Effects of age, gender, and blood pressure on myogenic responses of mesenteric arteries from C57BL/6 mice

Sphingosine-1-phosphate-dependent activation of p38 MAPK maintains elevated peripheral resistance in heart failure through increased myogenic vasoconstriction

RATIONALE

Mechanisms underlying vasomotor abnormalities and increased peripheral resistance exacerbating heart failure (HF) are poorly understood.

OBJECTIVE

To explore the role and molecular basis of myogenic responses in HF.

METHODS AND RESULTS

10 weeks old C57Bl6 mice underwent experimental myocardial infarction (MI) or sham surgery. At 1 to 12 weeks postoperative, mice underwent hemodynamic studies, mesenteric, cerebral, and cremaster artery perfusion myography and Western blot. Organ weights and hemodynamics confirmed HF and increased peripheral resistance after MI. Myogenic responses, ie, pressure-induced vasoconstriction, were increased as early as 1 week after MI and remained elevated. Vasoconstrictor responses to phenylephrine were decreased 1 week after MI, but not at 2 to 6 weeks after MI, whereas those to endothelin (ET)-1 and sphingosine-1-phosphate (S1P) were increased at all time points after MI. An antagonist (JTE-013) for the most abundant S1P receptor detected in mesenteric arteries (S1P(2)R) abolished the enhanced myogenic responses of HF, with significantly less effect on controls. Mice with genetic absence of sphingosine-kinases or S1P(2)R (Sphk1(-/-); Sphk1(-/-)/Sphk2(+/-); S1P(2)R(-/-)) did not manifest enhanced myogenic responses after MI. Mesenteric arteries from HF mice exhibited increased phosphorylation of myosin light chain, with deactivation of its phosphatase (MLCP). Among known S1P-responsive regulators of MLCP, GTP-Rho levels were unexpectedly reduced in HF, whereas levels of activated p38 MAPK and ERK1/2 (extracellular signal-regulated kinase 1/2) were increased. Inhibiting p38 MAPK abolished the myogenic responses of animals with HF, with little effect on controls.

CONCLUSIONS

Rho-independent p38 MAPK-mediated deactivation of MLCP underlies S1P/S1P(2)R-regulated increases in myogenic vasoconstriction observed in HF. Therapeutic targeting of these findings in HF models deserves study.

Cardiovascular sex differences influencing microvascular exchange

The vital role of the cardiovascular (CV) system is maintenance of body functions via the matching of exchange to tissue metabolic demand. Sex-specific differences in the regulatory mechanisms of CV function and the metabolic requirements of men and women, respectively, have been identified and appreciated. This review focuses on sex differences of parameters influencing exchange at the point of union between blood and tissue, the microvasculature. Microvascular architecture, blood pressure (hydrostatic and oncotic), and vascular permeability, therefore, are discussed in the specific context of sex in health and disorders. It is notable that when sex differences exist, they are generally subtle but significant. In the aggregate, though, they can give rise to profoundly different phenotypes. The postulated mechanisms responsible for sex differences are attributed to genomics, epigenetics, and sex hormones. Depending on specific circumstances, the effect of the combined factors can range from insignificant to lethal. Identifying and understanding key signalling mechanisms bridging genomics/sex hormones and microvascular exchange properties within the scope of this review holds significant promise for sex-specific prevention and treatment of vascular barrier dysfunction.

Oxygen sensing and conducted vasomotor responses in mouse cremaster arterioles in situ

This study examines mechanisms by which changes in tissue oxygen tension elicit vasomotor responses and whether localized changes in oxygen tension initiates conducted vasomotor responses in mouse cremaster arterioles. Intravital microscopy was used to visualize the mouse cremaster microcirculation. The cremaster was superfused with Krebs' solution with different oxygen tensions, and a gas exchange chamber was used to induce localized changes in oxygen tension. In arterioles where red blood cells were removed by buffer perfusion, arterioles responded with same magnitudes of vasodilatation (DeltaD = 16.0 +/- 4.9 microm) when changing from high (PO(2) = 242.5 +/- 13.3 mm Hg) to low (PO(2) = 22.5 +/- 4.8 mm Hg) oxygen tension as seen in the intact cremaster circulation (DeltaD = 18.7 +/- 1.0 microm). Blockade of NO synthases by L: -NAME and adenosine receptors by DPCPX had no effects on vasomotor responses to low or high oxygen. Induction of localized low (PO(2) = 23.3 +/- 5.7 mmHg) or high (PO(2) = 300.0 +/- 25.7 mm Hg) oxygen tension caused vasodilatation or -constriction locally and at a site 1,000 microm upstream (distantly). Glibenclamide blocker of ATP-sensitive K(+) channels inhibited vasodilatation and -constriction to low (PO(2) = 16.0 +/- 6.4 mm Hg) and high (PO(2) = 337.4 +/- 12.8 mm Hg) oxygen tension. 1) ATP-sensitive K(+) channels seem to mediate, at least in part, vasodilatation and vasoconstriction to low and high oxygen tension; 2) Red blood cells are not necessary for inducing vasodilatation and vasoconstriction to low or high oxygen tension; 3) localized changes in the oxygen tension cause vasomotor responses, which are conducted upstream along arterioles in mouse cremaster microcirculation.

