Novel role of mechanosensitive AT1B receptors in myogenic vasoconstriction

Arteries are finely tuned thermosensors regulating myogenic tone and blood flow

In response to changing blood pressure, arteries adjust their caliber to control perfusion. This vital autoregulatory property, termed vascular myogenic tone, stabilizes downstream capillary pressure. We discovered that tissue temperature critically determines myogenic tone. Heating steeply activates tone in skeletal muscle, gut, brain and skin arteries with temperature coefficients ( Q ₁₀ ) of ∼11-20. Further, arterial thermosensitivity is tuned to resting tissue temperatures, making myogenic tone sensitive to small thermal fluctuations. Interestingly, temperature and intraluminal pressure are sensed largely independently and integrated to trigger myogenic tone. We show that TRPV1 and TRPM4 mediate heat-induced tone in skeletal muscle arteries. Variations in tissue temperature are known to alter vascular conductance; remarkably, thermosensitive tone counterbalances this effect, thus protecting capillary integrity and fluid balance. In conclusion, thermosensitive myogenic tone is a fundamental homeostatic mechanism regulating tissue perfusion.

One-Sentence Summary

Arterial blood pressure and temperature are integrated via thermosensitve ion channels to produce myogenic tone.

Vascular mechanotransduction

This review aims to survey the current state of mechanotransduction in vascular smooth muscle cells (VSMCs) and endothelial cells (ECs), including their sensing of mechanical stimuli and transduction of mechanical signals that result in the acute functional modulation and longer-term transcriptomic and epigenetic regulation of blood vessels. The mechanosensors discussed include ion channels, plasma membrane-associated structures and receptors, and junction proteins. The mechanosignaling pathways presented include the cytoskeleton, integrins, extracellular matrix, and intracellular signaling molecules. These are followed by discussions on mechanical regulation of transcriptome and epigenetics, relevance of mechanotransduction to health and disease, and interactions between VSMCs and ECs. Throughout this review, we offer suggestions for specific topics that require further understanding. In the closing section on conclusions and perspectives, we summarize what is known and point out the need to treat the vasculature as a system, including not only VSMCs and ECs but also the extracellular matrix and other types of cells such as resident macrophages and pericytes, so that we can fully understand the physiology and pathophysiology of the blood vessel as a whole, thus enhancing the comprehension, diagnosis, treatment, and prevention of vascular diseases.

Angiotensin II Type 1A Receptor Expressed in Smooth Muscle Cells is Required for Hypertensive Vascular Remodeling in Mice Infused With Angiotensin II

BACKGROUND

Ang II (angiotensin II) type 1 (AT1) receptors play a critical role in cardiovascular diseases such as hypertension. Rodents have 2 types of AT1 receptor (AT1A and AT1B) of which knock-in Tagln-mediated smooth muscle AT1A silencing attenuated Ang II-induced hypertension. Although vascular remodeling, a significant contributor to organ damage, occurs concurrently with hypertension in Ang II-infused mice, the contribution of smooth muscle AT1A in this process remains unexplored. Accordingly, it is hypothesized that smooth muscle AT1A receptors exclusively contribute to both medial thickening and adventitial fibrosis regardless of the presence of hypertension.

METHODS

About 1 µg/kg per minute Ang II was infused for 2 weeks in 2 distinct AT1A receptor silenced mice, knock-in Tagln-mediated constitutive smooth muscle AT1A receptor silenced mice, and Myh11-mediated inducible smooth muscle AT1A together with global AT1B silenced mice for evaluation of hypertensive cardiovascular remodeling.

RESULTS

Medial thickness, adventitial collagen deposition, and immune cell infiltration in aorta were increased in control mice but not in both smooth muscle AT1A receptor silenced mice. Coronary arterial perivascular fibrosis in response to Ang II infusion was also attenuated in both AT1A receptor silenced mice. Ang II-induced cardiac hypertrophy was attenuated in constitutive smooth muscle AT1A receptor silenced mice. However, Ang II-induced cardiac hypertrophy and hypertension were not altered in inducible smooth muscle AT1A receptor silenced mice.

CONCLUSIONS

Smooth muscle AT1A receptors mediate Ang II-induced vascular remodeling including medial hypertrophy and inflammatory perivascular fibrosis regardless of the presence of hypertension. Our data suggest an independent etiology of blood pressure elevation and hypertensive vascular remodeling in response to Ang II.

Arterial myogenic response and aging

The arterial myogenic response is an inherent property of resistance arteries. Myogenic tone is crucial for maintaining a relatively constant blood flow in response to changes in intraluminal pressure and protects delicate organs from excessive blood flow. Although this fundamental physiological phenomenon has been extensively studied, the underlying molecular mechanisms are largely unknown. Recent studies identified a crucial role of mechano-activated angiotensin II type 1 receptors (AT1R) in this process. The development of myogenic response is affected by aging. In this review, we summarize recent progress made to understand the role of AT1R and other mechanosensors in the control of arterial myogenic response. We discuss age-related alterations in myogenic response and possible underlying mechanisms and implications for healthy aging.

