Rat Skeletal Muscle Arterioles Research Articles

Conducted dilatation to ATP and K+ in rat skeletal muscle arterioles.

AimDuring exercise in humans, circulating levels of ATP and K+ increase at a time when blood flow increases to satisfy metabolic demand. Both molecules can activate arteriolar K+ channels to stimulate vasodilatation; here, it is established whether conducted dilatation is observed in a skeletal muscle bed.MethodsIsolated and cannulated rat cremaster arterioles were used to assess both local and conducted responses. Agents were either added to the bath, focally pulse‐ejected to the downstream end of arterioles, or in triple‐cannulated arterioles, luminally perfused into the downstream branches to assess both local and conducted responses.ResultsThe endothelium‐dependent agonist ACh and the KATP channel opener levcromakalim each stimulated both local and conducted vasodilatation. Focal, bolus delivery of ATP (10 μ m) or KCl (33 mm) to the outside of arterioles stimulated a biphasic vasomotor response: rapid vasoconstriction followed by dilatation as each washed away. At lower concentrations of KCl (19 mm), constriction was avoided, and instead, Ba2+‐sensitive local dilatation and conducted dilatation were both observed. Luminal perfusion of ATP avoided constriction and activated P2Y1 receptors stimulating vasodilatation secondary to opening of KC a channels. In triple‐cannulated arterioles, either ATP (10 μ m) or K+ (15 mm) luminally perfused into daughter branches of a bifurcation stimulated local dilatation which conducted into the parent arteriole.ConclusionThe recognized physiological autocrine and paracrine mediators ATP and K+ each act to evoke both local and conducted vasodilatation in rat cremaster arterioles. Therefore, in situations when circulating levels are raised, such as during exercise, these agents can act as important regulators of blood flow.

Lysophosphatidic acid induces integrin activation in vascular smooth muscle and alters arteriolar myogenic vasoconstriction.

In vascular smooth muscle cells (VSMC) increased integrin adhesion to extracellular matrix (ECM) proteins, as well as the production of reactive oxygen species (ROS) are strongly stimulated by lysophosphatidic acid (LPA). We hypothesized that LPA-induced generation of ROS increases integrin adhesion to the ECM. Using atomic force microscopy (AFM) we determined the effects of LPA on integrin adhesion to fibronectin (FN) in VSMC isolated from rat (Sprague–Dawley) skeletal muscle arterioles. In VSMC, exposure to LPA (2 μM) doubled integrin-FN adhesion compared to control cells (P < 0.05). LPA-induced integrin-FN adhesion was reduced by pre-incubation with antibodies against β1 and β3 integrins (50 μg/ml) by 66% (P < 0.05). Inhibition of LPA signaling via blockade of the LPA G-protein coupled receptors LPAR1 and LPAR3 with 10 μM Ki16425 reduced the LPA-enhanced adhesion of VSCM to FN by 40% (P < 0.05). Suppression of ROS with tempol (250 μM) or apocynin (300 μM) also reduced the LPA-induced FN adhesion by 47% (P < 0.05) and 59% (P < 0.05), respectively. Using confocal microscopy, we observed that blockade of LPA signaling, with Ki16425, reduced ROS by 45% (P < 0.05), to levels similar to control VSMC unexposed to LPA. In intact isolated arterioles, LPA (2 μM) exposure augmented the myogenic constriction response to step increases in intraluminal pressure (between 40 and 100 mm Hg) by 71% (P < 0.05). The blockade of LPA signaling, with Ki16425, decreased the LPA-enhanced myogenic constriction by 58% (P < 0.05). Similarly, blockade of LPA-induced ROS release with tempol or gp91 ds-tat decreased the LPA-enhanced myogenic constriction by 56% (P < 0.05) and 55% (P < 0.05), respectively. These results indicate that, in VSMC, LPA-induced integrin activation involves the G-protein coupled receptors LPAR1 and LPAR3, and the production of ROS, and that LPA may play an important role in the control of myogenic behavior in resistance vessels through ROS modulation of integrin activity.