Aging and muscle fiber type alter K+ channel contributions to the myogenic response in skeletal muscle arterioles

Aging diminishes myogenic tone in arterioles from skeletal muscle. Recent evidence indicates that both large-conductance Ca2+-activated (BKCa) and voltage-dependent (KV) K+ channels mediate negative feedback control of the myogenic response. Thus we tested the hypothesis that aging increases the contributions of KV and BKCa channels to myogenic regulation of vascular tone. Because myogenic responsiveness differs between oxidative and glycolytic muscles, we predicted that KV and BKCa channel contributions to myogenic responsiveness vary with fiber type. Myogenic responses of first-order arterioles from the gastrocnemius and soleus muscles of 4- and 24-mo-old Fischer 344 rats were evaluated in the presence and absence of 4-aminopyridine (5 mM) or iberiotoxin (30 nM), inhibitors of KV and BKCa, respectively. 4-Aminopyridine enhanced myogenic tone with aging and normalized age-related differences in both muscle types. By contrast, iberiotoxin eliminated age-related differences in soleus arterioles and had no effect in gastrocnemius vessels. KV1.5 is an integral component of KV channels in vascular smooth muscle; therefore, we determined the relative protein expression of KV1.5, as well as BKCa, in soleus and gastrocnemius arterioles. Immunoblot analysis revealed no differences in KV1.5 protein with aging or between variant fiber types, whereas BKCa protein levels declined with age in arterioles from both muscle groups. Collectively, these results suggest that the contribution of BKCa to myogenic regulation of vascular tone changes with age in soleus muscle arterioles, whereas increased KV channel expression and negative feedback regulation of myogenic tone increases with advancing age in arterioles from both oxidative and glycolytic muscles.

Impaired vascular function in mice with an active cytomegalovirus infection

Human cytomegalovirus (CMV) is implicated in vascular complications through endothelial dysfunction. However, the effect of in vivo infections on vascular function in isolated arteries has not been examined. In pregnancy, systemic and uterine vascular adaptations accommodate increased blood volume through several mechanisms, including decreased sensitivity to vasoconstrictors and increased production of endothelial-dependent vasodilators. We hypothesized that an active in vivo CMV infection would reduce vasodilation of isolated arteries to the endothelial-dependent vasodilator methacholine and increase vasoconstriction to the α1-adrenergic receptor agonist phenylephrine and that these CMV-induced changes would be accentuated in late pregnancy. A mouse CMV infection model was used to study vascular responses in isolated mesenteric and uterine arteries from nonpregnant and late pregnant mice. In the mouse, CMV is not transmitted to the fetus. Accordingly, there was no evidence of active infection in any fetus examined, even though an active infection was found in salivary glands, uterine and mesenteric arteries, and placentas. Contrary to our hypothesis, increased endothelial-dependent vasodilation was found in mesenteric arteries from infected compared with uninfected nonpregnant and pregnant mice These data implicate active CMV infections in hypotensive disorders. Similarly, increased vasodilation was found in uterine arteries from infected vs. uninfected nonpregnant mice. However, this was completely reversed in infected compared with uninfected late pregnant mice in which vasodilation in uterine arteries was significantly reduced. Uterine arteries from infected pregnant mice also showed increased vasoconstriction to phenylephrine. Maternal infection led to decreased placental weights but had no effect on fetal weights in late pregnancy. These novel data demonstrate abnormal systemic and uterine vascular responses during an active CMV infection in both nonpregnant and late pregnant mice. Importantly, despite reduced placental weights, fetal weights were maintained, suggesting effective intrauterine compensation in the mouse model.

