Enhanced interaction between renovascular alpha(2)-adrenoceptors and angiotensin II receptors in genetic hypertension

Role of α2-Adrenoceptors in Hypertension: Focus on Renal Sympathetic Neurotransmitter Release, Inflammation, and Sodium Homeostasis

The kidney is extensively innervated by sympathetic nerves playing an important role in the regulation of blood pressure homeostasis. Sympathetic nerve activity is ultimately controlled by the central nervous system (CNS). Norepinephrine, the main sympathetic neurotransmitter, is released at prejunctional neuroeffector junctions in the kidney and modulates renin release, renal vascular resistance, sodium and water handling, and immune cell response. Under physiological conditions, renal sympathetic nerve activity (RSNA) is modulated by peripheral mechanisms such as the renorenal reflex, a complex interaction between efferent sympathetic nerves, central mechanism, and afferent sensory nerves. RSNA is increased in hypertension and, therefore, critical for the perpetuation of hypertension and the development of hypertensive kidney disease. Renal sympathetic neurotransmission is not only regulated by RSNA but also by prejunctional α2-adrenoceptors. Prejunctional α2-adrenoceptors serve as autoreceptors which, when activated by norepinephrine, inhibit the subsequent release of norepinephrine induced by a sympathetic nerve impulse. Deletion of α2-adrenoceptors aggravates hypertension ultimately by modulating renal pressor response and sodium handling. α2-adrenoceptors are also expressed in the vasculature, renal tubules, and immune cells and exert thereby effects related to vascular tone, sodium excretion, and inflammation. In the present review, we highlight the role of α2-adrenoceptors on renal sympathetic neurotransmission and its impact on hypertension. Moreover, we focus on physiological and pathophysiological functions mediated by non-adrenergic α2-adrenoceptors. In detail, we discuss the effects of sympathetic norepinephrine release and α2-adrenoceptor activation on renal sodium transporters, on renal vascular tone, and on immune cells in the context of hypertension and kidney disease.

α 2-Adrenoceptors: Challenges and Opportunities-Enlightenment from the Kidney

It was indeed a Don Quixote-like pursuit of the mechanism of essential hypertension when we serendipitously discovered α₂-adrenoceptors (α₂-ARs) in skin-lightening experiments in the frog. Now α₂-ARs lurk on the horizon involving hypertension causality, renal denervation for hypertension, injury from falling in the elderly and prazosin's mechanism of action in anxiety states such as posttraumatic stress disorder (PTSD). Our goal here is to focus on this horizon and bring into clear view the role of α₂-AR-mediated mechanisms in these seemingly unrelated conditions. Our narrative begins with an explanation of how experiments in isolated perfused kidneys led to the discovery of a sodium-retaining process, a fundamental mechanism of hypertension, mediated by α₂-ARs. In this model system and in the setting of furosemide-induced sodium excretion, α₂-AR activation inhibited adenylate cyclase, suppressed cAMP formation, and caused sodium retention. Further investigations led to the realization that renal α₂-AR expression in hypertensive animals is elevated, thus supporting a key role for kidney α₂-ARs in the pathophysiology of essential hypertension. Subsequent studies clarified the molecular pathways by which α₂-ARs activate prohypertensive biochemical systems. While investigating the role of α₁-adrenoceptors (α₁-ARs) versus α₂-ARs in renal sympathetic neurotransmission, we noted an astonishing result: in the kidney α₁-ARs suppress the postjunctional expression of α₂-ARs. Here, we describe how this finding relates to a broader understanding of the role of α₂-ARs in diverse disease states. Because of the capacity for qualitative and quantitative monitoring of α₂-AR-induced regulatory mechanisms in the kidney, we looked to the kidney and found enlightenment.