Hydrogen peroxide elicits constriction of skeletal muscle arterioles by activating the arachidonic acid pathway.

AimsThe molecular mechanisms of the vasoconstrictor responses evoked by hydrogen peroxide (H2O2) have not been clearly elucidated in skeletal muscle arterioles.Methods and ResultsChanges in diameter of isolated, cannulated and pressurized gracilis muscle arterioles (GAs) of Wistar-Kyoto rats were determined under various test conditions. H2O2 (10–100 µM) evoked concentration-dependent constrictions in the GAs, which were inhibited by endothelium removal, or by antagonists of phospholipase A (PLA; 100 µM 7,7-dimethyl-(5Z,8Z)-eicosadienoic acid), protein kinase C (PKC; 10 µM chelerythrine), phospholipase C (PLC; 10 µM U-73122), or Src family tyrosine kinase (Src kinase; 1 µM Src Inhibitor-1). Antagonists of thromboxane A2 (TXA2; 1 µM SQ-29548) or the non-specific cyclooxygenase (COX) inhibitor indomethacin (10 µM) converted constrictions to dilations. The COX-1 inhibitor (SC-560, 1 µM) demonstrated a greater reduction in constriction and conversion to dilation than that of COX-2 (celecoxib, 3 µM). H2O2 did not elicit significant changes in arteriolar Ca2+ levels measured with Fura-2.ConclusionsThese data suggest that H2O2 activates the endothelial Src kinase/PLC/PKC/PLA pathway, ultimately leading to the synthesis and release of TXA2 by COX-1, thereby increasing the Ca2+ sensitivity of the vascular smooth muscle cells and eliciting constriction in rat skeletal muscle arterioles.

N‐cadherins Serve as a Mechanotransducer in Mid‐Cerebral Artery Smooth Muscle

Similar to the extracellular matrix‐integrin‐cytoskeleton axis, cell‐to‐cell N‐cadherin‐adhesion sites are linked to the cytoskeleton. We investigated the hypothesis that these sites also act as sites of mechanotransduction in vascular smooth muscle cells (VSMC). Consistent with our previous observations that N‐cadherin‐directed antibodies inhibited the myogenic response in rat skeletal muscle arterioles, knock‐down of N‐cadherin expression with specific si‐RNA also abolished myogenic tone in rat mid‐cerebral arteries (MCA). Immunolabeling of N‐cadherin demonstrated it to be distributed along the borders between adjacent VSMCs. In VSMCs isolated from MCA and transfected with N‐cadherin‐GFP, clustering of N‐cadherin‐GFP formed around an N‐cadherin‐coated microsphere (10 μm) brought into contact with the cell using an atomic force microscope (AFM). After 20 min of contact with the cell surface, application of a pulling force (1 nN; via the AFM) to the microsphere resulted in a VSMC contractile response. The contractile response was characterized by the VSMC generating a force to oppose the pulling force, and by recruitment of actin‐RFP and RhoA‐GFP to the N‐cadherin adhesion sites. Collectively, these data strongly support an important role for cell‐to‐cell N‐cadherin adhesion sites in mediating VSM mechanotransduction. (1P01HL095486 to G.A.M. and AHA 0835676N to Z.S.)

Angiotensin II Induces Vascular Endocannabinoid Release, Which Attenuates Its Vasoconstrictor Effect via CB1 Cannabinoid Receptors

