR antagonist MK912 (3 x 10(-10) M) inhibited constriction to UK14304 in vibrated but not control arteries, reversing the vibration-induced increase in alpha2-AR activity. Moderate cooling (to 28 degrees C) increased constriction to the alpha2-AR agonist in control and vibrated arteries, but the magnitude of the amplification was less in vibrated compared with control arteries. Endothelium-dependent relaxation to acetylcholine was similar in control and vibrated arteries. Based on these results, we conclude that a single exposure to vibration caused a persistent increase in alpha2C-AR-mediated vasoconstriction, which may contribute to the pathogenesis of vibration-induced vascular disease.

Reactive oxygen species from smooth muscle mitochondria initiate cold-induced constriction of cutaneous arteries

Cold constricts cutaneous blood vessels by selectively increasing the activity of smooth muscle alpha2-adrenoceptors (alpha2-ARs). In mouse tail arteries, alpha2-AR constriction is mediated by alpha2A-ARs at 37 degrees C, whereas the cold-induced augmentation in alpha2-AR activity is mediated entirely by alpha2C-ARs. Cold causes translocation of alpha2C-ARs from the trans-Golgi to the plasma membrane, mediated by cold-induced activation of RhoA and Rho kinase. The present experiments analyzed the mechanisms underlying these responses. Mouse tail arteries were studied in a pressure myograph. Cooling the arteries (28 degrees C) caused a rapid increase in reactive oxygen species (ROS) in smooth muscle cells, determined by confocal microscopy of arteries loaded with the ROS-sensitive probes, dichlorodihydrofluorescein or reduced Mitotracker Red. The inhibitor of mitochondrial complex I rotenone (10 micromol/l), the antioxidant N-acetylcysteine (NAC; 20 mmol/l), or the cell-permeable mimic of superoxide dismutase MnTMPyP (50 micromol/l) did not affect vasoconstriction to alpha2-AR stimulation (UK-14304) at 37 degrees C but dramatically inhibited the response at 28 degrees C. Indeed, these ROS inhibitors abolished the cold-induced increase in alpha2-AR constrictor activity. NAC (20 mmol/l) or MnTMPyP (50 micromol/l) also abolished the cold-induced activation of RhoA in human cultured vascular smooth muscle cells and the cold-induced mobilization of alpha2C-ARs to the cell surface in human embryonic kidney 293 cells transfected with the receptor. The combined results suggest that cold-induced constriction is mediated by redox signaling in smooth muscle cells, initiated by mitochondrial generation of ROS, which stimulate RhoA/Rho kinase signaling and the subsequent mobilization of alpha2C-ARs to the cell surface. Altered activity of ROS may contribute to cold-induced vasospasm occurring in Raynaud's phenomenon.

Rho Kinase Mediates Cold-Induced Constriction of Cutaneous Arteries

Cold-induced vasoconstriction in cutaneous blood vessels is mediated in part by increased activity of vascular smooth muscle α2-adrenoceptors (VSM α2-ARs). In mouse cutaneous arteries, α2C-ARs are normally silent at 37°C but mediate cold-induced augmentation of α2-AR responsiveness. In transfected HEK293 cells, this functional rescue is mediated by cold-induced translocation of α2C-ARs from the Golgi to the plasma membrane. Experiments were performed to determine the role of Rho/Rho kinase signaling in this process. Inhibition of Rho kinase (fasudil, Y27632 or H-1152) did not affect constriction of isolated mouse tail arteries to the α2-AR agonist UK 14 304 at 37°C but dramatically reduced the augmented responses to the agonist at 28°C. After Rho kinase inhibition, cooling no longer increased constriction evoked by α2-AR stimulation. Cooling (to 28°C) activated Rho in VSM cells and increased the calcium sensitivity of constriction in α toxin-permeabilized arteries. Stimulation of α2-ARs in VSM cells had no effect on Rho activity or calcium sensitivity at 37°C or 28°C. In HEK293 cells transfected with α2C-ARs, cooling (to 28°C) stimulated the translocation of α2C-ARs to the plasma membrane and this effect was prevented by inhibition of Rho kinase, using fasudil or RNA interference. Consistent with inhibition of the spatial rescue of α2C-ARs, fasudil inhibited α2-AR–mediated mobilization of calcium in tail arteries at 28°C but not 37°C. Therefore, cold-induced activation of Rho/Rho kinase can mediate cold-induced constriction in cutaneous arteries by enabling translocation of α2C-ARs to the plasma membrane and by increasing the calcium sensitivity of the contractile process.