Sex differences in limb vasoconstriction responses to increases in transmural pressures

Women compared with men are more likely to have orthostatic intolerance. The purpose of this study was to examine whether sex affects limb vasoconstrictor response to increases in transmural pressure. Brachial and femoral mean blood velocity (MBV) and diameter (Doppler Ultrasound) were measured in 10 women and 10 men as transmural pressure was altered by applying local suction (-25, -50, -75, and -100 mmHg) via pressurized-limb tanks for 1 min to a single arm and leg. With the abrupt application of forearm suction (-75 and -100 mmHg), women compared with men had a greater initial rise in MBV (peak), followed by a quicker dynamic rate of velocity reduction. In the leg, women had a tendency for higher peak MBV but had similar dynamic velocity reductions compared with men. After 60 s of suction, women compared with men had attenuated reductions in brachial flow and conductance (-8.05 +/- 1.71 vs. -16.25 +/- 1.71 ml/min; -0.12 +/- 0.03 vs. -0.20 +/- 0.03 ml x min(-1) x mmHg(-1); main effect, P < 0.05), as well as attenuated femoral flow and conductance to sustained leg negative pressure at -100 mmHg (P < 0.05). When the data were expressed as percent change, women compared with men continued to have attenuated brachial flow responses (-24 +/- 2 vs. -36 +/- 2%, main effect, P < 0.05), with a trend toward attenuation at the highest leg pressure (-25 +/- 11 vs. -46 +/- 4%; P = 0.08). These sex differences remained after normalizing the flow responses by limb volume (percent change). Our findings suggest that young women compared with men have attenuated brachial and femoral vasoconstrictor responses to increases in transmural pressure, which may have implications for the greater incidence of orthostatic intolerance in women.

Enhanced capacitative calcium entry and sarcoplasmic-reticulum calcium storage capacity with advanced age in murine mesenteric arterial smooth muscle cells

Intracellular Ca(2+) signaling is important to perfusion pressure related arterial reactivity and to vascular disorders including hypertension, angina and ischemic stroke. We have recently shown that advancing-age leads to calcium signaling adaptations in mesenteric arterial myocytes from C57 BL/6 mice [Corsso, C.D., Ostrovskaya, O., McAllister, C.E., Murray, K., Hatton, W.J., Gurney, A.M., Spencer, N.J., Wilson, S.M., 2006. Effects of aging on Ca(2+) signaling in murine mesenteric arterial smooth muscle cells. Mech. Ageing Dev. 127, 315-323)] which may contribute to decrements in perfusion pressure related arterial contractility others have shown occur. Even still, the mechanisms underlying the changes in Ca(2+) signaling and arterial reactivity are unresolved. Ca(2+) transport and storage capabilities are thought to contribute to age-related Ca(2+) signaling dysfunctions in other cell types. The present studies were therefore designed to test the hypothesis that cytosolic and compartmental Ca(2+) homeostasis in mesenteric arterial myocytes changes with advanced age. The hypothesis was tested by performing digitalized fluorescence microscopy on mesenteric arterial myocytes isolated from 5- to 6-month and 29- to 30-month-old C57Bl/6 mice. The data provide evidence that with advanced age capacitative Ca(2+) entry and sarcoplasmic reticulum Ca(2+) storage are increased although sarcoplasmic reticulum Ca(2+) uptake and plasma membrane Ca(2+) extrusion are unaltered. Overall, the studies begin to resolve the mechanisms associated with age-related alterations in mesenteric arterial smooth muscle Ca(2+) signaling and their physiological consequences.

Cardioprotective and vasodilatory actions of glucagon-like peptide 1 receptor are mediated through both glucagon-like peptide 1 receptor-dependent and -independent pathways

BACKGROUND

The glucagon-like peptide 1 receptor (GLP-1R) is believed to mediate glucoregulatory and cardiovascular effects of the incretin hormone GLP-1(7-36) (GLP-1), which is rapidly degraded by dipeptidyl peptidase-4 (DPP-4) to GLP-1(9-36), a truncated metabolite generally thought to be inactive. Novel drugs for the treatment of diabetes include analogues of GLP-1 and inhibitors of DPP-4; however, the cardiovascular effects of distinct GLP-1 peptides have received limited attention.