In the vascular system angiotensin II (Ang II) causes vasoconstriction via the activation of type 1 angiotensin receptors. Earlier reports have shown that in cellular expression systems diacylglycerol produced during type 1 angiotensin receptor signaling can be converted to 2-arachidonoylglycerol, an important endocannabinoid. Because activation of CB(1) cannabinoid receptors (CB(1)R) induces vasodilation and reduces blood pressure, we have tested the hypothesis that Ang II-induced 2-arachidonoylglycerol release can modulate its vasoconstrictor action in vascular tissue. Rat and mouse skeletal muscle arterioles and mouse saphenous arteries were isolated, pressurized, and subjected to microangiometry. Vascular expression of CB(1)R was demonstrated using Western blot and RT-PCR. In accordance with the functional relevance of these receptors WIN55212, a CB(1)R agonist, caused vasodilation, which was absent in CB(1)R knock-out mice. Inhibition of CB(1)Rs using O2050, a neutral antagonist, enhanced the vasoconstrictor effect of Ang II in wild type but not in CB(1)R knock-out mice. Inverse agonists of CB(1)R (SR141716 and AM251) and inhibition of diacylglycerol lipase using tetrahydrolipstatin also augmented the Ang II-induced vasoconstriction, suggesting that endocannabinoid release modulates this process via CB(1)R activation. This effect was independent of nitric-oxide synthase activity and endothelial function. These data demonstrate that Ang II stimulates vascular endocannabinoid formation, which attenuates its vasoconstrictor effect, suggesting that endocannabinoid release from the vascular wall and CB(1)R activation reduces the vasoconstrictor and hypertensive effects of Ang II.

Effects of aging, TNF-α, and exercise training on angiotensin II-induced vasoconstriction of rat skeletal muscle arterioles

Skeletal muscle vascular resistance during physical exertion is higher with old age. The purpose of this study was to determine whether 1) aging enhances angiotensin II (ANG II)-induced vasoconstriction; 2) the proinflammatory cytokine tumor necrosis factor (TNF)-α contributes to alterations in ANG II-mediated vasoconstriction with aging; 3) exercise training attenuates putative age-associated increases in ANG II-mediated vasoconstriction; and 4) the mechanism(s) through which aging and exercise training alters ANG II-induced vasoconstriction in skeletal muscle arterioles. Male Fischer 344 rats were assigned to four groups: young sedentary (4 mo), old sedentary (24 mo), young trained, and old trained. In a separate group of young sedentary and old sedentary animals, a TNF type 1 receptor inhibitor was administered subcutaneously for 10 wk. First-order arterioles were isolated from soleus and gastrocnemius muscles for in vitro experimentation. Old age augmented ANG II-induced vasoconstriction in both soleus (young: 27 ± 3%; old: 38 ± 4%) and gastrocnemius (young: 42 ± 6%; old: 64 ± 9%) muscle arterioles; this augmented vasoconstriction was abolished with the removal of the endothelium, N(G)-nitro-l-arginine methyl ester, and chronic inhibition of TNF-α. In addition, exercise training ameliorated the age-induced increase in ANG II vasoconstriction. These findings demonstrate that old age enhances and exercise training diminishes ANG II-induced vasoconstrictor responses in skeletal muscle arterioles through an endothelium-dependent nitric oxide synthase signaling pathway. In addition, the enhancement of ANG II vasoconstriction with old age appears to be related to a proinflammatory state.

A Simple “Streak Length Method” for Quantifying and Characterizing Red Blood Cell Velocity Profiles and Blood Flow in Rat Skeletal Muscle Arterioles

To develop a valid experimental method for quantifying blood flow in continuously branching skeletal muscle arterioles, and to derive an empirical relationship between velocity ratio (V(Max)/V(Mean)) and arteriolar diameter. We evaluated arteriolar trees using IVVM of rat gluteus maximus muscle and developed a method to acquire single fluorescent-labeled RBC velocities across arteriolar lumens to create velocity profiles. These data were used to calculate the blood flow for 37 vessel segments (diameters: 21-115 μm). Mass balance at arteriolar bifurcations had 0.6 ± 3.2% error. Velocity ratios ranged from 1.35 to 1.98 and were positively correlated with diameter (p < 0.0001), and V(RBC) profiles were blunted with decreasing diameter. We present a means for quantifying blood flow in continuously branching skeletal muscle arterioles. Further, we provide an equation for calculating velocity ratios based on arteriolar diameter, which may be used by others for blood flow calculations.