Myogenic Vasoconstriction Requires Canonical Gq/11 Signaling of the Angiotensin II Type 1 Receptor

Background Blood pressure and tissue perfusion are controlled in part by the level of intrinsic (myogenic) arterial tone. However, many of the molecular determinants of this response are unknown. We previously found that mice with targeted disruption of the gene encoding the angiotensin II type 1a receptor (AT1AR) (Agtr1a), the major murine angiotensin II type 1 receptor (AT1R) isoform, showed reduced myogenic tone; however, uncontrolled genetic events (in this case, gene ablation) can lead to phenotypes that are difficult or impossible to interpret. Methods and Results We tested the mechanosensitive function of AT1R using tamoxifen-inducible smooth muscle-specific AT1aR knockout (smooth muscle-Agtr1a^-/-) mice and studied downstream signaling cascades mediated by G_q/11 and/or β-arrestins. FR900359, Sar1Ile4Ile8-angiotensin II (SII), TRV120027 and TRV120055 were used as selective G_q/11 inhibitor and biased agonists to activate noncanonical β-arrestin and canonical G_q/11 signaling of the AT1R, respectively. Myogenic and Ang II-induced constrictions were diminished in the perfused renal vasculature, mesenteric and cerebral arteries of smooth muscle-Agtr1a^-/- mice. Similar effects were observed in arteries of global mutant Agtr1a^-/- but not Agtr1b^-/- mice. FR900359 decreased myogenic tone and angiotensin II-induced constrictions whereas selective biased targeting of AT1R-β-arrestin signaling pathways had no effects. Conclusions This study demonstrates that myogenic arterial constriction requires G_q/11-dependent signaling pathways of mechanoactivated AT1R but not G protein-independent, noncanonical pathways in smooth muscle cells.

TRPV1 in arteries enables a rapid myogenic tone

KEY POINTS

We explored the physiological role of TRPV1 in vascular smooth muscle. TRPV1 antagonists dilated skeletal muscle arterioles both ex vivo and in vivo, increased coronary perfusion and decreased systemic blood pressure. Stretch of arteriolar myocytes and increases in intraluminal pressure in arteries triggered rapid Ca²⁺ signaling and vasoconstriction respectively. Pharmacologic and/or genetic disruption of TRPV1 significantly inhibited the magnitude and rate of these responses. Furthermore, disrupting TRPV1 blunted the rapid vasodilation evoked by arterial constriction. Pharmacological experiments identified key roles for phospholipase C and protein kinase C, combined with temperature, in TRPV1-dependent arterial tone. These results show that TRPV1 in arteriolar myocytes dynamically regulates myogenic tone and blood flow in the heart and skeletal muscle.

ABSTRACT

Arterioles maintain blow flow by adjusting their diameter in response to changes in local blood pressure. In this process called the myogenic response, a vascular smooth muscle mechanosensor controls tone predominantly through altering the membrane potential. In general, myogenic responses occur slowly (minutes). In the heart and skeletal muscle, however, tone is activated rapidly (tens of seconds) and terminated by brief (100 ms) arterial constrictions. Previously, we identified extensive expression of TRPV1 in the smooth muscle of arterioles supplying skeletal muscle, heart and fat. Here we reveal a critical role for TRPV1 in the rapid myogenic tone of these tissues. TRPV1 antagonists dilated skeletal muscle arterioles in vitro and in vivo, increased coronary flow in isolated hearts, and transiently decreased blood pressure. All of these pharmacologic effects were abolished by genetic disruption of TRPV1. Stretch of isolated vascular smooth muscle cells or raised intravascular pressure in arteries triggered Ca²⁺ signaling and vasoconstriction. The majority of these stretch-responses were TRPV1-mediated, with the remaining tone being inhibited by the TRPM4 antagonist, 9-phenantrol. Notably, tone developed more quickly in arteries from wild-type compared with TRPV1-null mice. Furthermore, the immediate vasodilation following brief constriction of arterioles depended on TRPV1, consistent with a rapid deactivation of TRPV1. Pharmacologic experiments revealed that membrane stretch activates phospholipase C/protein kinase C signaling combined with heat to activate TRPV1, and in turn, L-type Ca²⁺ channels. These results suggest a critical role, for TRPV1 in the dynamic regulation of myogenic tone and blood flow in the heart and skeletal muscle. This article is protected by copyright. All rights reserved.

Mechanical activation of the angiotensin II type 1 receptor contributes to abdominal aortic aneurysm formation

Objective

The angiotensin II type 1 receptor (AT1R) can be activated under conditions of mechanical stretch in some cellular systems. Whether this activity influences signaling within the abdominal aorta to promote to abdominal aortic aneurysm (AAA) development remains unknown. We evaluated the hypothesis that mechanical AT1R activation can occur under conditions of hypertension (HTN) and contribute to AAA formation.

Methods

BPH/2 mice, which demonstrate spontaneous neurogenic, low-renin HTN, and normotensive BPN/3 mice underwent AAA induction via the calcium chloride model, with or without an osmotic minipump delivering 30 mg/kg/d of the AT1R blocker Losartan. Systolic blood pressure (SBP) was measured at baseline and weekly via a tail cuff. The aortic diameter (AoD) was measured at baseline and terminal surgery at 21 days by digital microscopy. Aortic tissue was harvested for immunoblotting (phosphorylated extracellular signal-regulated kinase-1 and -2 [pERK1/2] to ERK1/2 ratio) and expressed as the fold-change from the BPN/3 control mice. Aortic vascular smooth muscle cells (VSMCs) underwent stretch with or without Losartan (1 μM) treatment to assess the mechanical stimulation of ERK1/2 activity. Statistical analysis of the blood pressure, AoD, and VSMC ERK1/2 activity was performed using analysis of variance. However, the data distribution was determined to be log-normal (Shapiro-Wilk test) for ERK1/2 activity. Therefore, it was logarithmically transformed before analysis of variance.