METHODS AND RESULTS

Here, we show that endothelium and cardiac and vascular myocytes express a functional GLP-1R as GLP-1 administration increased glucose uptake, cAMP and cGMP release, left ventricular developed pressure, and coronary flow in isolated mouse hearts. GLP-1 also increased functional recovery and cardiomyocyte viability after ischemia-reperfusion injury of isolated hearts and dilated preconstricted arteries from wild-type mice. Unexpectedly, many of these actions of GLP-1 were preserved in Glp1r(-/-) mice. Furthermore, GLP-1(9-36) administration during reperfusion reduced ischemic damage after ischemia-reperfusion and increased cGMP release, vasodilatation, and coronary flow in wild-type and Glp1r(-/-) mice, with modest effects on glucose uptake. Studies using a DPP-4-resistant GLP-1R agonist and inhibitors of DPP-4 and nitric oxide synthase showed that the effects of GLP-1(7-36) were partly mediated by GLP-1(9-36) through a nitric oxide synthase-requiring mechanism that is independent of the known GLP-1R.

CONCLUSIONS

These data describe cardioprotective actions of GLP-1(7-36) mediated through the known GLP-1R and novel cardiac and vascular actions of GLP-1(7-36) and its metabolite GLP-1(9-36) independent of the known GLP-1R. Our data suggest that the extent to which GLP-1 is metabolized to GLP-1(9-36) may have functional implications in the cardiovascular system.

Wavelet-based spectrum analysis of sacral skin blood flow response to alternating pressure

OBJECTIVES

To provide insight into the physiologic mechanisms associated with alternating pressure, using wavelet analysis of skin blood flow (SBF) oscillations, and to determine whether the application of alternating pressure induces myogenic responses, thereby enhancing SBF as compared with constant loading.

DESIGN

Repeated-measures design.

SETTING

University research laboratory.

PARTICIPANTS

Healthy, young adults (N=10; 5 men, 5 women; mean age +/- standard deviation, 30.0+/-3.1 y).

INTERVENTION

Alternating pressure for 20 minutes (four 5-min cycles with either 60 mmHg or 3 mmHg) and constant loading for 20 minutes at 30 mmHg on the skin over the sacrum.

MAIN OUTCOME MEASURES

A laser Doppler flowmeter was used to measure sacral SBF response to both alternating pressure and constant loading. Wavelet-based spectrum analysis of SBF oscillations was used to assess underlying physiologic mechanisms including endothelium-related metabolic (.008-.02 Hz), neurogenic (.02-.05 Hz), and myogenic (.05-.15 Hz) controls.

RESULTS

Alternating pressure stimulated an increase in sacral SBF of compressed soft tissues as compared with constant loading (P<.01). SBF during the high-pressure phase of 4 alternating pressure cycles showed an increasing trend. An increase in power in metabolic frequency range and a decrease in power in the myogenic frequency range during alternating pressure were observed compared with SBF prior to loading. Power increased in the myogenic frequency range during the low-pressure phase of alternating pressure and decreased during the high-pressure phase.

CONCLUSIONS

SBF control mechanisms, as assessed by the characteristic frequencies embedded in SBF oscillations, show different responses to 2 loading pressures with the same average pressure but different patterns. Our study suggests that optimization of operating parameters and configurations of alternating pressure support surfaces to compensate for impaired SBF control mechanisms in pathologic populations may be possible using wavelet analysis of blood flow oscillations.

Functional characterization and sex differences in small mesenteric arteries of the estrogen receptor-β knockout mouse

The role of the estrogen receptor (ER) subtypes in the modulation of vascular function is poorly understood. The aim of this study was to characterize ex vivo the functional properties of small arteries and their response to estrogens in the mesenteric circulation of female and male ER-β knockout mice (β-ERKO) and their wild-type (WT) littermates. Responses to changes in intraluminal flow and pressure were obtained before and after incubation with 17β-estradiol or ER-α agonist propyl-pyrazole-triol (3 h; 10 nM). Cumulative concentration-response curves to acetylcholine, norepinephrine, and passive distensibility were compared with respect to sex and genotype. The collagen and elastin content within the vascular wall and ER expression were also determined. Endothelial morphology was visualized by scanning electron microscopy. 17β-Estradiol and propyl-pyrazole-triol-treated arteries from female β-ERKO and WT mice showed enhanced flow-mediated dilation, but this was not evident in males. Distensibility was decreased in arteries from β-ERKO females. Sex differences in myogenic tone were observed in 17β-estradiol-treated arteries, but were similar between β-ERKO and WT mice. Acetylcholine- and norepinephrine-induced responses were similar between groups and sexes. ER-α was similarly expressed in the endothelium and media of arteries from all groups studied, as well as ER-β in WT animals. Endothelial morphology was similar in arteries from animals of both sexes and genotype; however, arterial elastin content was decreased, and collagen content was increased in β-ERKO male compared with WT male and with β-ERKO female. We suggest that ERs play a sex-specific role in estrogen-mediated flow responses and distensibility, and that deletion of ER-β affects artery structure but only in male animals. Further studies in β-ERKO mice with established hypertension and in α-ERKO mice are warranted.