Differential effects of diet-induced obesity on BKCa β1-subunit expression and function in rat skeletal muscle arterioles and small cerebral arteries

Mechanisms underlying obesity-related vascular dysfunction are unclear. This study examined the effect of diet-induced obesity on expression and function of large conductance Ca(2+)-activated potassium channel (BK(Ca)) in rat pressurized small resistance vessels with myogenic tone. Male Sprague-Dawley rats fed a cafeteria-style high fat diet (HFD; ∼30% energy from fat) for 16-20 wk were ∼30% heavier than controls fed standard chow (∼13% fat). Obesity did not alter BK(Ca) α-subunit function or α-subunit protein or mRNA expression in vessels isolated from the cremaster muscle or middle-cerebral circulations. In contrast, BK(Ca) β(1)-subunit protein expression and function were significantly reduced in cremaster muscle arterioles but increased in middle-cerebral arteries from obese animals. Immunohistochemistry showed α- and β(1)-subunits were present exclusively in the smooth muscle of both vessels. Cremaster muscle arterioles from obese animals showed significantly increased medial thickness, and media-to-lumen ratio and pressurized arterioles showed increased myogenic tone at 30 mmHg, but not at 50-120 mmHg. Myogenic tone was not affected by obesity in middle-cerebral arteries. The BK(Ca) antagonist iberiotoxin constricted both cremaster muscle and middle-cerebral arterioles from control rats; this effect of iberiotoxin was abolished in cremaster muscle arteries only from obese rats. Diet-induced obesity has contrasting effects on BK(Ca) function in different vascular beds, through differential effects on β(1)-subunit expression. However, these alterations in BK(Ca) function had little effect on overall myogenic tone, suggesting that the mechanisms controlling myogenic tone can be altered and compensate for altered BK(Ca) expression and function.

Time-dependent action of carbon monoxide on the newborn cerebrovascular circulation

Carbon monoxide (CO) causes cerebral arteriolar dilation in newborn pigs by the activation of large-conductance Ca(2+)-activated K(+) channels. In adult rat cerebral and skeletal muscle arterioles, CO has been reported to produce constriction caused by the inhibition of nitric oxide (NO) synthase (NOS). We hypothesized that, in contrast to dilation to acute CO, more prolonged exposure of newborn cerebral arterioles to elevated CO produces constriction by reducing NO. In piglets with closed cranial windows, pial arteriolar responses to isoproterenol (10(-6) M), sodium nitroprusside (SNP; 10(-7) and 3 x 10(-7) M), and L-arginine ethyl ester (L-Arg; 10(-5) and 10(-4) M) were determined before and after 2 h of treatment with CO. CO (10(-7) M) caused transient dilation and had no further effects. CO (2 x 10(-7) and 10(-6) M) initially caused vasodilation, but over the 2-h exposure, pial arterioles constricted and removal of the CO caused dilation. Exposure to elevated CO (2 h) did not alter dilation to SNP or isoproterenol. Conversely, the NOS substrate L-Arg caused dilation before CO that was progressively lost over 90 min of elevated CO. If NO was held constant, CO caused dilation that was sustained for 2 h. We conclude that in neonates, cerebral arteriole responses to CO are biphasic: dilation to acute elevation with subsequent constriction from NOS inhibition after more prolonged exposure. As a result, short episodic production of CO allows function as a dilator gasotransmitter, whereas prolonged elevation can reduce NO to elevate cerebrovascular tone. The interaction between heme oxygenase/CO and NOS/NO could form a negative feedback system in the control of cerebral vascular tone.

