Maturation is associated with changes in rat cerebral artery structure, biomechanical properties and tone.

AIM: This study evaluated the hypothesis that physiological maturation affects cerebral artery smooth muscle-endothelial interactions involved in pressure-induced tone and alters the dimensional and biomechanical properties of small posterior cerebral arteries (PCA). METHODS: Secondary branches of PCA from young (4-5 weeks old, n=11), adult (14-16 weeks old, n=11) and mature (44-47 weeks old, n=11) male Sprague-Dawley rats were isolated, cannulated, pressurized and subjected to a range of intraluminal pressures (10-110 mmHg) to determine tone with and without pharmacologic nitric oxide synthase (NOS) inhibition. Measurements of passive lumen diameter and wall thickness as a function of pressure were used to determine changes in structure, distensibility and wall stress; histological analysis was performed on vessel cross-sections to assess collagen and elastin contents. RESULTS: Although pressure-dependent tone decreased significantly during ageing, differences between groups were abolished by NOS inhibition. Vessel diameters increased in adult vs. young rats (at 90 mmHg, 233 ± 6.0 µm vs. 192 ± 4.5 µm; P<0.05), possibly secondary to somatic growth. Further ageing was associated with reductions in lumen diameter (207 ± 6.5 µm; P<0.05), increased wall and media thickness (and wall/lumen ratio) and cross-sectional area. Distensibility and wall collagen were unchanged, although elastin content was significantly reduced. CONCLUSIONS: Maturation is associated with differences in PCA dimensional properties that indicate a pattern of initial outward eutrophic, followed by inward hypertrophic remodelling. Functionally, the contribution of basal NO increases with age in a way that reduces pressure-dependent tone and diminishes vasodilator reserve.

Effects of etonogestrel treatment in the reproductive organs and uterine arteries of nonoophorectomized guinea pigs.

The endometria of women treated with long-term progestin-only contraceptives (LTPOCs) display abnormally enlarged, fragile blood vessels, decreased endometrial blood flow, oxidative stress, and unpredictable focal abnormal endometrial bleeding. Because human studies on the effects of LTPOC treatment are constrained for ethical and practical reasons, we assessed the suitability of nonoophorectomized guinea pigs (GPs) to best mimic the hormonal milieu of women. The present study demonstrates that treatment of GPs parallels the morphological changes following LTPOC treatment of the human endometrium and ovaries. Specifically, treatment resulted in larger hyperemic, uteri compared with controls. Histopathologic and immunohistochemical analysis demonstrated fewer endometrial glands, decreased luminal mucus, increased numbers of blood vessels, and focal hemorrhage. While increased staining for the cell mitosis marker, Ki67, was present in the zona functionalis, no such increase occurred in the basalis. Lastly, effects on vasomotor features of uterine arteries suggest changes that favor increased resistance and reduced blood flow promoting decreased ability to withstand elevations in transmural pressure.

Uterine venous permeability in the rat is altered in response to pregnancy, vascular endothelial growth factor, and venous constriction.

Venoarterial communication is a potential short-loop signaling pathway for the local control of uteroplacental perfusion. As this pathway is permeability-dependent, this study investigated the effects of molecular weight, gestation, vascular endothelial growth factor (VEGF), wall tension, and constriction on solute flux across the venous wall. Experiments utilized fluorimetry to quantitate solute flux (3- and 70-kDa dextran) in isolated segments of uterine vein from virgin (NP) and late-pregnant (LP; day 20) rats as a function of endothelial surface area and time. Uterine veins were > 10-fold more permeable to the 3- versus 70-kDa dextran in both NP and LP groups. Flux was increased during gestation ( 2.5-fold), and by VEGF (NP, 3 kDa: 3.3-fold, 70 kDa: 4.8-fold; LP, 3-kDa: 3.3-fold, 70 kDa: 7.4-fold). Permeability to the 3 kDa dextran correlated directly with wall tension (r2 = .74 in both groups), whereas permeability to both dextrans correlated inversely with constriction (NP: r2 = .85 and .76; LP: r2 = .89 and .79, respectively). Uterine veins demonstrate permeability to 3- and 70-kDa tracers resulting from molecular weight dependence and an apparent difference in transport mechanisms. Permeability is enhanced by gestational remodeling and subject to modulation by placental factors, indicating the presence of a regulated pathway for the transfer of molecules across the venous wall.