RACK1 regulates angiotensin II-induced contractions of SHR preglomerular vascular smooth muscle cells

The preglomerular microcirculation of spontaneously hypertensive rats (SHR) is hypersensitive to angiotensin (ANG) II, and studies have shown that this is likely due to enhanced coincident signaling between G protein subunits αq (Gαq; released by ANG II) and βγ (Gβγ; released by Gi-coupled receptors) to active phospholipase C (PLC). Here we investigated the molecular basis for the enhanced coincident signaling between Gβγ and Gαq in SHR preglomerular vascular smooth muscle cells (PGVSMCs). Because receptor for activated C kinase 1 (RACK1; a scaffolding protein) organizes interactions between Gβγ, Gαq, and PLC, we included RACK1 in this investigation. Cell fractionation studies demonstrated increased levels of membrane (but not cytosolic) Gβ, Gαq, PLCβ3, and RACK1 in SHR PGVSMCs compared with Wistar-Kyoto rat PGVSMCs. In SHR PGVSMCs, coimmunoprecipitation demonstrated RACK1 binding to Gβ and PLCβ3, but only at cell membranes. Pertussis toxin (which blocks Gβγ) and U73122 (which blocks PLC) reduced membrane RACK1; however, RACK1 knockdown (shRNA) did not affect membrane levels of Gβ, Gαq, or PLCβ3 In a novel gel contraction assay, RACK1 knockdown in SHR PGVSMCs attenuated contractions to ANG II and abrogated the ability of neuropeptide Y (which signals via Gβγ) to enhance ANG II-induced contractions. We conclude that in SHR PGVSMCs the enlarged pool of Gβγ and PLCβ3 recruits RACK1 to membranes and RACK1 then organizes signaling. Consequently, knockdown of RACK1 prevents coincident signaling between ANG II and the Gi pathway. This is the first study to implicate RACK1 in vascular smooth muscle cell contraction and suggests that RACK1 inhibitors could be effective cardiovascular drugs.

Neurohormonal interactions on the renal oxygen delivery and consumption in haemorrhagic shock-induced acute kidney injury

Haemorrhagic shock is a common cause of acute kidney injury (AKI), which is a major risk factor for developing chronic kidney disease. The mechanism is superficially straightforward. An arterial pressure below the kidney's autoregulatory region leads to a direct reduction in filtration pressure and perfusion, which in turn cause renal failure with reduced glomerular filtration rate and AKI because of hypoxia. However, the kidney's situation is further worsened by the hormonal and neural reactions to reduced perfusion pressure. There are three major systems working to maintain arterial pressure in shock: sympathetic signalling, the renin-angiotensin system and vasopressin. These work to retain electrolytes and water and to increase peripheral resistance and cardiac output. In the kidney, the increased electrolyte reabsorption consumes oxygen. At the same time, at the signalling level seen in shock, all of these hormones reduce renal perfusion and thereby oxygen delivery. This creates an exaggerated hypoxic situation that is liable to worsen the AKI. The present review will examine this mechanistic background and identify a number of areas that require further studies. At this time, the ideal treatment of haemorrhagic shock appears to be slow fluid resuscitation, possibly with hyperosmolar sodium, low chloride and no artificial colloids. From the standpoint of the kidney, renin-angiotensin system inhibitors appear fruitful for further study.

Sitagliptin augments angiotensin II-induced renal vasoconstriction in kidneys from rats with metabolic syndrome

