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

Effects of dexmedetomidine on porcine pulmonary artery vascular smooth muscle

Background

The α₂-receptor agonist dexmedetomidine (Dex) has been shown to produce sedative and analgesic effects not only with systemic administration but also when administered in the extradural space and around peripheral nerves. The effects and mechanism of action of Dex on pulmonary arteries, however, have not been determined. This study therefore aimed to investigate the effect of Dex on pulmonary arterial vascular smooth muscle by evaluating changes in isometric contraction tension. We then attempted to determine the effects of Dex on depolarization stimulation and receptor stimulation.

Methods

Endothelium-denuded porcine pulmonary arteries were sliced into 2- to 3-mm rings. We then exposed them to certain substances at various concentrations under different conditions of baseline stimulation (with KCl, adrenaline, caffeine, or histamine) and to α₂-receptor stimulants or antagonists, or α₁-receptor antagonists (imidazoline, yohimbine, rauwolscine, prazosin), and different conditions of Ca²⁺ depletion of the intracellular reservoir or extracellular stores. We measured the changes in isometric contraction tension with each addition or change in conditions.

Results

Dex enhanced the contraction induced by high-concentration KCl stimulation. Dex-induced enhancement of contraction induced by high-concentration KCl was completely suppressed by yohimbine and rauwolscine, which are α₂-receptor antagonists, but not by prazosin. Dex, imidazoline, yohimbine, and rauwolscine reduced the increases in contraction tension induced by the receptor stimulant adrenaline. Dex suppressed the adrenaline-induced increases in contraction tension after depletion of the Ca²⁺ reservoir. In the absence of extracellular Ca²⁺, Dex suppressed the adrenaline- and histamine-induced increases in contraction tension but did not affect caffeine-induced increases.

Conclusions

Dex-enhanced, high-concentration KCl-induced contraction was mediated by α₂-receptors. Adrenaline-induced contraction was suppressed by the α₂-receptor stimulant Dex and α₂-receptor antagonists yohimbine and rauwolscine, suggesting that the effect of Dex on adrenaline-induced contraction is attributable to its α₂-receptor-blocking action. Dex inhibited receptor-activated Ca²⁺ channels and phosphatidylinositol-1,4,5-triphosphate-induced Ca²⁺ release but not Ca²⁺-induced Ca²⁺ release.

Smooth Muscle Ion Channels and Regulation of Vascular Tone in Resistance Arteries and Arterioles

Vascular tone of resistance arteries and arterioles determines peripheral vascular resistance, contributing to the regulation of blood pressure and blood flow to, and within the body's tissues and organs. Ion channels in the plasma membrane and endoplasmic reticulum of vascular smooth muscle cells (SMCs) in these blood vessels importantly contribute to the regulation of intracellular Ca2+ concentration, the primary determinant of SMC contractile activity and vascular tone. Ion channels provide the main source of activator Ca2+ that determines vascular tone, and strongly contribute to setting and regulating membrane potential, which, in turn, regulates the open-state-probability of voltage gated Ca2+ channels (VGCCs), the primary source of Ca2+ in resistance artery and arteriolar SMCs. Ion channel function is also modulated by vasoconstrictors and vasodilators, contributing to all aspects of the regulation of vascular tone. This review will focus on the physiology of VGCCs, voltage-gated K+ (KV) channels, large-conductance Ca2+-activated K+ (BKCa) channels, strong-inward-rectifier K+ (KIR) channels, ATP-sensitive K+ (KATP) channels, ryanodine receptors (RyRs), inositol 1,4,5-trisphosphate receptors (IP3Rs), and a variety of transient receptor potential (TRP) channels that contribute to pressure-induced myogenic tone in resistance arteries and arterioles, the modulation of the function of these ion channels by vasoconstrictors and vasodilators, their role in the functional regulation of tissue blood flow and their dysfunction in diseases such as hypertension, obesity, and diabetes. © 2017 American Physiological Society. Compr Physiol 7:485-581, 2017.

Association of adrenergic receptor gene polymorphisms in gallbladder cancer susceptibility in a North Indian population

PURPOSE

Gallbladder cancer (GBC), the most common gastrointestinal and biliary tract malignancy, often coincides with gallstone disease (GSD). The genetic variants of adrenergic receptor (ADR) have been previously reported to be associated with hypomotility disorder of cardiovascular system and GSD. Since GSD may function as GBC precursor, the present study aimed to investigate the association of common functional genetic variants of ADRA2A C-1291G, ADRβ3 T190C or Trp64Arg, and ADRβ1 C1165G or Arg389Gly with GBC and GSD susceptibility.