Loss of SM-B myosin affects muscle shortening velocity and maximal force development.

We used an exon-specific gene-targeting strategy to generate a mouse model deficient only in the SM-B myosin isoform. Here we show that deletion of exon-5B (specific for SM-B) in the gene for the heavy chain of smooth muscle myosin results in a complete loss of SM-B myosin and switching of splicing to the SM-A isoform, without affecting SM1 and SM2 myosin content. Loss of SM-B myosin does not affect survival or cause any overt smooth muscle pathology. Physiological analysis reveals that absence of SM-B myosin results in a significant decrease in maximal force generation and velocity of shortening in smooth muscle tissues. This is the first in vivo study to demonstrate a functional role for the SM-B myosin isoform. We conclude that the extra seven-residue insert in the surface loop 1 of SM-B myosin is a critical determinant of crossbridge cycling and velocity of shortening.

Differential expression and regulation of the vascular endothelial growth factor receptors neuropilin-1 and neuropilin-2 in rat uterus.

Vascular endothelial growth factor (VEGF) is a potent modulator of vascular remodeling and angiogenesis in the uterus. Recently, neuropilins (Npn), semaphorin receptors associated with neuronal guidance, were demonstrated to bind VEGF isoforms with high affinity, facilitating VEGF(165) binding to the tyrosine kinase receptor VEGFR2. The current studies examined rat uterus neuropilin expression and regulation. Npn-1 and Npn-2 transcripts and 135-kDa proteins were observed in uterine extracts. Both uterine vascular endothelial cells and glandular epithelium expressed Npn-1 immunoreactivity, whereas Npn-2 was restricted to the glandular epithelium. In hormone-replaced ovariectomized animals, progesterone increased uterine 6.5-kb Npn-1 messenger RNA (mRNA) expression approximately 2-fold compared with that in tissues from ovariectomized controls. 17ss-Estradiol alone had no effect, but blunted the progesterone response; by contrast, Npn-2 mRNA expression was decreased by estrogen. VEGFR2 mRNA was coregulated with Npn-1. Consistent with these results, Npn-1 mRNA expression was augmented nearly 7- and 4-fold at metestrus and diestrus, respectively, during periods of high progesterone; Npn-2 mRNA expression was not significantly altered during the estrous cycle. The regulated expression and differential localization of neuropilins in the rat uterus suggest that these receptors may participate in hormonally regulated changes occurring throughout the female reproductive cycle.

Estrogen augments the vasodilatory effects of vascular endothelial growth factor in the uterine circulation of the rat.

OBJECTIVE: Pregnancy augments uterine artery vasodilatation in response to vascular endothelial growth factor, although the underlying mechanism is not known. The aim of this study was to test the hypothesis that estrogen and progesterone, the primary sex steroids of pregnancy, are responsible for this effect through increased endothelial secretion of nitric oxide. STUDY DESIGN: Adult female Sprague-Dawley rats underwent oophorectomy at 9 weeks of age with concomitant placement of 21-day timed-release pellets containing either 17beta-estradiol (n = 6) or progesterone (n = 6), or a combination of these (n = 6). Control rats also underwent oophorectomy but did not receive hormone replacement (n = 6). Two to 3 weeks after oophorectomy the rats were killed and the main uterine artery was dissected free, cannulated in an arteriograph, and pressurized to 50 mm Hg. After constriction with phenylephrine, concentration-response curves to vascular endothelial growth factor (0.1-20 nmol/L) were performed to compare arterial sensitivity to and maximal effects of vascular endothelial growth factor among the 4 treatment groups. Vessels were then treated with N omega-nitro-L -arginine (0.24 mmol/L), an inhibitor of nitric oxide synthase, and the maximally effective concentration of vascular endothelial growth factor was reapplied to evaluate the relative contribution of nitric oxide to the overall effect. RESULTS: Comparisons of the effective concentration of vascular endothelial growth factor that elicited 50% of the maximal dilatation revealed the vessels of the estrogen group to be approximately 10 times more sensitive than the control group (0.4 +/- 0.11 nmol/L vs 4.2 +/- 1.13 nmol/L, respectively; P <. 05). Responses of vessels from the progesterone and combined groups were intermediate (progesterone, 2.3 +/- 0.66 nmol/L; combined, 1.1 +/- 0.28 nmol/L). Maximal vasodilatory responses were greatest in the groups with treatment including estrogen (estrogen, 61% +/- 3. 1%; combined, 54% +/- 3.4%; progesterone, 42% +/- 5.8%, control, 40% +/- 3.5%; P <.05). Addition of N omega-nitro-L -arginine inhibited maximal vascular endothelial growth factor-induced dilatation by approximately 40% irrespective of treatment group. CONCLUSION: The presence of estrogen rather than progesterone leads to an enhancement of vascular endothelial growth factor-induced arterial dilatation during pregnancy. This effect results from a proportional increase in endothelial nitric oxide secretion, along with that of another, as yet unidentified vasodilatory substance.