Myogenic tone and reactivity of cerebral arteries in type II diabetic BBZDR/Wor rat

BBZDR/Wor rat is a new model of type II diabetes with spontaneous obesity and clinical characteristics close to human diabetes. In this study the time-course of cerebroarterial dysfunction was characterized. Posterior cerebral arteries from BBZDR/Wor rats and their age-matched lean controls were pressurized to 70 mm Hg in an arteriograph. Effects of intraluminal pressure and different pharmacological agents on myogenic tone were evaluated. Pressure-myogenic tone curves in diabetic arteries were similar to that in non-diabetic arteries at pre-diabetic age, showed leftward shift at 4 weeks and were significantly different with higher myogenic tone at 5 and 8 months of diabetes. Age-dependent decrease in myogenic tone was observed in non-diabetic arteries. Dilation to histamine was similar to that in non-diabetic arteries at pre-diabetic and at 4 weeks but significantly reduced at 5 and 8 months of diabetes. Bradykinin-mediated dilation was significantly reduced in early and chronic diabetes, whereas (+/-)-S-nitroso-N-acetylpenicillamine (SNAP)-mediated dilation was decreased modestly at 8 months of diabetes. Sensitivity and constriction to 5-hydroxytryptamine were increased in early and chronic diabetes. Responses to bradykinin and 5-hydroxytryptamine were decreased and increased, respectively. Myogenic tone was significantly less sensitive to (lower pIC(50)) U-73122 than normal arteries at 4 weeks and 8 months of diabetes suggesting an increased activation of phospholipase C (PLC). This study shows that pressure-mediated autoregulation of cerebral arteries in type II diabetes operates at higher resistance. Endothelium-dependent dilation was decreased with chronic diabetes with increased sensitivity to constrictor agonist. Endothelium-independent dilation was modestly affected. Arterial hyper-reactivity to pressure and constrictor agonist were likely due to increased PLC activation.

Paradoxical role of angiotensin II type 2 receptors in resistance arteries of old rats

The role of angiotensin II type 2 receptors (AT2Rs) remains a matter of controversy. Its vasodilatory and antitrophic properties are well accepted. Nevertheless, in hypertensive rats, AT2R stimulation induces a vasoconstriction counteracting flow-mediated dilation (FMD). This contraction is reversed by hydralazine. Because FMD is also decreased in aging, another risk factor for cardiovascular diseases, we hypothesized that AT2R function might be altered in old-rat resistance arteries. Mesenteric resistance arteries (250 mum in diameter) were isolated from old (24 months) and control (4 months) rats receiving hydralazine (16 mg/kg per day; 2 weeks) or water. FMD, NO-mediated dilation, and endothelial NO synthase expression were lower in old versus control rats. AT2R blockade improved FMD in old rats, suggesting that AT2R stimulation produced vasoconstriction. AT2R expression was higher in old rats and mainly located in the smooth muscle layer. In old rats, AT2R stimulation induced endothelium-independent contraction, which was suppressed by the antioxidant Tempol. Reactive oxygen species level was higher in old-rat arteries than in controls. Hydralazine improved FMD and NO-dependent dilation in old rats without change in AT2R expression and location. In old rats treated with hydralazine, reactive oxygen species level was reduced in endothelial and smooth muscle cells, and AT2R-dependent contraction was abolished. Thus, AT2R stimulation induced vasoconstriction through activation of reactive oxygen species production, contributing to decrease FMD in old-rat resistance arteries. Hydralazine suppressed AT2R-dependent reactive oxygen species production and AT2R-dependent contraction, improving FMD. Importantly, endothelial alterations in aging were reversible. These findings are important to consider in the choice of vasoactive drugs in aging.

Heterozygous CuZn superoxide dismutase deficiency produces a vascular phenotype with aging