Redox signaling of the arteriolar myogenic response

Arteriolar vascular smooth muscle cells (VSMCs) are mechanosensitive, constricting to elevations in transmural pressure (P(TM)). The goal of the present study was to determine using mouse isolated tail arterioles and arteries whether oxidant signaling regulates this myogenic response. In response to P(TM) elevation, VSMCs of arterioles but not arteries generated constriction and increased reactive oxygen species (ROS) activity (using the H(2)O(2)-sensitive probe dichlorodihydrofluorescein). Arterioles had increased expression of NADPH oxidase components compared with arteries. Inhibition of NADPH oxidase, using mice with targeted impairment of enzyme components (p47(phox) or rac1) or diphenyleneiodonium, prevented the pressure-induced generation of ROS. When ROS activity was inhibited, either by inhibiting NADPH oxidase or with N-acetylcysteine, the myogenic constriction was abolished. The myogenic constriction was also inhibited by catalase, which inactivates H(2)O(2), but was unaffected by a cell-permeant mimic of superoxide dismutase (MnTMPyP). alpha(1)-Adrenergic constriction was not associated with altered ROS activity and was not affected by inhibition of NADPH oxidase or ROS. Exogenous H(2)O(2) constricted VSMCs of arterioles but not arteries. Thus, NADPH oxidase and ROS, in particular H(2)O(2), contribute to the myogenic response of arteriolar VSMCs.

Redox regulation of vascular smooth muscle cell differentiation

Experiments were performed to determine the role of reactive oxygen species (ROS) in regulating vascular smooth muscle cell (VSMC) phenotype. After quiescence, cultured human VSMCs increased their expression of differentiation proteins (alpha-actin, calponin, and SM1 and SM2 myosin), but not beta-actin. ROS activity, determined using the H(2)O(2)-sensitive probe dichlorodihydrofluorescein (DCF), remained high in quiescent cells and was inhibited by catalase (3000 U/mL) or by N-acetylcysteine (NAC, 2 to 20 mmol/L). A superoxide dismutase mimic (SOD; MnTMPyP, 25 micromol/L) or SOD plus low concentrations of NAC (SODNAC2, 2 mmol/L) increased DCF fluorescence, which was inhibited by catalase or by NAC (10 to 20 mmol/L). Inhibition of ROS activity (by catalase or NAC) decreased the baseline expression of differentiation proteins, whereas elevation of ROS (by SOD or SODNAC2) increased expression of the differentiation markers. The latter effect was blocked by catalase or by NAC (10 to 20 mmol/L). None of the treatments altered beta-actin expression. SODNAC2-treated cells demonstrated contractions to endothelin that were absent in proliferating cells. p38 Mitogen-activated protein kinase (MAPK) activity was decreased when ROS activity was reduced (NAC, 10 mmol/L) and was augmented when ROS activity was increased (SODNAC2). Inhibition of p38 MAPK with pyridyl imidazole compound (SB202190, 2 to 10 micromol/L) reduced expression of differentiation proteins occurring under basal conditions and in response to SODNAC2. Transduction of VSMCs with an adenovirus encoding constitutively active MKK6, an activator of p38 MAPK, increased expression of differentiation proteins, whereas transduction with an adenovirus encoding dominant-negative p38 MAPK decreased expression of the differentiation proteins. These findings demonstrate that ROS can increase VSMC differentiation through a p38 MAPK-dependent pathway.