Increased Ca2+ sensitivity as a key mechanism of PKC-induced constriction in pressurized cerebral arteries.

The effects of activating protein kinase C (PKC) with indolactam V (Indo-V) and 1,2-dioctanoyl-sn-glycerol (DOG) on smooth muscle intracellular Ca2+ concentrations ([Ca2+]i) and arterial diameter were determined using ratiometric Ca2+ imaging and video edge detection of pressurized rat posterior cerebral arteries. Elevation of intraluminal pressure from 10 to 60 mmHg resulted in an increase in [Ca2+]i from 74 +/- 5 to 219 +/- 8 nM and myogenic constriction. Application of Indo-V (0.01-3 microM) or DOG (0.1-30 microM) induced constriction and decreased [Ca2+]i to 140 +/- 11 and 127 +/- 12 nM, respectively, at the highest concentrations used. In the presence of Indo-V, the dihydropyridine Ca2+-channel-blocker nisoldipine produced nearly maximum dilation and decreased [Ca2+]i to 97 +/- 7 nM. In alpha-toxin-permeabilized arteries, the constrictor effects of Indo-V and DOG were not observed in the absence of Ca2+. Both PKC activators significantly increased the degree of constriction of permeabilized arteries at different [Ca2+]i. We conclude that 1) Indo-V- or DOG-induced constriction of pressurized arteries requires Ca2+ influx through voltage-dependent Ca2+ channels, and 2) PKC-induced constriction of pressurized rat cerebral arteries is associated with a decrease in [Ca2+]i, suggesting an increase in the Ca2+ sensitivity of the contractile process.

Intolerance to volume expansion: a theorized mechanism for the development of preeclampsia.

We present a theorized mechanism for the development of preeclampsia, suggesting that one important underlying pathophysiologic mechanism is intolerance to volume expansion. The stage is set for this intolerance by chronic volume constriction, which leads to a requirement for increased basal peripheral vasoconstrictor tone to maintain blood pressure and allow for continued perfusion of the upright hominid head. In pregnancy, volume expansion signaled by the placenta cannot be accommodated by the constricted vascular system. The inability of the normally adaptive endothelial vasodilatory mechanisms to overcome the chronic vasoconstrictor tone leads to endothelial damage, exacerbation of vasoconstriction, and clinical hypertension. Disease resolution, characterized by diuresis, occurs with the elimination of the placenta-derived drive to retain volume. The reason preeclampsia does not recur uniformly with subsequent pregnancy is permanent restructuring of the maternal cardiovascular system with pregnancy that allows for greater plasma volume expansion in future gestations.

Vascular smooth muscle actin cytoskeleton in cerebral artery forced dilatation.

BACKGROUND AND PURPOSE: We investigated the role of actin polymerization in regulating arterial diameter in response to increasing pressure and modulating forced dilatation of cerebral arteries at pressures above the upper limit of autoregulation. METHODS: Posterior cerebral arteries (n = 12) were isolated and pressurized in a special arteriograph that allowed control of intravascular pressure and measurement of lumen diameter. Intact arteries in the absence (control) or presence of 3.0 mumol/L cytochalasin B (CB), an inhibitor of actin polymerization, were subjected to stepwise increases in pressure from 75 to 200 mm Hg. Lumen diameter was continuously recorded, as was the pressure at which forced dilatation (loss of tone) occurred. After a period of time at 200 mm Hg, pressure was returned to 75 mm Hg and the extent of tone recovery was evaluated. RESULTS: Arteries with and without CB developed a similar amount of tone during equilibration at 75 mm Hg: percent tone = 27 +/- 3% for control versus 29 +/- 4% for CB arteries (P > 0.05). However, arteries in the presence of CB could not withstand pressure as well and underwent FD at significantly lower pressures: 168 +/- 5 mm Hg for control versus 142 +/- 5 mm Hg for CB arteries (P < 0.01). The amount of tone that arteries regained after FD when pressure was returned to 75 mm Hg was also less in CB arteries: percent tone = 34 +/- 3% for control versus 11 +/- 2% for CB arteries (P < 0.01). CONCLUSIONS: Cytoskeletal integrity appears important for maintaining cerebral arterial diameter during changing intravascular pressure. In addition, the process of actin polymerization may be a significant contributor to development of myogenic tone after forced dilatation.