1. Dipeptidyl peptidase (DPP) IV inhibitors enhance renovascular responses to angiotensin (Ang) II in spontaneously hypertensive rats (SHR), but not Wistar-Kyoto rats. Because DPPIV inhibitors are often used in metabolic syndrome, it is important to determine whether DPPIV inhibition in this setting enhances renovascular responses to AngII. 2. Six-week-old Lean-ZSF1 rats (harbouring SHR genes, but without metabolic syndrome; n = 11) and Obese-ZSF1 rats (harbouring SHR genes and expressing metabolic syndrome; n = 10) were provided food and water ad libitum, and metabolic parameters and renovascular responses to AngII were assessed when the animals were 7 and 8 weeks of age, respectively. 3. At 7 weeks of age, compared with Lean-ZSF1, Obese-ZSF1 demonstrated significant (P < 0.05) increases in bodyweight (262 +/- 8 vs 310 +/- 13 g), plasma glucose (112 +/- 4 vs 153 +/- 9 mg/dL), haemoglobin A1c (4.7 +/- 0.1 vs 5.8 +/- 0.4%), urinary glucose excretion (0.021 +/- 0.003 vs 6.70 +/- 1.80 g/kg bodyweight per 24 h) and urinary protein excretion (100 +/- 7 vs 313 +/- 77 mg/kg bodyweight per 24 h). Mean blood pressure was high (133 +/- 7 mmHg) in both strains. 4. At 8 weeks of age, kidneys were isolated and perfused. In Lean-ZSF1 rats, renovascular responses (i.e. changes in perfusion pressure) to physiological levels of AngII (0.1 nmol/L) were 3.4 +/- 1.3 and 18.2 +/- 5.9 mmHg in untreated (n = 5) and 1 micromol/L sitagliptin-treated (n = 6) kidneys, respectively. In Obese-ZSF1 rats, renovascular responses to AngII were 5.5 +/- 1.3 and 17.8 +/- 8.2 mmHg in untreated (n = 4) and sitagliptin-treated (n = 6) kidneys, respectively. Analysis of variance revealed a significant (P = 0.0367) effect of sitagliptin on renovascular responses to AngII that was independent of strain. 5. In conclusion, sitagliptin enhances renovascular responses to AngII in rats harbouring SHR genes and this effect persists in rats with diabetic nephropathy and metabolic syndrome.

Adenosine triphosphate increases the reactivity of the afferent arteriole to low concentrations of norepinephrine

Adenosine triphosphate (ATP) and norepinephrine (NE) interact in the control of blood flow in the kidney. A combined effect of NE and ATP has not been previously investigated at the level of the afferent arteriole (Af). We studied the effects of ATP on the contractile response of the Af to NE. Vascular reactivity to ATP, NE, and their combination was investigated in isolated perfused Af from mice. The roles of alpha-adrenoceptors and P2-ATP-receptors were investigated by use of specific agonists and antagonists. Cytosolic calcium was measured using the fluorescent calcium dye fura-2. ATP in concentrations from 10(-12) to 10(-4) mol/l induced transient contractions. NE constricted the Af in a dose-dependent manner and induced significant contractions at > 10(-7) mol/l. Treatment with ATP (10(-8) and 10(-6) mol/l) increased the NE response. Diameters were reduced by 20% already at 10(-11) mol/l NE during ATP treatment of 10(-6) mol/l. ATP increased the calcium response to NE significantly at 10(-8) and 10(-7)mol/l NE. The P2-type ATP receptor blocker pyridoxal-phosphate-6-azophenyl-2',4'-disulfonic acid (PPADS) (10(-5) mol/l) abolished the sensitization of the NE response by ATP. The alpha(1)-blocker prazosin (10(-7) mol/l) inhibited the ATP effect, as did the alpha 2-blocker yohimbine (10(-7) mol/l). Neither the phenylephrine- nor clonidine-induced concentration response curves was affected by ATP in the bath solution. Costimulation with ATP enhances the response of the Af to NE. This effect is mediated by increased cytosolic calcium. The enhancing effect involves P2-type ATP receptors and both alpha (1)- and alpha 2-adrenoceptors.

Rho kinase contributes to basal vascular tone in humans: role of endothelium-derived nitric oxide

Our objective was to determine the role of the Rho-associated kinase (ROK) for the regulation of FBF (FBF) and to unmask a potential role of ROK for the regulation of endothelium-derived nitric oxide (NO). Moreover, the effect of fasudil on the constrictor response to endothelin-1 was recorded. Regarding background, phosphorylation of the myosin light chain (MLC) determines the calcium sensitivity of the contractile apparatus. MLC phosphorylation depends on the activity of the MLC kinase and the MLC phosphatase. The latter enzyme is inhibited through phosphorylation by ROK. ROK has been suggested to inhibit NO generation, possibly via the inhibition of the Akt pathway. In this study, the effect of intra-arterial infusion of the ROK inhibitor fasudil on FBF in 12 healthy volunteers was examined by venous occlusion plethysmography. To unmask the role of NO, fasudil was infused during NO clamp. As a result, fasudil markedly increased FBF in a dose-dependent manner from 2.34 ± 0.21 to 6.96 ± 0.93 ml/100 ml forearm volume at 80 μg/min ( P < 0.001). At 1,600 μg/min, fasudil reduced systolic, diastolic, and mean arterial pressure while increasing heart rate. Fasudil abolished the vasoconstrictor effect of endothelin-1. The vascular response to fasudil (80 μmol/min) was blunted during NO clamp (104 ± 18% vs. 244 ± 48% for NO clamp + fasudil vs. fasudil alone; data as ratio between infused and noninfused arm with baseline = 0%, P < 0.05). In conclusion, 1) basal peripheral and systemic vascular tone depends on ROK; 2) a significant portion of fasudil-induced vasodilation is mediated by NO, suggesting that vascular bioavailable NO is negatively regulated by ROK; and 3) the constrictor response to endothelin involves the activation of ROK.