Results

At baseline, the SBP was elevated in the BPH/2 mice relative to the BPN/3 mice (P < .05). Losartan treatment significantly reduced the SBP in both mouse strains (P < .05). AAA induction did not affect the SBP. At 21 days after induction, the percentage of increase in the AoD from baseline was significantly greater in the BPH/2 mice than in the BPN/3 mice (101.28% ± 4.19% vs 75.59% ± 1.67% above baseline; P < .05). Losartan treatment significantly attenuated AAA growth in both BPH/2 and BPN/3 mice (33.88% ± 2.97% and 43.96% ± 3.05% above baseline, respectively; P < .05). ERK1/2 activity was increased approximately fivefold in the BPH/2 control mice relative to the BPN/3 control mice (P < .05). In the BPH/2 and BPN/3 mice with AAA, ERK1/2 activity was significantly increased relative to the respective baseline control (P < .05) and effectively reduced by concomitant Losartan therapy (P < .05). Biaxial stretch of the VSMCs in the absence of angiotensin II demonstrated increased ERK1/2 activation (P < .05 vs static control), which was significantly inhibited by Losartan.

Conclusions

In BPH/2 mice with spontaneous neurogenic, low-renin HTN, AAA growth was amplified compared with the normotensive control and was effectively attenuated using Losartan. ERK1/2 activity was significantly elevated in the BPH/2 mice and after AAA induction in the normotensive and hypertensive mice but was attenuated by Losartan treatment. These data suggest that AT1R activation contributes to AAA development. Therefore, further investigation into this signaling pathway could establish targets for pharmacotherapeutic engineering to slow AAA growth. (JVS-Vascular Science 2021;2:194-206.).

Clinical Relevance

Hypertension (HTN) and abdominal aortic aneurysm (AAA) have been epidemiologically linked for decades; however, a biomechanical link has not yet been identified. Using a murine model of spontaneous neurogenic HTN experimentally demonstrated to have low circulating renin, mechanical activation of the angiotensin II type 1 receptor (AT1R) was identified with elevated blood pressure and AAA induction. HTN amplified AAA growth. However, more importantly, blocking the activation of AT1R with the angiotensin receptor blocker Losartan effectively abrogated AAA development. Although inhibiting the production of angiotensin II has previously been unsuccessful in altering AAA growth, the results from the present study suggest that blocking the activation of AT1R through direct ligand binding or mechanical stimulation might alter aortic wall signaling and warrants further investigation.

What Evolutionary Evidence Implies About the Identity of the Mechanoelectrical Couplers in Vascular Smooth Muscle Cells

Loss of pressure-induced vasoconstriction increases susceptibility to renal and cerebral vascular injury. Favored paradigms underlying initiation of the response include transient receptor potential channels coupled to G protein-coupled receptors or integrins as transducers. Degenerin channels may also mediate the response. This review addresses the 1) evolutionary role of these molecules in mechanosensing, 2) limitations to identifying mechanosensitive molecules, and 3) paradigm shifting molecular model for a VSMC mechanosensor.

Transient receptor potential channel-dependent myogenic responsiveness in small-sized resistance arteries

It is well documented that the inherent ability of small arteries and arterioles to regulate intraluminal diameter in response to alterations in intravascular pressure determines peripheral vascular resistance and blood flow (termed myogenic response or pressure-induced vasoconstriction/dilation). This autoregulatory property of resistance arteries is primarily originated from mechanosensitive vascular smooth muscle cells (VSMCs). There are diverse biological apparatuses in the plasma membrane of VSMCs that sense mechanical stimuli and generate intracellular signals for the contractility of VSMCs. Although the roles of transient receptor potential (TRP) channels in pressure-induced vasoconstriction are not fully understood to date, TRP channels that are directly activated by mechanical stimuli (e.g., stretch of VSMCs) or indirectly evoked by intracellular molecules (e.g., inositol trisphosphate) provide the major sources of Ca²⁺ (e.g., Ca²⁺ influx or release from the sarcoplasmic reticulum) and in turn, evoke vascular reactivity. This review sought to summarize mounting evidence over several decades that the activation of TRP canonical, TRP melastatin, TRP vanilloid, and TRP polycystin channels contributes to myogenic vasoconstriction.

Nox2 signaling and muscle fiber remodeling are attenuated by losartan administration during skeletal muscle unloading

Reduced mechanical loading results in atrophy of skeletal muscle fibers. Increased reactive oxygen species (ROS) are causal in sarcolemmal dislocation of nNOS and FoxO3a activation. The Nox2 isoform of NADPH oxidase and mitochondria release ROS during disuse in skeletal muscle. Activation of the angiotensin II type 1 receptor (AT1R) can elicit Nox2 complex formation. The AT1R blocker losartan was used to test the hypothesis that AT1R activation drives Nox2 assembly, nNOS dislocation, FoxO3a activation, and thus alterations in morphology in the unloaded rat soleus. Male Fischer 344 rats were divided into four groups: ambulatory control (CON), ambulatory + losartan (40 mg kg^-1  day^-1 ) (CONL), 7 days of tail-traction hindlimb unloading (HU), and HU + losartan (HUL). Losartan attenuated unloading-induced loss of muscle fiber cross-sectional area (CSA) and fiber-type shift. Losartan mitigated unloading-induced elevation of ROS levels and upregulation of Nox2. Furthermore, AT1R blockade abrogated nNOS dislocation away from the sarcolemma and elevation of nuclear FoxO3a. We conclude that AT1R blockade attenuates disuse remodeling by inhibiting Nox2, thereby lessening nNOS dislocation and activation of FoxO3a.