Effect of advanced glycation end products on dilation of rat skeletal muscle arterioles

Post translational protein modification, including the formation and accumulation of advanced glycation endproducts (AGE), may contribute to vascular dysfunction in diabetes. This study investigated the effect of AGE on dilation of arterioles possessing myogenic tone. AGE were manufactured by incubating 250 mM glucose‐6‐phosphate with 10 mg/ml BSA in PBS for 60 days. The effects of the modified protein were studied on isolated, pressurized (70 mmHg) first‐order arterioles from rat cremaster muscle. The AGE preparation inhibited dilation of the arterioles caused by both acetylcholine (ACh; endothelium‐dependent) and sodium nitroprusside (SNP), but had no effect on adenosine‐induced vasodilation. Iberiotoxin (0.1 μM), an antagonist of large‐conductance, Ca2+‐activated K+ channels (BKCa), inhibited ACh‐induced dilation. The combination of iberiotoxin and AGE did not cause a greater inhibition of ACh‐induced vasodilation than the application of iberiotoxin or AGE alone. The inhibitory effect of AGE on ACh‐ and SNP‐induced vasodilation was abolished by pre‐incubation of the arterioles individually with the NOS inhibitor L‐NAME (100 μM), the anti‐oxidant lipoic acid (2 μM) or the NADPH oxidase inhibitor apocynin (500 μM). Taken together, these observations suggest that AGE inhibit ACh‐ and SNP‐induced dilation of the arterioles by inhibiting BKCa, through generation of reactive oxygen species.

A Fibronectin Fragment Elicits Vasodilatation and Alters Myogenic Responsiveness of Skeletal Muscle Arterioles

Interactions between integrin‐extracellular matrix proteins are important modulators of vascular tone that are altered during vascular remodeling, wound healing, and inflammation. Degraded collagen, which contains integrin‐binding sequences Arg‐Gly‐Asp (RGD), has been shown to modulate vascular tone by altering calcium entry. In this study, we investigated the effects of a proteolytic fragment of fibronectin (FN120) on vascular tone in isolated, cannulated, and pressurized rat skeletal muscle arterioles. Abluminal incubation with FN120 (1, 10, 100 μM) for 15 minutes elicited a dose‐dependent, transient dilatation at 90 cmH2O intraluminal pressure, with 10 μM FN120 producing significant dilation (n=9, p=0.03). FN120 (10 μM) also significantly increased the steady‐state myogenic index (−0.3; control: −0.13; n=9, p=0.02), as measured over the pressure range of 60–180 cmH2O. After wash‐off and re‐application of FN120, the time to peak transient dilation at 60 cmH2O was markedly shortened (from 8 to 1.5 minutes for 10 μM FN120), compared to the first application. To determine if these effects depended on FN‐integrin interactions, arterioles were pre‐incubated with a β1 integrin function blocking antibody (26 μg/mL) prior to addition of 10 μM FN120. The β1 integrin antibody blunted the steady‐state dilation to FN120 (25% decrease; p=0.039) and attenuated the increased myogenic index (−0.3 to −0.1; n=3, p=0.03). Our data suggest that the interaction between this FN fragment and β1 integrin may be important components of vascular remodeling. Supported by NIH 072989 to MJD.

Setting the “tone” for aging in the skeletal muscle microcirculation

the ability to regulate vasomotor tone in resistance arteries and arterioles plays an integral role in mediating the circulatory adjustments that occur during exercise. During exercise, skeletal muscle blood flow must increase to actively contracting skeletal muscle to supply oxygen and nutrients

Potentiation of large conductance, Ca2+‐activated K+ (BK) channels by α5β1 integrin activation in arteriolar smooth muscle