Involvement of G-protein βγ subunits on the influence of inhibitory α2-autoreceptors on the angiotensin AT1-receptor modulation of noradrenaline release in the rat vas deferens

The influence of alpha2-autoreceptors on the facilitation of [3H]-noradrenaline release mediated by angiotensin II was studied in prostatic portions of rat vas deferens preincubated with [3H]-noradrenaline. Angiotensin II enhanced tritium overflow evoked by trains of 100 pulses at 8 Hz, an effect that was attenuated by the AT1-receptor antagonist losartan (0.3-1 microM), at concentrations suggesting the involvement of the AT1B subtype. The effect of angiotensin II was also attenuated by inhibition of phospholipase C (PLC) and protein kinase C (PKC) indicating that prejunctional AT1-receptors are coupled to the PLC-PKC pathway. Angiotensin II (0.3-100 nM) enhanced tritium overflow more markedly, up to 64%, under conditions that favor alpha2-autoinhibition, observed when stimulation consisted of 100 pulses at 8 Hz, than under poor alpha2-autoinhibition conditions, only up to 14%, observed when alpha2-adrenoceptors were blocked with yohimbine (1 microM) or when stimulation consisted of 20 pulses at 50 Hz. Activation of PKC with 12-myristate 13-acetate (PMA, 0.1-3 microM) also enhanced tritium overflow more markedly under strong alpha2-autoinhibition conditions. Inhibition of Gi/o-proteins with pertussis toxin (8 microg/ml) or blockade of Gbetagamma subunits with the anti-betagamma peptide MPS-Phos (30 microM) attenuated the effects of angiotensin II and PMA. The results indicate that activation of AT1-receptors coupled to the PLC-PKC pathway enhances noradrenaline release, an effect that is markedly favoured by an ongoing activation of alpha2-autoreceptors. Interaction between alpha2-adrenoceptors and AT1-receptors seems to involve the betagamma subunits released from the Gi/o-proteins coupled to alpha2-adrenoceptors and protein kinase C activated by AT1-receptors.

Role of renal sympathetic nerves in regulating renovascular responses to angiotensin II in spontaneously hypertensive rats

The purpose of this study was to test the hypothesis that renal sympathetic nerves modulate angiotensin II-induced renal vasoconstriction in kidneys from genetically hypertensive rats via Y1 receptors activating the Gi pathway. In isolated, perfused kidneys from spontaneously hypertensive rats, the naturally occurring renal sympathetic cotransmitter neuropeptide Y at 6 nM enhanced angiotensin II (0.3 nM)-induced changes in perfusion pressure by 47 +/- 7 mm Hg, and this effect was inhibited by BIBP3226 [N2-(diphenylacetyl)-N-[(4-hydroxyphenyl)-methyl]-D-arginine amide)], a selective Y1 receptor antagonist (1 microM). We next examined whether periarterial nerve stimulation (5 Hz) enhances renal vascular responses to a physiological level of angiotensin II (100 pM). Kidneys were pretreated with prazosin (a selective alpha1-adrenoceptor antagonist) to block nerve stimulation-induced changes in perfusion pressure. In kidneys from spontaneously hypertensive rats, but not normotensive rats, periarterial nerve stimulation significantly augmented angiotensin II-induced changes in perfusion pressure (177 +/- 26% of response in absence of stimulation). BIBP3226, but not rauwolscine (a selective alpha2-adrenoceptor antagonist), abolished periarterial nerve stimulation-induced enhancement of angiotensin II-mediated renal vasoconstriction. Pretreatment of hypertensive animals with pertussis toxin 3 days prior to kidney perfusion significantly (p < 0.000001) decreased mean blood pressure (203 +/- 2 versus 145 +/- 6 mm Hg in nonpretreated versus pertussis toxin-pretreated spontaneously hypertensive rats) and abolished periarterial nerve stimulation-induced enhancement of angiotensin II-mediated renal vasoconstriction. We conclude that, in spontaneously hypertensive rats but not normotensive rats, sympathetic nerve stimulation enhances renal vascular responses to physiological levels of angiotensin II via a mechanism mainly involving Y1 receptors coupled to Gi proteins.