Differential expression of angiotensin II type 1 receptor subtypes within the cerebral microvasculature

Arteries and arterioles constrict in response to intraluminal pressure to generate myogenic tone, but the molecular nature of the vascular force-sensing mechanism is not fully characterized. Here, we investigated the role of angiotensin II type 1 receptors (AT1Rs) on vascular smooth muscle cells in the development of myogenic tone in cerebral parenchymal arterioles from mice. We found that pretreatment with the AT1R blocker losartan inhibited the development of myogenic tone in these vessels but did not alter the luminal diameter of arterioles with preestablished tone. Rodents express two AT1R isotypes: AT1Ra and AT1Rb. We previously demonstrated that AT1Rb is expressed at much higher levels compared with AT1Ra in cerebral pial arteries and is required for myogenic contractility in these vessels, whereas AT1Ra is unnecessary for this function. Here, we found that AT1Ra and AT1Rb are expressed at similar levels in parenchymal arterioles and that genetic knockout of AT1Ra blunted the ability of these vessels to generate myogenic tone. We also found that AT1Rb and total AT1R expression levels are much lower in parenchymal arterioles compared with pial arteries and that parenchymal arterioles are less sensitive to the vasoconstrictive effects of the endogenous AT1R ligand angiotensin II (ANG II). We conclude that 1) AT1Rs are critical for the initiation, but not the maintenance, of myogenic tone in parenchymal arterioles, and 2) lower levels of AT1Rb and total AT1R in parenchymal arterioles compared with pial arteries result in differences in myogenic and ANG II-induced vasoconstriction between these vascular segments.NEW & NOTEWORTHY Myogenic tone is critical for appropriate regulation of cerebral blood flow, but the mechanisms used by vascular smooth muscle cells to detect changes in intraluminal pressure are not fully characterized. Here, we demonstrate angiotensin II receptor type 1 (AT1R) is indispensable to initiation, but not maintenance, of myogenic tone in cerebral parenchymal arterioles. Furthermore, we demonstrate differences in AT1R expression levels lead to critical differences in contractile regulation between parenchymal arterioles and cerebral pial arteries.

Canonical Transient Receptor Potential Channels and Vascular Smooth Muscle Cell Plasticity

Vascular smooth muscle cells (VSMCs) play a pivotal role in the stability and tonic regulation of vascular homeostasis. VSMCs can switch back and forth between highly proliferative (synthetic) and fully differentiated (contractile) phenotypes in response to changes in the vessel environment. Abnormal phenotypic switching of VSMCs is a distinctive characteristic of vascular disorders, including atherosclerosis, pulmonary hypertension, stroke, and peripheral artery disease; however, how the control of VSMC phenotypic switching is dysregulated under pathological conditions remains obscure. Canonical transient receptor potential (TRPC) channels have attracted attention as a key regulator of pathological phenotype switching in VSMCs. Several TRPC subfamily member proteins—especially TRPC1 and TRPC6—are upregulated in pathological VSMCs, and pharmacological inhibition of TRPC channel activity has been reported to improve hypertensive vascular remodeling in rodents. This review summarizes the current understanding of the role of TRPC channels in cardiovascular plasticity, including our recent finding that TRPC6 participates in aberrant VSMC phenotype switching under ischemic conditions, and discusses the therapeutic potential of TRPC channels.

Helix 8 is the essential structural motif of mechanosensitive GPCRs

G-protein coupled receptors (GPCRs) are versatile cellular sensors for chemical stimuli, but also serve as mechanosensors involved in various (patho)physiological settings like vascular regulation, cardiac hypertrophy and preeclampsia. However, the molecular mechanisms underlying mechanically induced GPCR activation have remained elusive. Here we show that mechanosensitive histamine H₁ receptors (H₁Rs) are endothelial sensors of fluid shear stress and contribute to flow-induced vasodilation. At the molecular level, we observe that H₁Rs undergo stimulus-specific patterns of conformational changes suggesting that mechanical forces and agonists induce distinct active receptor conformations. GPCRs lacking C-terminal helix 8 (H8) are not mechanosensitive, and transfer of H8 to non-responsive GPCRs confers, while removal of H8 precludes, mechanosensitivity. Moreover, disrupting H8 structural integrity by amino acid exchanges impairs mechanosensitivity. Altogether, H8 is the essential structural motif endowing GPCRs with mechanosensitivity. These findings provide a mechanistic basis for a better understanding of the roles of mechanosensitive GPCRs in (patho)physiology.

GPCRs are versatile cellular sensors for chemical stimuli but the molecular mechanisms underlying mechanically induced GPCR activation have remained elusive. Here authors identify the C-terminal helix 8 (H8) as the essential structural motif endowing H₁R and other GPCRs with mechanosensitivity.

Angiotensin II Signal Transduction: An Update on Mechanisms of Physiology and Pathophysiology

The renin-angiotensin-aldosterone system plays crucial roles in cardiovascular physiology and pathophysiology. However, many of the signaling mechanisms have been unclear. The angiotensin II (ANG II) type 1 receptor (AT₁R) is believed to mediate most functions of ANG II in the system. AT₁R utilizes various signal transduction cascades causing hypertension, cardiovascular remodeling, and end organ damage. Moreover, functional cross-talk between AT₁R signaling pathways and other signaling pathways have been recognized. Accumulating evidence reveals the complexity of ANG II signal transduction in pathophysiology of the vasculature, heart, kidney, and brain, as well as several pathophysiological features, including inflammation, metabolic dysfunction, and aging. In this review, we provide a comprehensive update of the ANG II receptor signaling events and their functional significances for potential translation into therapeutic strategies. AT₁R remains central to the system in mediating physiological and pathophysiological functions of ANG II, and participation of specific signaling pathways becomes much clearer. There are still certain limitations and many controversies, and several noteworthy new concepts require further support. However, it is expected that rigorous translational research of the ANG II signaling pathways including those in large animals and humans will contribute to establishing effective new therapies against various diseases.