Injury/degradation of the extracellular matrix (ECM) is associated with vascular wall remodelling and impaired reactivity, a process in which altered ECM-integrin interactions play key roles. Previously, we found that peptides containing the RGD integrin-binding sequence produce sustained vasodilatation of rat skeletal muscle arterioles. Here, we tested the hypothesis that RGD ligands work through alpha5beta1 integrin to modulate the activity of large conductance, Ca(2+)-activated K(+) (BK) channels in arteriolar smooth muscle. K(+) currents were recorded in single arteriolar myocytes using whole-cell and single-channel patch clamp methods. Activation of alpha5beta1 integrin by an appropriate, insoluble alpha5beta1 antibody resulted in a 30-50% increase in the amplitude of iberiotoxin (IBTX)-sensitive, whole-cell K(+) current. Current potentiation occurred 1-8 min after bead-antibody application to the cell surface. Similarly, the endogenous alpha5beta1 integrin ligand fibronectin (FN) potentiated IBTX-sensitive K(+) current by 26%. Current potentiation was blocked by the c-Src inhibitor PP2 but not by PP3 (0.1-1 mum). In cell-attached patches, number of open channels x open probability (NP(o)) of a 230-250 pS K(+) channel was significantly increased after FN application locally to the external surface of cell-attached patches through the recording pipette. In excised, inside-out patches, the same method of FN application led to large, significant increases in NP(o) and caused a leftward shift in the NP(o)-voltage relationship at constant [Ca(2+)]. PP2 (but not PP3) nearly abolished the effect of FN on channel activity, suggesting that signalling between the integrin and channel involved an increase in Ca(2+)sensitivity of the channel via a membrane-delimited pathway. The effects of alpha5beta1 integrin activation on both whole-cell and single-channel BK currents could be reproduced in HEK 293 cells expressing the BK channel alpha-subunit. This is the first demonstration at the single-channel level that integrin signalling can regulate an ion channel. Our results show that alpha5beta1 integrin activation potentiates BK channel activity in vascular smooth muscle through both Ca(2+)- and c-Src-dependent mechanisms. This mechanism is likely to play a role in the arteriolar dilatation and impaired vascular reactivity associated with ECM degradation.

Integrin‐Fibronectin (FN) Mediated Mechanotransduction of Microvascular Smooth Muscle Cells (VSMC) Involves Activation of RhoA and PKC‐α

Integrin‐mediated mechanotransduction plays an important role in the arteriolar myogenic response. Diverse signaling pathways are believed to be involved in this process. To investigate the activation of RhoA and PKC‐α during VSMC mechanotransduction, we utilized atomic force microscopy (AFM) to mechanically stimulate single VSMC cells. Confocal microscopy was used to visualize RhoA and PKC‐α in live VSMCs isolated from rat skeletal muscle arterioles, 70~100 μm Diameter. VSMCs were transfected with either a FRET sensor of RhoA activation or GFP‐tagged PKC‐α. The tips of the AFM probes were fused with a FN‐coated bead (5 μm Dia). The FN‐coated beads were placed in contact (20 minutes) with the VSMC membrane that leads to the formation of an integrin‐mediated cell adhesion complex. A controlled pulling force was applied through the bead adhesion site using the AFM while simultaneously recording the signal from the RhoA‐FRET sensor or the GFP‐PKC‐α. Following pulling, a significant increase in the FRET signal for the RhoA sensor was detected. Also, the pulling was accompanied by translocation of GFP‐PKC‐α to the cell membrane. These changes began to occur within seconds after force application and were observed throughout the cell. Collectively, these data suggested that RhoA and PKC were activated by pulling force applied through integrin‐FN focal adhesion. (Supported by NIH HL‐062863 to GAM and AHA 0765481Z to ZS)

Potentiation of BK Channels by α5β1 Integrin Activation in Arteriolar Smooth Muscle