Pre- and postjunctional effects of angiotensin II in hypertension due to adenosine receptor blockade

Prejunctional facilitation of [3H]noradrenaline release induced by sympathetic nerve stimulation and postjunctional contractile effects of angiotensin II were studied in the mesenteric artery and vein of 1,3-dipropyl-8-sulfophenylxanthine (DPSPX)-hypertensive rats. Male Wistar rats received infusions of saline or DPSPX (90 microg/kg/h) i.p.. Blood pressure was determined by tail-cuff. The prejunctional effect of angiotensin II was similar in artery and vein preparations of control rats and was increased in DPSPX-hypertensive rats. In contrast, the contractile effect of angiotensin II was much more pronounced in the mesenteric vein than in the mesenteric artery of control rats and was markedly reduced in DPSPX-hypertensive rats. We conclude that (1) the increased prejunctional effect of angiotensin II may contribute to, while (2) the decreased contractile effect of angiotensin II may attenuate DPSPX-induced hypertension. This study also supports the hypothesis that pre- and postjunctional angiotensin II receptors are different.

Mechanism of the Vascular Angiotensin II/α2-Adrenoceptor Interaction

alpha(2)-Adrenoceptors potentiate vascular responses to angiotensin II. The goal of this study was to test the hypothesis that the phospholipase C (PLC)/protein kinase C (PKC)/c-src/phosphatidylinositol 3-kinase (PI3K) pathway contributes to the vascular angiotensin II/alpha(2)-adrenoceptor interaction. In rats in vivo, intrarenal infusions of angiotensin II (10 ng/kg/min) increased renal vascular resistance by 5.8 +/- 0.5 units, and this response was enhanced (p < 0.05) to 9.1 +/- 1.2 units by UK-14,304 [5-bromo-N-(4,5-dihydro-1H-imidazol-2-yl)-6-quinoxalinamine; 3 microg/kg/min; alpha(2)-adrenoceptor agonist]. Intrarenal infusions of U-73122 [1-[6-[[(17beta)-3-methoxyestra-1,3,5(10)-trien-17-yl]amino]-hexyl]-1H-pyrrole-2,5-dione; 3 microg/min; PLC inhibitor], GF109203X [bisindolylmaleimide I; 10 microg/min; PKC inhibitor], CGP77675 [1-(2-{4-[4-amino-5-(3-methoxyphenyl)pyrrolo[2,3-d]pyrimidin-7-yl]phenyl}ethyl)piperidin-4-ol; 5 microg/min; c-src inhibitor], and wortmannin (1 microg/min; PI3K inhibitor) abolished the angiotensin II/alpha(2)-adrenoceptor interaction. In isolated perfused rat kidneys, angiotensin II (0.3, 1, and 3 nM) increased perfusion pressure (by 15 +/- 8, 39 +/- 4, and 93 +/- 9 mm Hg, respectively), and UK-14,304 (1 microM) potentiated these responses (to 36 +/- 4, 67 +/- 7, and 135 +/- 17 mm Hg, respectively). This angiotensin II/alpha(2)-adrenoceptor interaction was abolished by U-73122 (10 microM), GF109203X (3 microM), CGP77675 (5 microM), and wortmannin (0.2 microM). Preglomerular microvascular smooth muscle cells expressed phospholipase (PLC)-beta(2), PLC-beta(3), c-src, phospho(tyrosine 416)-c-src, and PI3K. In these cells, angiotensin II (0.1 microM) and UK-14,304 (1 microM) per se did not increase phospho-c-src; however, the combination of angiotensin II plus UK-14,304 doubled phospho-c-src, and this interaction was abolished by U-73122 (10 microM) and GF109203X (3 microM). In conclusion, the PLC/PKC/c-src/PI3K pathway may contribute importantly to the interaction between alpha(2)-adrenoceptors and angiotensin II on renal vascular resistance.