The goal of this study was to test the hypothesis that loss of a single copy of the gene for CuZn superoxide dismutase (CuZnSOD) increases vascular superoxide levels and produces vascular dysfunction with aging. Responses of carotid arteries from young (7 months) and old (22 to 24 months of age) heterozygous CuZnSOD-deficient (CuZnSOD(+/-)) mice and their wild-type (CuZnSOD(+/+)) littermates were examined in vitro. Total superoxide dismutase activity in aorta was reduced by approximately 30% (P<0.05) in CuZnSOD(+/-) mice compared with wild-type mice. Responses to acetylcholine (an endothelium-dependent agonist) produced relaxation that was similar (P>0.05) in carotid arteries from young wild-type, young CuZnSOD(+/-), and old wild-type mice. In contrast, relaxation to acetylcholine was markedly impaired in old CuZnSOD(+/-) mice (eg, 100 micromol/L acetylcholine produced 51+/-5% and 96+/-5% relaxation in vessels from old CuZnSOD(+/-) and old wild-type mice, respectively). This effect was selective, because relaxation to nitroprusside (an endothelium-independent agonist) was not affected by either CuZnSOD genotype or aging. The impaired response to acetylcholine in old CuZnSOD(+/-) mice was restored toward normal with either tempol (a scavenger of superoxide; 1 mmol/L) or PJ34 (an inhibitor of poly-ADP-ribose polymerase; 3 micromol/L). Vascular superoxide levels were increased in aorta in old CuZnSOD(+/+) mice and increased further in CuZnSOD(+/-) mice with aging. These findings provide the first direct evidence that normal CuZnSOD expression protects endothelial function and that deficiency in a single copy of the gene that encodes CuZnSOD produces increases in superoxide and marked impairment of endothelial function with aging.

P2X1 receptors mediate sympathetic postjunctional Ca2+ transients in mesenteric small arteries

Brief, spatially localized Ca(2+) transients occur in the smooth muscle adjacent to perivascular nerves of small arteries during neurogenic contractions. We named these "junctional Ca(2+) transients" (jCaTs) and postulated that they arose from Ca(2+) entering smooth muscle cells through P2X(1) receptors activated by neurally released ATP. Nevertheless, the lack of potent, subtype-selective P2X-receptor antagonists made determining the exact molecular identity of the channels difficult. Here we used small, pressurized mesenteric arteries from P2X(1)-receptor-deficient mice (KO) to test the hypothesis that jCaTs arise from Ca(2+) entering the smooth muscle cell via P2X(1) receptors. In wild-type (WT) arteries, confocal microscopy of fluo-4 fluorescence during electrical field stimulation (EFS) of perivascular sympathetic nerves revealed jCaTs in the smooth muscle cells adjacent to the perivascular nerves, similar to those reported previously in rat arteries, and alpha-latrotoxin (2.5 nM) markedly increased the frequency of "spontaneous" jCaTs. In the KO arteries, however, neither EFS nor alpha-latrotoxin elicited any jCaTs. A potent P2X-receptor agonist, alpha,beta-methylene ATP (10.0 microM), elicited strong contractions and increased intracellular Ca(2+) concentration in WT arteries but elicited neither in KO arteries. A biphasic vasoconstriction in response to EFS was observed in WT arteries. In KO arteries, however, the initial rapid, transient component of the biphasic vasoconstriction was absent. The data support the hypothesis that jCaTs represent Ca(2+) that enters the smooth muscle cells through P2X(1) receptors activated by neurally released ATP and that this Ca(2+) is involved in the initial rapid component of the sympathetic neurogenic contraction.

Deletion of Endothelial Cell Endothelin B Receptors Does Not Affect Blood Pressure or Sensitivity to Salt

Endothelin B receptors in different tissues regulate diverse physiological responses including vasoconstriction, vasodilatation, clearance of endothelin-1, and renal tubular sodium reabsorption. To examine the role of endothelial cell endothelin B receptors in these processes, we generated endothelial cell-specific endothelin B receptor knockout mice using a Cre- loxP approach. We have demonstrated loss of endothelial cell endothelin B receptor expression and function and preservation of nonendothelial endothelin B receptor-mediated responses through binding and functional assays. Ablation of endothelin B receptors exclusively from endothelial cells produces endothelial dysfunction in the absence of hypertension, with evidence of decreased endogenous release of NO and increased plasma endothelin-1. In contrast to models of total endothelin B receptor ablation, the blood pressure response to a high-salt diet is unchanged in endothelial cell–specific endothelin B receptor knockouts compared with control floxed mice. These findings suggest that the endothelial cell endothelin B receptor mediates a tonic vasodilator effect and that nonendothelial cell endothelin B receptors are important for the regulation of blood pressure.