METHODS

The present study included a total of 400 histologically confirmed GBC, 230 GSD, and 268 healthy controls. The ADRA2A C-1291G, ADRβ3 T190C, and ADRβ1 C1165G polymorphisms were determined by PCR-RFLP. Statistical analysis was performed by using SPSS version 16.

RESULTS

ADRβ3 T190C polymorphism was significantly associated with increased risk of GBC (CT: Pcorr = 0.015, OR 2.87; CC: Pcorr = 0.038, OR 10.33; C allele: Pcorr = 0.014, OR 2.7; CT + CC: Pcorr = 0.017, OR 3.05). These associations existed even after gallstone and gender stratification. Similarly, ADRβ3 T190C polymorphism was also associated with GSD risk, though limited only to female GSD patients. In contrary, ADRA2A C-1291G conferred a marginally increased risk only in GSD patients. ADRβ1 C1165G polymorphism was not associated with GBC and GSD susceptibility when compared to controls.

CONCLUSION

ADRβ3 T190C polymorphism is significantly associated with GBC and GSD susceptibility. The ADRβ3 T190C may be involved in the pathophysiology of GBC by both gallstone-dependent pathway and by some other independent mechanisms.

A polymorphism in the protein kinase C gene PRKCB is associated with α2-adrenoceptor-mediated vasoconstriction

OBJECTIVES

α2-Adrenoceptors (α2-AR) mediate both constriction and dilatation of blood vessels. There is considerable interindividual variability in dorsal hand vein (DHV) constriction responses to α2-AR agonist activation. Genetic factors appear to contribute significantly to this variation. The present study was designed to identify the genetic factors contributing toward the interindividual variability in α2-AR-mediated vascular constriction induced by the selective α2-AR agonist dexmedetomidine.

METHODS

DHV constriction responses to a local infusion of dexmedetomidine were assessed by measuring changes in vein diameter with a linear variable differential transformer. The outcome variable for constriction was log-transformed dexmedetomidine ED50. A genome-wide association study (GWAS) of 433 378 single-nucleotide polymorphisms (SNPs) was carried out for determining the sensitivity of DHV responses in 64 healthy Finnish individuals. Twenty SNPs were selected on the basis of the GWAS results and their associations with the ED50 of dexmedetomidine were tested in an independent North American study population of 68 healthy individuals.

RESULTS

In both study populations (GWAS and replication samples), the SNP rs9922316 in the gene for protein kinase C type β was consistently associated with dexmedetomidine ED50 for DHV constriction (unadjusted P=0.00016 for the combined population).

CONCLUSION

Genetic variation in protein kinase C type β may contribute toward the interindividual variation in DHV constriction responses to α2-AR activation by the agonist dexmedetomidine.

Dexmedetomidine-induced contraction of isolated rat aorta is dependent on extracellular calcium concentration

Background

Dexmedetomidine is a highly selective α₂-adrenoceptor agonist that is widely used for sedation and analgesia during the perioperative period. Intravenous administration of dexmedetomidine induces transient hypertension due to vasoconstriction via the activation of the α₂-adrenoceptor on vascular smooth muscle. The goal of this in vitro study is to investigate the calcium-dependent mechanism underlying dexmedetomidine-induced contraction of isolated endothelium-denuded rat aorta.

Methods

Isolated endothelium-denuded rat thoracic aortic rings were suspended for isometric tension recording. Cumulative dexmedetomidine concentration-response curves were generated in the presence or absence of the following inhibitors: α₂-adrenoceptor inhibitor rauwolscine; voltage-operated calcium channel blocker verapamil (5 × 10^-7, 10^-6 and 5 × 10^-5 M); purported inositol 1,4,5-trisphosphate receptor blocker 2-aminoethoxydiphenylborate (5 × 10^-6, 10^-5 and 5 × 10^-5 M); phospholipase C inhibitor U-73122 (10^-6 and 3 × 10^-6 M); and store-operated calcium channel inhibitor gadolinium chloride hexahydrate (Gd³⁺; 5 × 10^-6 M). Dexmedetomidine concentration-response curves were also generated in low calcium concentrations (1 mM) and calcium-free Krebs solution.

Results

Rauwolscine, verapamil, and 2-aminoethoxydiphenylborate attenuated dexmedetomidine-induced contraction in a concentration-dependent manner. Low calcium concentrations attenuated dexmedetomidine-induced contraction, and calcium-free Krebs solution nearly abolished dexmedetomidine-induced contraction. However, U-73122 and Gd³⁺ had no effect on dexmedetomidine-induced contraction.