Temperature and protein kinase C modulate myofilament Ca2+ sensitivity in pressurized rat cerebral arteries.

The effects of pharmacological activation and inhibition of protein kinase C (PKC) and temperature on the relationship between cytoplasmic Ca2+ and lumen diameter were studied in pressurized (50 mmHg) rat posterior cerebral arteries permeabilized with alpha-toxin. Increasing Ca2+ concentrations (30 nM-10 microM, 22 degrees C) induced stable, concentration-dependent constrictions with a half-maximal effective concentration (EC50) of 112 nM. The maximal constriction was 80% of baseline diameter and 157% of that during depolarization of nonpermeabilized vessels with 124 mM KCl. Elevation of temperature to 37 degrees C increased the EC50 to 246 nM and enhanced the steepness of concentration-response curves. Exposure of permeabilized arteries to indolactam V, an activator of PKC, resulted in a significant myofilament Ca2+ sensitization (e.g., EC50 at 5 microM = 126 nM) without changing efficacy. The effects of calphostin C, a PKC inhibitor, on Ca2+ sensitivity were minimal; however, the amplitude of Ca2+-induced constrictions in both control and indolactam-treated arteries was suppressed in a concentration-dependent manner. Thus 1) temperature is an important variable in studies of arterial Ca2+ sensitivity, and 2) changes in PKC activity can significantly alter both myofilament sensitivity to and constrictor efficacy of cytosolic Ca2+.

Estrogen replacement modulates resistance artery smooth muscle and endothelial alpha2-adrenoceptor reactivity.

The purpose of this study was to evaluate the effects of estrogen replacement on ovariectomized rats on the reactivity to alpha2-adrenoceptor activation, and to analyze the role of the endothelium in modulating this response. Third order branches of the superior mesenteric artery from ovariectomized untreated (OvX) and estrogen-replaced (E2) Sprague-Dawley rats were cannulated and pressurized to 50 mmHg. Under relaxed conditions (0.1 mM papaverine), there were no differences in lumen diameter. Intact vessels from E2 rats were unresponsive to clonidine (0.01-10 microM); incubation in indomethacin (1 microM), a cyclooxygenase inhibitor, produced intermediate constriction that was significantly augmented by L-NNA (0.3 mM), a NO synthase inhibitor, or by endothelial denudation. Conversely, intact vessels from OvX animals constricted to clonidine in a concentration-dependent manner. This effect was significantly diminished by endothelial removal or indomethacin, but was not affected by L-NNA. Yohimbine (1 microM), an beta2 receptor antagonist, significantly diminished arterial sensitivity to, and efficacy of clonidine. These results suggest that estrogen replacement enhanced vasoconstriction induced by smooth muscle alpha2 adrenoceptor activation, although this effect was obscured in intact vessels due to an overriding influence of endothelial dilator substances, primarily NO. In arteries from OvX animals, smooth muscle was less sensitive to alpha2 agonist stimulation, however, the release of a vasoconstrictor prostanoid from the endothelium was predominant, and induced significant vasoconstriction.

Flow decreases myogenic reactivity of mesenteric arteries from pregnant rats.