Effects of aging on Ca2+ signaling in murine mesenteric arterial smooth muscle cells

Pathophysiological changes in arterial smooth muscle structure and function occur with aging and there are a number of reports illustrating reductions in vascular responsiveness with aging. While much is known about arterial remodeling and functional adaptations with aging, very little is known about the biophysical adaptations in individual arterial myocytes. Cytosolic Ca2+ signaling, involving activation of L-type Ca2+ channels on the plasma membrane as well as InsP3 and ryanodine receptors on the sarcoplasmic reticulum, is integral to vascular tone and reactivity. Thus, we tested the hypothesis that aging results in reductions in the functional expression of L-type channels and temporal aspects of ryanodine receptor and InsP3 receptor Ca2+ signaling, in mesenteric arterial smooth muscle cells isolated from 6 and 30 months old C57Bl/6 mice. Comparisons of L-type current activity were made using dialyzed, whole-cell voltage-clamp techniques and Ba2+ as charge carrier. Ca2+ signaling was measured using fura-2 fluorescence microscopy techniques. Cell morphological changes were also investigated using electrophysiological and immunocytochemical approaches. The amplitudes of L-type Ca2+ currents were increased in older mice, but this was associated with membrane surface area increases of approximately 50%, due to increases in cell length not cell width. Consequently, L-type Ca2+ current densities were preserved with age, indicating functional channel expression was unchanged. In contrast, aging was associated with decrements in Ca2+ signaling in response to either ryanodine receptor stimulation by caffeine or InsP3 receptor activation with phenylephrine. These changes with aging may be related to the previously reported depression in myogenic reactivity.

Increased myogenic tone in 7-month-old adult male but not female offspring from rat dams exposed to hypoxia during pregnancy

Intrauterine growth restriction (IUGR) increases the risk of cardiovascular disease later in life. Vascular dysfunction occurs in adult offspring from animal models of IUGR including maternal undernutrition, but the influence of reduced fetal oxygen supply on adult vascular function is unclear. Myogenic responses, essential for vascular tone regulation, have not been evaluated in these offspring. We hypothesized that 7-mo-old offspring from hypoxic (12% O(2); H) or nutrient-restricted (40% of control; NR) rat dams would show greater myogenic responses than their 4-mo-old littermates or control (C) offspring through impaired modulation by vasodilators. Growth restriction occurred in male H (P < 0.01), male NR (P < 0.01), and female NR (P < 0.02), but not female H, offspring. Myogenic responses in mesenteric arteries from males but not females were increased at 7 mo in H (P < 0.01) and NR (P < 0.05) vs. C offspring. There was less modulation of myogenic responses after inhibition of nitric oxide synthase (P < 0.05), prostaglandin H synthase (P < 0.005), or both enzymes (P < 0.001) in arteries from 7-mo male H vs. C offspring. Thus reduced vasodilator modulation may explain elevated myogenic responses in 7-mo male H offspring. In contrast, there was increased modulation of myogenic responses in arteries from 7-mo female H vs. C or NR offspring after inhibition of both enzymes (P < 0.05). Thus increased vasodilator modulation may maintain myogenic responses in female H offspring at control levels. In summary, vascular responses in adult offspring from adverse intrauterine environments are impaired in a gender-specific, age-dependent, and maternal insult-dependent manner, with males more profoundly affected.

alphavbeta3- and alpha5beta1-integrin blockade inhibits myogenic constriction of skeletal muscle resistance arterioles

In isolated resistance arterioles with spontaneous tone, ligation of alpha4beta1- and alpha5beta1-integrins induces vasoconstriction whereas ligation of alphavbeta3-integrin induces vasodilation. However, whether integrins directly participate in myogenic constriction to pressure elevation is not known. To answer this question, isolated rat skeletal muscle arterioles were exposed to step increments in pressure in the absence or presence of peptides and function-blocking antibodies known to bind alpha4beta1-, alpha5beta1-, or alphavbeta3-integrins while vessel diameter was continually monitored. Myogenic constriction, as assessed by the ability of isolated arterioles to reduce their diameter in response to two consecutive increments in intraluminal pressure (90-110 and 110-130 cmH2O), was not affected by treatment with any of the control peptides (RAD, LEV), a control antibody (anti-rat major histocompatibility complex), an alpha4beta1-integrin-binding peptide (LDV), or an anti-alpha4-integrin antibody. In contrast, alpha5beta1-integrin blockade with either anti-alpha5- or anti-beta1-integrin antibody caused a significant inhibition of myogenic constriction. Also, both RGD peptide and anti-beta3-integrin antibody inhibited myogenic constriction. These results indicate that alpha5beta1- and alphavbeta3-integrins are necessary for myogenic constriction and further suggest that integrins are part of the mechanosensory apparatus responsible for the ability of vascular smooth muscle cells to detect and/or respond to changes in intraluminal pressure.