Arterial smooth muscle cell PKD2 (TRPP1) channels regulate systemic blood pressure

Systemic blood pressure is determined, in part, by arterial smooth muscle cells (myocytes). Several Transient Receptor Potential (TRP) channels are proposed to be expressed in arterial myocytes, but it is unclear if these proteins control physiological blood pressure and contribute to hypertension in vivo. We generated the first inducible, smooth muscle-specific knockout mice for a TRP channel, namely for PKD2 (TRPP1), to investigate arterial myocyte and blood pressure regulation by this protein. Using this model, we show that intravascular pressure and α₁-adrenoceptors activate PKD2 channels in arterial myocytes of different systemic organs. PKD2 channel activation in arterial myocytes leads to an inward Na⁺ current, membrane depolarization and vasoconstriction. Inducible, smooth muscle cell-specific PKD2 knockout lowers both physiological blood pressure and hypertension and prevents pathological arterial remodeling during hypertension. Thus, arterial myocyte PKD2 controls systemic blood pressure and targeting this TRP channel reduces high blood pressure.

Loss of Vascular Myogenic Tone in miR-143/145 Knockout Mice Is Associated With Hypertension-Induced Vascular Lesions in Small Mesenteric Arteries

OBJECTIVE

Pressure-induced myogenic tone is involved in autoregulation of local blood flow and confers protection against excessive pressure levels in small arteries and capillaries. Myogenic tone is dependent on smooth muscle microRNAs (miRNAs), but the identity of these miRNAs is unclear. Furthermore, the consequences of altered myogenic tone for hypertension-induced damage to small arteries are not well understood.

APPROACH AND RESULTS

The importance of smooth muscle-enriched microRNAs, miR-143/145, for myogenic tone was evaluated in miR-143/145 knockout mice. Furthermore, hypertension-induced vascular injury was evaluated in mesenteric arteries in vivo after angiotensin II infusion. Myogenic tone was abolished in miR-143/145 knockout mesenteric arteries, whereas contraction in response to calyculin A and potassium chloride was reduced by ≈30%. Furthermore, myogenic responsiveness was potentiated by angiotensin II in wild-type but not in knockout mice. Angiotensin II administration in vivo elevated systemic blood pressure in both genotypes. Hypertensive knockout mice developed severe vascular lesions characterized by vascular inflammation, adventitial fibrosis, and neointimal hyperplasia in small mesenteric arteries. This was associated with depolymerization of actin filaments and fragmentation of the elastic laminae at the sites of vascular lesions.

CONCLUSIONS

This study demonstrates that miR-143/145 expression is essential for myogenic responsiveness. During hypertension, loss of myogenic tone results in potentially damaging levels of mechanical stress and detrimental effects on small arteries. The results presented herein provide novel insights into the pathogenesis of vascular disease and emphasize the importance of controlling mechanical factors to maintain structural integrity of the vascular wall.

Angiotensin II Type 1 Receptor Mechanoactivation Involves RGS5 (Regulator of G Protein Signaling 5) in Skeletal Muscle Arteries: Impaired Trafficking of RGS5 in Hypertension

Studies suggest that arteriolar pressure-induced vasoconstriction can be initiated by GPCRs (G protein-coupled receptors), including the AT₁R (angiotensin II type 1 receptor). This raises the question, are such mechanisms regulated by negative feedback? The present studies examined whether RGS (regulators of G protein signaling) proteins in vascular smooth muscle cells are colocalized with the AT₁R when activated by mechanical stress or angiotensin II and whether this modulates AT₁R-mediated vasoconstriction. To determine whether activation of the AT₁R recruits RGS5, an in situ proximity ligation assay was performed in primary cultures of cremaster muscle arteriolar vascular smooth muscle cells treated with angiotensin II or hypotonic solution in the absence or presence of candesartan (an AT₁R blocker). Proximity ligation assay results revealed a concentration-dependent increase in trafficking/translocation of RGS5 toward the activated AT₁R, which was attenuated by candesartan. In intact arterioles, knockdown of RGS5 enhanced constriction to angiotensin II and augmented myogenic responses to increased intraluminal pressure. Myogenic constriction was attenuated to a higher degree by candesartan in RGS5 siRNA-transfected arterioles, consistent with RGS5 contributing to downregulation of AT₁R-mediated signaling. Further, translocation of RGS5 was impaired in vascular smooth muscle cells of spontaneously hypertensive rats. This is consistent with dysregulated (RGS5-mediated) AT₁R signaling that could contribute to excessive vasoconstriction in hypertension. In intact vessels, candesartan reduced myogenic vasoconstriction to a greater extent in spontaneously hypertensive rats compared with controls. Collectively, these findings suggest that AT₁R activation results in translocation of RGS5 toward the plasma membrane, limiting AT₁R-mediated vasoconstriction through its role in G_q/11 protein-dependent signaling.

Molecular Sensors of Blood Flow in Endothelial Cells

Mechanical stress from blood flow has a significant effect on endothelial physiology, with a key role in initiating vasoregulatory signals. Disturbances in blood flow, such as in regions of disease-associated stenosis, arterial branch points, and sharp turns, can induce proatherogenic phenotypes in endothelial cells. The disruption of vascular homeostasis as a result of endothelial dysfunction may contribute to early and late stages of atherosclerosis, the underlying cause of coronary artery disease. In-depth knowledge of the mechanobiology of endothelial cells is essential to identifying mechanosensory complexes involved in the pathogenesis of atherosclerosis. In this review, we describe different blood flow patterns and summarize current knowledge on mechanosensory molecules regulating endothelial vasoregulatory functions, with clinical implications. Such information may help in the search for novel therapeutic approaches.