OBJECTIVE: To determine whether pregnancy alters the response of small mesenteric arteries to increased pressure (myogenic reactivity) and flow. METHODS: Mesenteric arteries (300 microns) from cycling nonpregnant (NP, n = 6) and late pregnant (20 days, LP, n = 6) Sprague-Dawley rats were dissected and mounted on an arteriograph system designed for the precise measurement of pressure and flow. Myogenic reactivity was measured as the percentage constriction after a pressure increase to 75 mmHg in the absence and presence of flow (60 microL/minute). RESULTS: In the absence of flow, there was no difference in myogenic reactivity in arteries from NP versus LP animals (NP, 8.4 +/- 1.4%; LP, 11.0 +/- 1.6%; not significant). In the presence of flow, myogenic reactivity was decreased in arteries from LP rats, but was unchanged in arteries from NP rats (NP, 13.2 +/- 1.1%; LP, 2.5 +/- 2.9%; P < .05). The differential group effect appeared to result not from differences in arterial response to changes in pressure or flow alone, but rather from the interaction between pressure and flow. CONCLUSION: These results suggest that pregnancy alters the interaction of the physical forces of pressure and flow on the arterial wall in a manner consistent with decreased vascular resistance.

Pregnancy augments uteroplacental vascular endothelial growth factor gene expression and vasodilator effects.

This study examined a potential role for vascular endothelial growth factor (VEGF) in uterine artery remodeling and vasodilation during pregnancy. Reverse transcription-polymerase chain reaction (RT-PCR) was used to detect VEGF mRNA in uterine tissues from nonpregnant (NP), midpregnant (MP, 15-16 days), and late-pregnant (LP, 19-21 days) rats and in placentas from MP and LP rats. VEGF mRNA levels in uteri and placentas were determined by Northern blotting, and the vasorelaxant activity of recombinant human VEGF (rh-VEGF) was tested and compared in isolated uterine arteries from LP and NP animals. VEGF120 and VEGF164 were the major isoforms detected in uterine tissues; all members of the VEGF family (VEGF120, VEGF164, VEGF188, and VEGF205) were expressed in LP placentas. VEGF mRNA levels increased 60% in MP and 80% in LP above those in NP (P < 0.05) in uterine tissues; VEGF mRNA levels were also detectable in placentas and elevated approximately fivefold in LP vs. MP tissues (P < 0.01). Phenylephrine-preconstricted uterine arcuate arteries (NP and LP) dilated in response to rhVEGF, an effect that was completely abolished by endothelial denudation or pretreatment with genistein, a tyrosine kinase inhibitor. The magnitude of dilation to an intermediate concentration of rhVEGF (1 nM) was greater in LP than in NP vessels (55 +/- 8 vs. 24 +/- 11%; P < 0.05), and this effect was diminished comparably in both groups (approximately 60% by N omega-nitro-L-arginine, an inhibitor of nitric oxide synthesis. These results suggest that VEGF may play a role in the vascular remodeling and vasodilation that lead to decreased uterine vascular resistance and increased uterine blood flow during pregnancy.

Myoendometrial versus placental uterine arteries: structural, mechanical, and functional differences in late-pregnant rabbits.

OBJECTIVE: This study compared late-pregnant radial uterine arteries that supplied the placenta versus the myoendometrium to evaluate differences in active and passive mechanical properties. STUDY DESIGN: Pressurized segments of placental versus myoendometrial radial uterine arteries from late-pregnant (day 28 to 30) New Zealand White rabbits (n = 12) were compared in vitro for differences in luminal diameter, wall thickness, distensibility, and intrinsic tone as a function of transmural pressure. RESULTS: Both types of arteries responded to increased transmural pressure with active vasoconstriction; however, the amount of tone present in myoendometrial arteries was significantly greater than in placental arteries (percent tone at 75 mm Hg = 39% +/- 3% for myoendometrial versus 31% +/- 2% for placental arteries, p < 0.01). Measurements of unpressurized, fully relaxed arteries revealed that placental arteries were 38% larger in diameter and had thicker walls than myoendometrial arteries did. However, myoendometrial arteries were significantly more distensible at transmural pressures >5 mm Hg. CONCLUSIONS: The increased size and diminished tone of placental compared with adjacent myoendometrial arteries would favor increased blood flow to the placenta; differences in size and passive mechanical properties suggest that a localized factor(s) originating from the fetus or placenta contributes to the gestational enlargement of those arteries that perfuse the placenta.

Estrogen replacement attenuates resistance artery adrenergic sensitivity via endothelial vasodilators.