Effects of age on brachial artery myogenic responses in humans

The myogenic response, the inherent ability of blood vessels to rapidly respond to changes in transmural pressure, is involved in local blood flow autoregulation. Animal studies suggest that aging impairs the myogenic response. The purpose of this study was to compare the effects of changes in transmural pressure on mean blood velocity (MBV, cm/s) in young and older subjects. Twelve younger men and women (25 ± 1 yr) were gender and body composition matched to twelve older men and women (65 ± 1 yr). A specially designed tank raised or lowered forearm pressure by 50 mmHg within 0.2 s. Brachial artery MBV was measured directly above the site of forearm pressure change using Doppler methods. In response to increasing transmural pressure (i.e., release of +50 mmHg), older subjects compared with younger subjects had significantly lower peak MBV (Δ 12.43 ± 1.16 vs. Δ 17.97 ± 2.01 cm/s; P < 0.05), reduced rates in the dynamic fall of MBV after peak values were achieved (vasoconstriction) (−1.88 ± 0.17 vs. −2.90 ± 0.28 cm·s−1·s−1; P < 0.05), and lower MBV values with sustained suction. In response to decreasing transmural pressure (i.e., change to +50 mmHg), there was a significantly greater increase in MBV (Δ peak flow from trough 7.71 ± 1.32 vs. 4.38 ± 0.71 cm/s; P < 0.05) and a trend toward a greater rate of rise in MBV (vasodilation; 1.61 ± 0.29 vs. 0.96 ± 0.21 cm·s−1·s−1; P = 0.08) in the older subjects. Older subjects compared with the younger subjects exhibited decreased dynamic vasoconstriction, enhanced steady-state constriction, as well as evidence for enhanced dynamic vasodilation responses to sustained alterations in forearm transmural pressure.

Vascular reactivity to norepinephrine and acetylcholine after chronic intermittent hypoxia in mice

This study assessed the early vascular reactivity changes in mice after exposure to 14 days intermittent hypoxia (IH) with active or inactive sympathetic nervous system (SNS). Hindquarters of mice exposed to 14 days of IH, sham exposed mice or unhandled mice were perfused at constant flow with Krebs-Albumin (5%). Changes in perfusion pressure were assessed after injection of several doses of norepinephrine in anaesthetized mice (active SNS) or in euthanized mice (inactive SNS). Response to several doses of acetylcholine was recorded after precontraction of hindquarter vascular bed by methoxamine in euthanized mice. Vasoconstrictor response was increased after IH for high dose of NE (50 microg) in euthanized mice and for all doses of NE (2-10-50 microg) in anaesthetized mice, but no change in vasodilatation was observed. These findings suggest that 14 days of IH altered vascular reactivity of mice hindquarter in an early pattern. Vasoconstriction was enhanced, particularly with active SNS, while there was no dysfunction of endothelium-relaxation.

Plasma membrane calcium ATPase overexpression in arterial smooth muscle increases vasomotor responsiveness and blood pressure

In vascular smooth muscle cells (SMCs), several mechanisms act in concert to regulate the intracellular calcium concentration [Ca2+]i, which may in turn affect vascular tone. One such mechanism is the extrusion of Ca2+ by the plasma membrane calcium ATPase (PMCA). To address, in particular, the role of the neuronal nitric oxide synthase (nNOS)-associating isoform PMCA4b in regulating vascular tone, a doxycycline-responsive transgene for human PMCA4b was overexpressed in arterial SMCs of mice. Overexpression of hPMCA4b resulted in a 2-fold increase in total aortic PMCA4 protein expression and significant real-time RT-PCR-documented differences in the levels of endogenous mouse PMCA1, PMCA4, SERCA2, and IP3R1 gene expression in arterial SMCs. Whereas no significant difference in basal [Ca2+]i or Ca2+ sensitivity was observed in vascular SMCs or mesenteric arteries, respectively, from hPMCA4b-overexpressing versus control mice, hPMCA4b-overexpressing mice revealed a reduced set-point and increased extent of myogenic response and heightened sensitivity to vasoconstrictors. Treatment of arteries with an nNOS inhibitor resulted in a reduced set-point and increased extent of the myogenic response in control but not hPMCA4b-overexpressing mice. Moreover, aortic SMCs from hPMCA4b-overexpressing mice exhibited reduced levels of cGMP under both basal and phenylephrine-stimulated conditions. These changes were associated with significant doxycycline-reversible elevations in blood pressure. Taken together, these data show that overexpression of hPMCA4b in arterial SMCs increases vascular reactivity and blood pressure, an effect that may be mediated in part by negative regulation of nNOS.