The angiotensin II receptor type 1b is the primary sensor of intraluminal pressure in cerebral artery smooth muscle cells

KEY POINTS

The angiotensin II receptor type 1b (AT1 Rb ) is the primary sensor of intraluminal pressure in cerebral arteries. Pressure or membrane-stretch induced stimulation of AT1 Rb activates the TRPM4 channel and results in inward transient cation currents that depolarize smooth muscle cells, leading to vasoconstriction. Activation of either AT1 Ra or AT1 Rb with angiotensin II stimulates TRPM4 currents in cerebral artery myocytes and vasoconstriction of cerebral arteries. The expression of AT1 Rb mRNA is ∼30-fold higher than AT1 Ra in whole cerebral arteries and ∼45-fold higher in isolated cerebral artery smooth muscle cells. Higher levels of expression are likely to account for the obligatory role of AT1 Rb for pressure-induced vasoconstriction. ABSTRACT: Myogenic vasoconstriction, which reflects the intrinsic ability of smooth muscle cells to contract in response to increases in intraluminal pressure, is critically important for the autoregulation of blood flow. In smooth muscle cells from cerebral arteries, increasing intraluminal pressure engages a signalling cascade that stimulates cation influx through transient receptor potential (TRP) melastatin 4 (TRPM4) channels to cause membrane depolarization and vasoconstriction. Substantial evidence indicates that the angiotensin II receptor type 1 (AT1 R) is inherently mechanosensitive and initiates this signalling pathway. Rodents express two types of AT1 R - AT1 Ra and AT1 Rb - and conflicting studies provide support for either isoform as the primary sensor of intraluminal pressure in peripheral arteries. We hypothesized that mechanical activation of AT1 Ra increases TRPM4 currents to induce myogenic constriction of cerebral arteries. However, we found that development of myogenic tone was greater in arteries from AT1 Ra knockout animals compared with controls. In patch-clamp experiments using native cerebral arterial myocytes, membrane stretch-induced cation currents were blocked by the TRPM4 inhibitor 9-phenanthrol in both groups. Further, the AT1 R blocker losartan (1 μm) diminished myogenic tone and blocked stretch-induced cation currents in cerebral arteries from both groups. Activation of AT1 R with angiotensin II (30 nm) also increased TRPM4 currents in smooth muscle cells and constricted cerebral arteries from both groups. Expression of AT1 Rb mRNA was ∼30-fold greater than AT1 Ra in cerebral arteries, and knockdown of AT1 Rb selectively diminished myogenic constriction. We conclude that AT1 Rb , acting upstream of TRPM4 channels, is the primary sensor of intraluminal pressure in cerebral artery smooth muscle cells.

Mechanical activation of angiotensin II type 1 receptors causes actin remodelling and myogenic responsiveness in skeletal muscle arterioles

KEY POINTS

Candesartan, an inverse agonist of the type 1 angiotensin II receptor (AT1 R), causes a concentration-dependent inhibition of pressure-dependent myogenic tone consistent with previous reports of mechanosensitivity of this G protein-coupled receptor. Mechanoactivation of the AT1 R occurs independently of local angiotensin II production and the type 2 angiotensin receptor. Mechanoactivation of the AT1 R stimulates actin polymerization by a protein kinase C-dependent mechanism, but independently of a change in intracellular Ca2+ . Using atomic force microscopy, changes in single vascular smooth muscle cell cortical actin are observed to remodel following mechanoactivation of the AT1 R.

ABSTRACT

The Gq/11 protein-coupled angiotensin II type 1 receptor (AT1 R) has been shown to be activated by mechanical stimuli. In the vascular system, evidence supports the AT1 R being a mechanosensor that contributes to arteriolar myogenic constriction. The aim of this study was to determine if AT1 R mechanoactivation affects myogenic constriction in skeletal muscle arterioles and to determine underlying cellular mechanisms. Using pressure myography to study rat isolated first-order cremaster muscle arterioles the AT1 R inhibitor candesartan (10-7 -10-5  m) showed partial but concentration-dependent inhibition of myogenic reactivity. Inhibition was demonstrated by a rightward shift in the pressure-diameter relationship over the intraluminal pressure range, 30-110 mmHg. Pressure-induced changes in global vascular smooth muscle intracellular Ca2+ (using Fura-2) were similar in the absence or presence of candesartan, indicating that AT1 R-mediated myogenic constriction relies on Ca2+ -independent downstream signalling. The diacylglycerol analogue 1-oleoyl-2-acetyl-sn-glycerol (OAG) reversed the inhibitory effect of candesartan, while this rescue effect was prevented by the protein kinase C (PKC) inhibitor GF 109203X. Both candesartan and PKC inhibition caused increased G-actin levels, as determined by Western blotting of vessel lysates, supporting involvement of cytoskeletal remodelling. At the single vascular smooth muscle cell level, atomic force microscopy showed that cell swelling (stretch) with hypotonic buffer also caused thickening of cortical actin fibres and this was blocked by candesartan. Collectively, the present studies support growing evidence for novel modes of activation of the AT1 R in arterioles and suggest that mechanically activated AT1 R generates diacylglycerol, which in turn activates PKC which induces the actin cytoskeleton reorganization that is required for pressure-induced vasoconstriction.