Conclusions

Taken together, these results suggest that dexmedetomidine-induced contraction is primarily dependent on extracellular calcium concentrations that contribute to calcium influx via voltage-operated calcium channels of isolated rat aortic smooth muscle. Dexmedetomidine-induced contraction is mediated by α₂-adrenoceptor stimulation. Dexmedetomidine-induced contraction appears to be partially mediated by calcium release from the sarcoplasmic reticulum.

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.

Dorsal hand vein responses to the α₁-adrenoceptor agonist phenylephrine do not predict responses to the α₂-adrenoceptor agonist dexmedetomidine

Significant inter-individual variability exists in responses of human dorsal hand veins to activation of α-adrenoceptors. Simultaneous graded infusions of the α₁- and α₂-adrenoceptor agonists phenylephrine (3.66-8000 ng/min) and dexmedetomidine (0.0128-1000 ng/min) were given into dorsal veins of both hands and responses of 75 subjects were analyzed to assess whether a subject's sensitivity to phenylephrine (ED(50)) predicts his sensitivity to dexmedetomidine. Individual ED(50) estimates of dexmedetomidine and phenylephrine ranged between 0.06-412 and 14.2-7450 ng/min and exhibited only a weak positive relationship (r² =0.074, P=0.018). Finger temperature, body mass index, age and phenylephrine sensitivity together accounted for about 30% of dexmedetomidine ED(50) variation (r² =0.315, P<0.001). The large inter-individual variability observed in the responses of dorsal hand veins to both α₁- and α₂-adrenoceptor agonists is not explained by some common factors; instead, dorsal hand vein responsivity is separately determined for both receptor mechanisms.

Dual alpha(2)-adrenergic agonist and alpha(1)-adrenergic antagonist actions of dexmedetomidine on human isolated endothelium-denuded gastroepiploic arteries

The actions of dexmedetomidine (DEX) on human vascular smooth muscle are unclear. We investigated its effects on isolated, endothelium-denuded human gastroepiploic arteries in vitro and compared them with clonidine (CLO). DEX had little direct effect on resting tension, whereas CLO produced small contractile responses, an effect which is blocked by the alpha(1)-adrenergic antagonist prazosin. DEX markedly enhanced the high K(+) (40 mmol/L)-induced contraction, and this effect was reversed by the alpha(2)-adrenergic antagonists yohimbine and rauwolscine but unaffected by prazosin. However, CLO had little effect on the K(+) contractions. Interestingly, larger concentrations (>10(-7) mol/L) of both alpha(2)-adrenergic stimulants significantly inhibited the contractions elicited by the alpha(1)-adrenergic agonist phenylephrine (10(-6) mol/L) and, to a lesser extent, those elicited by the alpha(1)/alpha(2)-agonist norepinephrine (10(-6) mol/L). These results suggest the possibility that DEX and CLO each have a high affinity for alpha(1)-adrenoceptors in human isolated gastroepiploic arteries, resulting in a reduced efficacy of alpha(1)-adrenergic activation by alpha-agonists. The differing affinities of the drugs for alpha(1)- and alpha(2)-adrenoceptors may help explain their additional actions: 1) DEX enhances the high K(+)-induced contraction presumably through alpha(2)-adrenoceptor activation, and 2) CLO acts on alpha(1)-adrenoceptors as a partial agonist when present alone.

IMPLICATIONS

Dexmedetomidine may not directly affect smooth muscle in human peripheral resistance vessels within the usual range of plasma concentrations (<10(-7) mol/L) achieved in clinical practice. However, in large doses, it could enhance the response to nonadrenergic vasoconstrictor agonists while antagonizing the vasoconstrictor response to alpha(1)-adrenoceptor agonists.

Mechanism of action of angiotensin II in human isolated subcutaneous resistance arteries