Enhanced Activation of RhoA by Angiotensin II in SHR Preglomerular Microvascular Smooth Muscle Cells

Angiotensin II causes a greater renal vasoconstriction in spontaneously hypertensive rats (SHR) than in normotensive Wistar Kyoto rats (WKY), and alpha2-adrenoceptor agonists potentiate angiotensin II-induced renal vasoconstriction more in SHR. Because angiotensin II activates RhoA, and RhoA contributes to vasoconstriction, we tested the hypothesis that the ability of angiotensin II to stimulate RhoA in preglomerular vascular smooth muscle cells and the ability of alpha2-adrenoceptor activation to potentiate this response are augmented in cells from SHR. In SHR preglomerular vascular smooth muscle cells, angiotensin II (1 micromol/L) greatly stimulated RhoA activity, and this effect was markedly potentiated by UK 14,304 (1 micromol/L; alpha2-adrenoceptor agonist) (fold increase from vehicle-treated cells: 9.0 +/- 2, 0.8 +/- 0.2, and 13.6 +/- 3.2 in cells treated with angiotensin II, UK 14,304, and angiotensin II + UK 14,304, respectively). In contrast, in WKY cells, angiotensin II only mildly activated RhoA (2.0 +/- 0.50), and this response was not enhanced by UK 14,304. The expression of Galpha12 and Galpha13, G-proteins thought to link G-protein-coupled receptors to RhoA, was not increased in SHR cells. We conclude that angiotensin II-induced activation of RhoA is much more robust in the preglomerular microcirculation of SHR compared with WKY and that this may contribute to the etiology of genetic hypertension.

alpha 2-Adrenoceptors potentiate angiotensin II- and vasopressin-induced renal vasoconstriction in spontaneously hypertensive rats

Hypertension in spontaneously hypertensive rats (SHRs) is due in part to enhanced effects of vasoactive peptides on the renal vasculature. We hypothesize that the G(i) signal transduction pathway enhances renovascular responses to vasoactive peptides in SHRs more so than in normotensive Wistar-Kyoto (WKY) rats. To test this hypothesis, we examined in isolated perfused kidneys from SHRs and WKY rats the renovascular responses (assessed as changes in renal perfusion pressure in mm Hg) to angiotensin II (10 nM) and vasopressin (3 nM) in the presence and absence of UK-14,304 [5-bromo-N-(4,5-dihydro-1H-imidazol-2-yl)-6-quinoxalinamine; an agonist that selectively activates the G(i) pathway by stimulating alpha(2)-adrenoceptors]. In SHR, but not WKY, kidneys, UK-14,304 (10 nM) enhanced (P < 0.05) renovascular responses to angiotensin II (control WKY, 43 +/- 6; UK-14,304-treated WKY, 52 +/- 19; control SHR, 66 +/- 17; UK-14,304-treated SHR, 125 +/- 16) and vasopressin (control WKY, 42 +/- 17; UK-14,304-treated WKY, 36 +/- 11; control SHR, 16 +/- 8; UK-14,304-treated SHR, 83 +/- 17). Pretreatment of SHRs with pertussis toxin (30 microg/kg, intravenously, 3-4 days before study) to inactivate G(i) blocked the effects of UK-14,304. Western blot analysis of receptor expression in whole kidney and preglomerular microvessels revealed similar levels of expression of AT(1), V(1a), and alpha(2A) receptors in SHRs compared with WKY rats. We conclude that activation of alpha(2)-adrenoceptors selectively enhances renovascular responses to angiotensin II and vasopressin in SHRs via an enhanced cross talk between the G(i) signal transduction pathway and signal transduction pathways activated by angiotensin II and vasopressin.