Role of angiotensin II type 1a receptor in renal injury induced by deoxycorticosterone acetate-salt hypertension

The aim of this study was to investigate the in vivo role of angiotensin II type 1a (AT1a) receptor in renal damage as a result of hypertension by using transgenic mice with AT1a receptor gene disruption. Transgenic mice that express human liver-type fatty acid binding protein (L-FABP) with or without disruption of the AT1a receptor gene (L-FABP^+/− AT1a^−/−, and L-FABP^+/− AT1a^+/+, respectively) were used with urinary L-FABP as an indicator of tubulointerstitial damage. Those female mice were administered subcutaneously deoxycorticosterone acetate (DOCA)–salt tablets plus drinking water that contained 1% saline for 28 d after uninephrectomy. In L-FABP^+/− AT1a^+/+ mice that received DOCA-salt treatment, hypertension was induced and slight expansion of glomerular area, glomerular sclerosis, and tubulointerstitial damage were observed. In L-FABP^+/− AT1a^−/− mice that received DOCA-salt treatment, hypertension was similarly induced and the degree of glomerular damage was significantly more severe than in L-FABP^+/− AT1a^+/+-DOCA mice. Urinary L-FABP levels were significantly higher in L-FABP^+/− AT1a^−/−-DOCA mice compared with those in L-FABP^+/− AT1a^+/+-DOCA mice. Hydralazine treatment significantly attenuated renal damage that was found in L-FABP^+/− AT1a^−/−-DOCA mice along with a reduction in blood pressure. In summary, activation of the AT1a receptor may contribute to maintenance of the glomerular structure against hypertensive renal damage.—Hisamichi, M., Kamijo-Ikemori, A., Sugaya, T., Ichikawa, D., Natsuki, T., Hoshino, S., Kimura, K., Shibagaki, Y. Role of angiotensin II type 1a receptor in renal injury induced by deoxycorticosterone acetate–salt hypertension.

Arterial Myogenic Activation through Smooth Muscle Filamin A

Mutations in the filamin A (FlnA) gene are frequently associated with severe arterial abnormalities, although the physiological role for this cytoskeletal element remains poorly understood in vascular cells. We used a conditional mouse model to selectively delete FlnA in smooth muscle (sm) cells at the adult stage, thus avoiding the developmental effects of the knockout. Basal blood pressure was significantly reduced in conscious smFlnA knockout mice. Remarkably, pressure-dependent tone of the resistance caudal artery was lost, whereas reactivity to vasoconstrictors was preserved. Impairment of the myogenic behavior was correlated with a lack of calcium influx in arterial myocytes upon an increase in intraluminal pressure. Notably, the stretch activation of CaV1.2 was blunted in the absence of smFlnA. In conclusion, FlnA is a critical upstream element of the signaling cascade underlying the myogenic tone. These findings allow a better understanding of the molecular basis of arterial autoregulation and associated disease states.

Salt, Angiotensin II, Superoxide, and Endothelial Function

Proper function of the vascular endothelium is essential for cardiovascular health, in large part due to its antiproliferative, antihypertrophic, and anti-inflammatory properties. Crucial to the protective role of the endothelium is the production and liberation of nitric oxide (NO), which not only acts as a potent vasodilator, but also reduces levels of reactive oxygen species, including superoxide anion (O2•-). Superoxide anion is highly injurious to the vasculature because it not only scavenges NO molecules, but has other damaging effects, including direct oxidative disruption of normal signaling mechanisms in the endothelium and vascular smooth muscle cells. The renin-angiotensin system plays a crucial role in the maintenance of normal blood pressure. This function is mediated via the peptide hormone angiotensin II (ANG II), which maintains normal blood volume by regulating Na+ excretion. However, elevation of ANG II above normal levels increases O2•- production, promotes oxidative stress and endothelial dysfunction, and plays a major role in multiple disease conditions. Elevated dietary salt intake also leads to oxidant stress and endothelial dysfunction, but these occur in the face of salt-induced ANG II suppression and reduced levels of circulating ANG II. While the effects of abnormally high levels of ANG II have been extensively studied, far less is known regarding the mechanisms of oxidant stress and endothelial dysfunction occurring in response to chronic exposure to abnormally low levels of ANG II. The current article focuses on the mechanisms and consequences of this less well understood relationship among salt, superoxide, and endothelial function.

Podocyte Purinergic P2X4 Channels Are Mechanotransducers That Mediate Cytoskeletal Disorganization

Podocytes are specialized, highly differentiated epithelial cells in the kidney glomerulus that are exposed to glomerular capillary pressure and possible increases in mechanical load. The proteins sensing mechanical forces in podocytes are unconfirmed, but the classic transient receptor potential channel 6 (TRPC6) interacting with the MEC-2 homolog podocin may form a mechanosensitive ion channel complex in podocytes. Here, we observed that podocytes respond to mechanical stimulation with increased intracellular calcium concentrations and increased inward cation currents. However, TRPC6-deficient podocytes responded in a manner similar to that of control podocytes, and mechanically induced currents were unaffected by genetic inactivation of TRPC1/3/6 or administration of the broad-range TRPC blocker SKF-96365. Instead, mechanically induced currents were significantly decreased by the specific P2X purinoceptor 4 (P2X4) blocker 5-BDBD. Moreover, mechanical P2X4 channel activation depended on cholesterol and podocin and was inhibited by stabilization of the actin cytoskeleton. Because P2X4 channels are not intrinsically mechanosensitive, we investigated whether podocytes release ATP upon mechanical stimulation using a fluorometric approach. Indeed, mechanically induced ATP release from podocytes was observed. Furthermore, 5-BDBD attenuated mechanically induced reorganization of the actin cytoskeleton. Altogether, our findings reveal a TRPC channel-independent role of P2X4 channels as mechanotransducers in podocytes.