Increased alpha2-adrenergic constriction of isolated arterioles in diffuse scleroderma

OBJECTIVE

Vasospasm and ischemic organ injury are important in the pathogenesis of systemic sclerosis (SSc; scleroderma). The present study was performed to determine whether SSc arterioles have an intrinsic disturbance in vasoconstrictor activity.

METHODS

Skin biopsy samples were obtained from the upper arm of 11 patients with diffuse SSc (clinically uninvolved skin) and 8 age- and sex-matched control subjects. Dermal arterioles were dissected from the biopsy sample and mounted in a myograph for continuous monitoring of arteriolar diameter. The resting internal diameter of control and SSc arterioles was similar (mean +/- SEM 164+/-15 micro and 166+/-18micro, respectively).

RESULTS

Dermal arterioles displayed no spontaneous constrictor activity in the absence of stimulation. Vasoconstriction in response to KCI, a receptor-independent activator of smooth muscle, or to phenylephrine, a selective alpha1-adrenergic receptor (alpha1-AR) agonist, was similar in control and SSc arterioles. However, constrictor responses to UK 14,304, a selective alpha2-AR agonist, were increased in SSc compared with control arterioles (maximal constriction responses of 25+/-5% and 67+/-4% [mean +/- SEM] in control and SSc arterioles, respectively; P = 0.000014). Mechanical denudation of the endothelium did not alter reactivity to alpha2-AR activation, indicating that the enhanced constriction in SSc was not mediated by changes in endothelial dilator activity. Indeed, in arterioles constricted with phenylephrine, the endothelial stimuli acetylcholine or bradykinin evoked endothelium-dependent relaxation that was similar in control and SSc arterioles.

CONCLUSIONS

Vascular smooth muscle in SSc arterioles displayed a selective increase in alpha2-AR reactivity. The endothelial dilator function appeared normal. Altered activity of smooth muscle alpha2-ARs may contribute to the vasospastic activity that is a prominent feature of the SSc disease process.

Silent alpha(2C)-adrenergic receptors enable cold-induced vasoconstriction in cutaneous arteries

Cold constricts cutaneous blood vessels by increasing the reactivity of smooth muscle alpha(2)-adrenergic receptors (alpha(2)-ARs). Experiments were performed to determine the role of alpha(2)-AR subtypes (alpha(2A)-, alpha(2B)-, alpha(2C)-ARs) in this response. Stimulation of alpha(1)-ARs by phenylephrine or alpha(2)-ARs by UK-14,304 caused constriction of isolated mouse tail arteries mounted in a pressurized myograph system. Compared with proximal arteries, distal arteries were more responsive to alpha(2)-AR activation but less responsive to activation of alpha(1)-ARs. Cold augmented constriction to alpha(2)-AR activation in distal arteries but did not affect the response to alpha(1)-AR stimulation or the level of myogenic tone. Western blot analysis demonstrated expression of alpha(2A)- and alpha(2C)-ARs in tail arteries: expression of alpha(2C)-ARs decreased in distal compared with proximal arteries, whereas expression of the glycosylated form of the alpha(2A)-AR increased in distal arteries. At 37 degrees C, alpha(2)-AR-induced vasoconstriction in distal arteries was inhibited by selective blockade of alpha(2A)-ARs (BRL-44408) but not by selective inhibition of alpha(2B)-ARs (ARC-239) or alpha(2C)-ARs (MK-912). In contrast, during cold exposure (28 degrees C), the augmented response to UK-14,304 was inhibited by the alpha(2C)-AR antagonist MK-912, which selectively abolished cold-induced amplification of the response. These experiments indicate that cold-induced amplification of alpha(2)-ARs is mediated by alpha(2C)-ARs that are normally silent in these cutaneous arteries. Blockade of alpha(2C)-ARs may prove an effective treatment for Raynaud's Phenomenon.