1. Human isolated subcutaneous arteries were mounted in a myograph and isometric tension measured. In some experiments, intracellular calcium [Ca(2+)]i was also measured using fura-2. 2. Angiotensin II (100 pM - 1 microM) increased [Ca(2+)]i and tone in a concentration-dependent manner. The effects of angiotensin II (100 nM) were inhibited by an AT1-receptor antagonist, candesartan (100 pM). 3. Ryanodine (10 microM), had no effect on angiotensin II-induced responses, but removal of extracellular Ca(2+) abolished angiotensin II-induced rise in [Ca(2+)]i and tone. Inhibition of Ca(2+) entry by Ni(2+) (2 mM), also inhibited angiotensin II responses. The dihydropyridine, L-type calcium channel antagonist, amlodipine (10 microM), only partially attenuated angiotensin II responses. 4. Inhibition of protein kinase C (PKC) by chelerythrine (1 microM), or by overnight exposure to a phorbol ester (PDBu; 500 nM) had no effect on angiotensin II-induced contraction. 5. Genistein (10 microM), a tyrosine kinase inhibitor, inhibited angiotensin II-induced contraction, but did not inhibit the rise in [Ca(2+)]i, suggesting that at this concentration it affected the calcium sensitivity of the contractile apparatus. Genistein did not affect responses to norepinephrine (NE) or high potassium (KPSS). 6. A selective MEK inhibitor, PD98059 (30 microM), inhibited both the angiotensin II-induced contraction and rise in [Ca(2+)]i, but had no effect on responses to NE or KPSS. 7. AT1 activation causes Ca(2+) influx via L-type calcium channels and a dihydropyridine-insensitive route, but does not release Ca(2+) from intracellular sites. Activation of tyrosine kinase(s) and the ERK 1/2 pathway, but not classical or novel PKC, also play a role in angiotensin II-induced contraction in human subcutaneous resistance arteries.

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.

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.

Increased vascular responsiveness to alpha 2-adrenergic stimulation during NOS inhibition-induced hypertension

Increased vascular resistance during systemic nitric oxide synthase (NOS) inhibition is dependent on adrenergic vasoconstriction. This study tested the hypothesis that increased vascular sensitivity to adrenergic agonists contributes to this vasoconstriction. Superior mesenteric arteries and thoracic aortae from male Sprague-Dawley rats drinking water containing N omega-nitro-L-arginine (L-NNA; 14 days, 60 mg.kg-1.day-1) and control rats were-cut into helical strips, and endothelium was removed for contractile experiments. L-NNA arteries were more sensitive to UK-14304 (alpha 2-adrenergic agonist) and norepinephrine (NE), whereas responses to phenylephrine (PE) were not different concentration causing 50% maximal response (EC50), L-NNA vs. control: UK-14304, 0.071 vs. 0.71 mumol/l; NE, 1.15 vs. 9.95 nmol/l]. Yohimbine, an alpha 2-selective antagonist, caused a concentration-dependent inhibition of contraction to NE only in L-NNA arteries (EC50 = 6.3 vs. 1.6 nmol/l at 1 nmol/l yohimbine), whereas prazosin shifted NE curves similarly in arteries from both groups. Yohimbine (10 nmol/l) inhibited contractions to UK-14304 (EC50 = 59 mumol/l vs. 17 mumol/l) but not contractions to PE, whereas prazosin inhibited both. These data indicate that L-NNA-induced hypertension leads to increased sensitivity of prazosin-sensitive alpha 2-adrenoceptors, an upregulation that could cause the increased vasoconstrictor response to NE in this model of hypertension.

Effects of noradrenaline and neuropeptide Y on rat mesenteric microvessel contraction

We have studied the contractile effects of the sympathetic transmitter noradrenaline and its cotransmitter neuropeptide Y (NPY) given alone and in combination on isolated rat mesenteric resistance vessels (200-300 microns diameter). Noradrenaline and NPY each concentration-dependently contracted rat mesenteric microvessels (EC50 approximately equal to 800 nM and 10 nM, respectively), but noradrenaline caused considerably greater maximal effects than NPY (14.3 mN vs. 3.5 mN). A low antagonistic potency of yohimbine indicated that the response to noradrenaline did not involve alpha 2-adrenoceptors, and the subtype-selective antagonists 5-methylurapidil, tamsulosin and chloroethylclonidine indicated mediation via an alpha 1A-adrenoceptor. Shallow Schild regressions for prazosin and 5-methylurapidil indicated that an alpha 1-adrenoceptor subtype with relatively low prazosin affinity might additionally be involved. Studies with the NPY analogues PYY, [Leu31, Pro34] NPY and NPY18-36 demonstrated that NPY acted via a Y1 NPY receptor. In addition to its direct vasoconstricting effects NPY also lowered the noradrenaline EC50 but did not appreciably affect maximal noradrenaline responses indicating possible potentiation. The potentiating NPY response occurred with similar agonist potency as the direct contractile NPY effects and also via a Y1 NPY receptor. The Ca2+ entry blocker nitrendipine (300 nM) reduced direct contractile responses to noradrenaline and NPY but did not affect the potentiation response to NPY.