PC-PLC/sphingomyelin synthase activity plays a central role in the development of myogenic tone in murine resistance arteries

Myogenic tone is an intrinsic property of the vasculature that contributes to blood pressure control and tissue perfusion. Earlier investigations assigned a key role in myogenic tone to phospholipase C (PLC) and its products, inositol 1,4,5-trisphosphate (IP3) and diacylglycerol (DAG). Here, we used the PLC inhibitor, U-73122, and two other, specific inhibitors of PLC subtypes (PI-PLC and PC-PLC) to delineate the role of PLC in myogenic tone of pressurized murine mesenteric arteries. U-73122 inhibited depolarization-induced contractions (high external K(+) concentration), thus confirming reports of nonspecific actions of U-73122 and its limited utility for studies of myogenic tone. Edelfosine, a specific inhibitor of PI-PLC, did not affect depolarization-induced contractions but modulated myogenic tone. Because PI-PLC produces IP3, we investigated the effect of blocking IP3 receptor-mediated Ca(2+) release on myogenic tone. Incubation of arteries with xestospongin C did not affect tone, consistent with the virtual absence of Ca(2+) waves in arteries with myogenic tone. D-609, an inhibitor of PC-PLC and sphingomyelin synthase, strongly inhibited myogenic tone and had no effect on depolarization-induced contraction. D-609 appeared to act by lowering cytoplasmic Ca(2+) concentration to levels below those that activate contraction. Importantly, incubation of pressurized arteries with a membrane-permeable analog of DAG induced vasoconstriction. The results therefore mandate a reexamination of the signaling pathways activated by the Bayliss mechanism. Our results suggest that PI-PLC and IP3 are not required in maintaining myogenic tone, but DAG, produced by PC-PLC and/or SM synthase, is likely through multiple mechanisms to increase Ca(2+) entry and promote vasoconstriction.

Cysteinyl leukotriene 1 receptors as novel mechanosensors mediating myogenic tone together with angiotensin II type 1 receptors-brief report

OBJECTIVE

Myogenic vasoconstriction is mediated by vascular smooth muscle cells of resistance arteries sensing mechanical stretch. Angiotensin II AT1 receptors and in particular AT1BRs in murine vascular smooth muscle cells have been characterized as mechanosensors that cannot fully account for myogenic vasoconstriction observed. Therefore, we aimed at uncovering novel vascular mechanosensors by expression profiling and functional characterization of candidate proteins.

APPROACH AND RESULTS

Analyzing myogenic tone of isolated murine mesenteric arteries of AT1A and AT1B receptor double gene-deficient (AT1A/1B (-/-)) mice ex vivo, we observed a decreased myogenic tone at high intraluminal pressures and an unexpected hyper-reactivity at low intraluminal pressures because of upregulation of cysteinyl leukotriene 1 receptors (CysLT1Rs). Pharmacological blockade of CysLT1Rs with pranlukast significantly reduced myogenic tone not only in AT1A/1B (-/-) but also in wild-type arteries. In wild-type arteries, additional blockade of angiotensin II AT1 receptors with candesartan resulted in an additive reduction of myogenic tone. Furthermore, calcium imaging experiments were performed with fura-2-loaded human embryonic kidney 293 cells overexpressing CysLT1Rs and with isolated mesenteric vascular smooth muscle cells. Hypo-osmotically induced membrane stretch provoked calcium transients that were significantly reduced by pranlukast. Incubations of isolated mesenteric vascular smooth muscle cells with the 5-lipoxygenase inhibitor zileuton had no effect. Furthermore, the Gq/11-protein inhibitor YM 254890 profoundly reduced myogenic tone to the same extent as induced by the application of pranlukast plus candesartan.

CONCLUSIONS

Here, we identify a novel, hitherto unappreciated role of CysLT1Rs in vascular regulation. We identified CysLT1Rs as novel mechanosensors in the vasculature involved in myogenic vasoconstriction. Moreover, our findings suggest that myogenic tone is determined by AT1 and CysLT1 receptors acting together as mechanosensors via Gq/11-protein activation.

Elevated testosterone levels during rat pregnancy cause hypersensitivity to angiotensin II and attenuation of endothelium-dependent vasodilation in uterine arteries

Elevated testosterone levels increase maternal blood pressure and decrease uterine blood flow in pregnancy, resulting in abnormal perinatal outcomes. We tested whether elevated testosterone alters uterine artery adaptations during pregnancy, and whether these alterations depend on endothelium-derived factors such as nitric oxide, endothelium-derived hyperpolarizing factor, and prostacyclin, or endothelium-independent mechanisms such as angiotensin II (Ang-II). Pregnant Sprague-Dawley rats were injected with vehicle (n=20) or testosterone propionate (0.5 mg/kg per day from gestation day 15 to 19; n=20). Plasma testosterone levels increased 2-fold in testosterone-injected rats compared with controls. Elevated testosterone significantly decreased placental and pup weights compared with controls. In endothelium-intact uterine arteries, contractile responses to thromboxane, phenylephrine, and Ang-II were greater in testosterone-treated rats compared with controls. In endothelium-denuded arteries, contractile responses to Ang-II (pD2=9.1±0.04 versus 8.7±0.04 in controls; P<0.05), but not thromboxane and phenylephrine, were greater in testosterone-treated rats. Ang-II type 1b receptor expression was increased, whereas Ang-II type 2 receptor was decreased in testosterone-exposed arteries. In endothelium-denuded arteries, relaxations to sodium nitroprusside were unaffected. Endothelium-dependent relaxation to acetylcholine was significantly lower in arteries from testosterone-treated dams (Emax=51.80±6.9% versus 91.98±1.4% in controls; P<0.05). The assessment of endothelial factors showed that nitric oxide-, endothelium-derived hyperpolarizing factor-, and prostacyclin-mediated relaxations were blunted in testosterone-treated dams. Endothelial nitric oxide synthase, small conductance calcium-activated potassium channel-3, and prostacyclin receptor expressions were significantly decreased in arteries from testosterone-treated dams. Hypoxia-inducible factor-1α, Ankrd37, and Egln were significantly increased in testosterone-exposed placentas. These results suggest that elevated maternal testosterone impairs uterine vascular function, which may lead to an increased vascular resistance and a decrease in uterine blood flow.