RGD peptides produce sustained vasodilation of rat skeletal muscle arterioles. We hypothesized that RGD ligands work through α5β1 integrin to modulate the activity of large conductance, Ca2+‐activated K+ (BK) channels in arteriolar smooth muscle. K+ currents were recorded in single arteriolar myocytes using whole‐cell and single‐channel patch clamp methods. Activation of α5β1 integrin by an insoluble antibody resulted in ∼30–50% increase in the amplitude of IBTX‐sensitive, whole‐cell K+ current. The endogenous α5β1 integrin ligand, fibronectin (FN; 10 μg/ml, 120 kD fragment), potentiated IBTX‐sensitive K+ current by ∼80%, an effect that was blocked by the c‐Src inhibitor, PP2 (0.1–1 μM). In cell‐attached or excised, inside‐out patches, the N•Po of a ∼240 pS K+ channel was significantly increased after FN application to the external surface of the patch through the recording pipette. FN also caused a leftward shift in the N•Po‐voltage relationship at constant [Ca2+], which was blocked by PP2. The results suggest that α5β1 integrin activation potentiates BK channel activity in vascular smooth muscle via a membrane‐delimited pathway through both Ca2+‐ and c‐Src‐dependent mechanisms.

CADHERINS PLAY A ROLE IN ARTERIOLAR MYOGENIC RESPONSIVENESS

The myogenic response is characterized as a vascular phenomenon in which blood vessels constrict to an increase in intraluminal pressure and dilate in response to a decrease in intraluminal pressure. This mechanism is important for the regulation of blood flow, capillary pressure and arterial pressure. The essential molecular components and signaling pathways have not been identified. Thus, the purpose of this study was to investigate the role of cadherins, a family of calcium‐dependent cell‐cell transmembrane adhesion proteins. Since the myogenic response is intrinsic to vascular smooth muscle, N‐Cadherin, the prominent vascular smooth muscle cadherin, was selected for investigation. In the present study, isolated rat skeletal muscle arterioles were exposed to a series of step increments in intraluminal pressure (60 mmHg to 100 mmHg in increments of 20 mmHg). Diameter responses to changes in pressure were recorded in the presence and absence of a synthetic cadherin inhibitory peptide (HAV) or an inhibitory antibody both of which are known to bind to N‐cadherin. In the absence of peptide and antibody, myogenic vasoconstriction was observed as pressure was increased (60–80 and 80–100 mmHg) whereas following inhibition of N‐cadherin the myogenic responses of the arteriole were abolished. A control peptide (HGV) or a function blocking VE‐cadherin antibody had no inhibitory effect on the myogenic response. Therefore, these data suggests that cadherins are an important component of the mechanotransduction pathway supporting arteriolar myogenic responsiveness.

High Glucose Concentrations via Activating Rho‐kinase Leads to Augmented and Sustained Angiotensin II‐induced Arteriolar Constrictions

We tested the hypothesis that functional availability of type 1 angiotensin II (Ang II) receptors (AT1) is enhanced and more sustained in arterioles exposed to high glucose concentration. In isolated, pressurized rat skeletal muscle arterioles (~ 150 μm) Ang II‐induced (10−8 M) constrictions were measured upon repeated applications in normal glucose (NG, 5.5 mM) or high glucose conditions (HG, 25 mM for 1 hour). Ang II‐induced constrictions to first applications were significantly augmented in HG arterioles (max: 74±6%), compared to NG vessels (max: 56±7%). Upon second applications Ang II‐induced constrictions decreased in NG arterioles (30±6%), but remained substantial in HG vessels (65±9%). In similar protocols constrictions to norepinephrine (10−7 M) were unaffected by HG exposure. In arterioles, HG caused Rho‐kinase activation, as detected by the increased membrane‐bound RhoA with Western blots. In the presence of Rho‐kinase inhibitor, Y27632 (1 μM) constrictions to first application of Ang II were not significantly different in NG and HG vessels. Moreover, Y27632 reduced constrictions to second applications of Ang II in HG arterioles to the level of NG vessels. These findings indicate that high glucose via activation of Rho‐kinase leads to augmented and maintained Ang II‐induced arteriolar constrictions, which is likely due to a sustained availability of AT1 receptors. (Grants: Hungarian Sci. Res. Funds/OTKA T48376; Am Heart Assoc., NE Aff. T‐0555897, T‐0735540, NIH HL43023).

Endothelin and Aged Blood Vessels

Age is one of the most important risk factors for cardiovascular disease. For example, a 30-year-old male smoker with untreated hypertension, diabetes, dyslipidemia, and a strong family history of premature coronary heart disease (CHD) would have a 10-year coronary heart disease event rate of only 16% (“intermediate risk”) by Framingham criteria; however, given exactly the same risk profile for a 60-year-old, the risk climbs to >50%. Aging, particularly sedentary aging, leads not only to the development of atherosclerosis but also to ventricular and arterial stiffening.1 This reduces functional capacity and contributes to chronic diseases of the elderly, such as systolic hypertension or heart failure with a preserved ejection fraction. In the search for the underlying mechanism for this effect of sedentary aging, one common feature that has been identified universally in older men is an increased baseline vascular tone,2 which has both hemodynamic and metabolic consequences.3 Although this age-related increase in vascular tone is mediated in part by a chronically elevated sympathetic α-adrenergic vasoconstriction,2 endothelial function also plays a critical role in vascular stability and the regulation of vasomotor function. The endothelium regulates vascular tone through the release of dilator and constrictor substances. Endothelin (ET)-1 is the most potent endothelium-derived constricting factor and influences peripheral vascular tone by interacting with ETA and ETB receptors on smooth muscle cells and the endothelium.4 Several animal studies have examined the profound effects of vascular aging on the ET-1 pathway and support a central role for ET-1 to explain the age-related increase in vascular tone.5–7 In the present issue of Hypertension , Van Guilder et al8 present new and important information regarding the role of aging and physical fitness on the vasomotor effects of ET-1. Using a cross-sectional design in a small but carefully studied …

Coupling a change in intraluminal pressure to vascular smooth muscle depolarization: still stretching for an explanation

pressure-induced (myogenic) constriction of arterioles is believed to be a fundamental contributor to the local regulation of microvascular blood flow and pressure. Myogenic constriction provides a level of tone upon which vasodilators can act to lower resistance and match blood flow to metabolic

Exercise training enhances flow-induced vasodilation in skeletal muscle resistance arteries of aged rats: role of PGI2 and nitric oxide

Flow-induced vasodilation is attenuated with old age in rat skeletal muscle arterioles. The purpose of this study was to determine whether diminished cyclooxygenase (COX) signaling contributes to the age-induced attenuation of flow-induced vasodilation in gastrocnemius muscle arterioles and to determine whether, and through which mechanism(s), exercise training restores this deficit in old rats. Fischer 344 rats (3 and 22 mo old) were assigned to a sedentary or exercise-trained group. First-order arterioles were isolated from the gastrocnemius muscles, cannulated, and pressurized to 70 cm H(2)O. Diameter changes were determined in response to graded increases in intraluminal flow in the presence and absence of nitric oxide synthase (NOS) inhibition [10(-5) M N(G)-nitro-L-arginine methyl ester (L-NAME)], COX inhibition (10(-5) M indomethacin), or combination NOS (10(-5) M L-NAME) plus COX (10(-5) M indomethacin) inhibition. Aging reduced flow-induced vasodilation in gastrocnemius muscle arterioles. Exercise training restored responsiveness to flow in arterioles of aged rats and enhanced flow-induced vasodilation in arterioles from young rats. L-NAME inhibition of flow-induced vasodilation was greater in arterioles from old rats compared with those from young rats and was increased after exercise training in arterioles from both young and old rats. Although the indomethacin-sensitive portion of flow-induced dilation was not altered by age or training, both COX-1 mRNA expression and PGI(2) production increased with training in arterioles from old rats. These data demonstrate that exercise training restores flow-induced vasodilation in gastrocnemius muscle arterioles from old rats and enhances flow-induced vasodilation in gastrocnemius muscle arterioles from young rats. In arterioles from both old and young rats, the exercise training-induced enhancement of flow-induced dilation occurs primarily through a NOS mechanism.