The objective of this study was to determine whether chronic estrogen replacement alters adrenergic constriction and endothelium-dependent dilation in resistance arteries from the rat. Resistance-sized (< 200 microns) mesenteric arteries from castrated female Sprague-Dawley rats with (E2; 21 day, 0.5-mg pellet) and without (OvX) estrogen replacement were removed for in vitro study on a pressurized arteriograph system. Sensitivity to alpha-adrenergic constriction and the role of the endothelium in its modulation and of agonist-provoked endothelium-dependent relaxation were determined. Estrogen-treated rats had decreased heart rate as well as systolic and diastolic blood pressure. Arteries from estrogen-replaced rats were fivefold less sensitive to alpha 1-adrenergic stimulation with phenylephrine (50% effective concentration: E2, 3.2 +/- 1.1 microM; OvX, 0.6 +/- 0.2 microM; P < 0.05). This difference was abolished by endothelial denudation, blockade of cyclooxygenase (1 microM ibuprofen), or nitric oxide synthase blockade (0.24 mM N omega-nitro-L-arginine). There was no difference in muscarinic agonist-provoked relaxation or vascular smooth muscle sensitivity to prostacyclin or sodium nitroprusside. These results indicate that estrogen replacement decreases resistance artery adrenergic sensitivity by increasing the basal release of relaxing factors from the endothelium. This effect on small artery function may produce dual cardioprotective effects by decreasing peripheral resistance, blood pressure, and the likelihood of thrombosis.

Gestation increases nitric oxide-mediated vasodilation in rat uterine arteries.

OBJECTIVE: The purpose of this study was to determine the influence of endothelium-released nitric oxide on uterine arterial tone and reactivity during pregnancy. STUDY DESIGN: The effects of pregnancy on endothelial function were evaluated in isolated pressurized rat uterine arteries from late-pregnant rats (day 19 to 20) versus age-matched nonpregnant controls. The effects of nitric oxide synthase inhibition (N(omega)-nitro-L-arginine) on arterial tone and reactivity under basal and activated (phenylephrine) conditions were determined, as was arterial reactivity to endothelium-dependent (acetylcholine) and endothelium-independent (sodium nitroprusside) vasodilators, by evaluating changes in lumen diameter. RESULTS: (1) Maximal constriction to N(omega)-nitro-L-arginine was significantly enhanced under basal (nonstimulated) conditions in arteries from late-pregnant versus nonpregnant rats (changes in lumen diameter 37% +/- 8% vs 9.3 +/- 6.2%, respectively, p < 0.05). (2) Nitric oxide synthase blockade with 1 nmol/L N(omega)-nitro-L-arginine significantly increased phenylephrine sensitivity in arteries from late-pregnant animals (median effective concentration 115 +/- 23 nmol/L vs 33 +/- 8 nmol/L, control vs treated vessels, p < 0.05) but was without statistically significant effect on arteries from nonpregnant animals (control 255 +/- 164 nmol/L, treated 250 +/- 102 nmol/L, p > 0.05). (3) The threshold concentration of acetylcholine required to elicit endothelium-dependent dilation was significantly lower in late-pregnant versus nonpregnant arteries (1.4 +/- 0.2 nmol/L vs 12.2 +/- 3.8 nmol/L, p < 0.05). (4) Vascular smooth muscle sensitivity to an exogenous nitrodilator (sodium nitroprusside) was identical in late-pregnant versus nonpregnant vessels. CONCLUSION: Endothelial vasodilator influences are augmented during pregnancy under basal, activated (phenylephrine), and chemically provoked (acetylcholine) conditions in uterine arteries by enhanced release of nitric oxide.

High glucose concentrations dilate cerebral arteries and diminish myogenic tone through an endothelial mechanism.

BACKGROUND AND PURPOSE: Diabetes is associated with cerebrovascular disease and impaired autoregulation of cerebral blood flow. The purpose of this study was to determine the effect of acute glucose exposure on basal tone and myogenic reactivity of isolated rat cerebral arteries. METHODS: Posterior cerebral arteries (PCAs, n = 38) were dissected from male Wistar rats and mounted on glass cannulas in a system that allowed control of transmural pressure (TMP) and measurement of lumen diameter. Arteries were exposed to various concentrations of glucose, and the amount of basal tone and reactivity to TMP was measured. The effect of elevated glucose on cerebral endothelial modulation of basal tone was determined by mechanical denudation and the use of inhibitors of both nitric oxide and prostaglandin synthesis. RESULTS: Arteries exposed to 44 versus 5.5 mmol/L glucose developed significantly less intrinsic tone (percent tone, 2 +/- 1% versus 28 +/- 2%; P < .01) and responded passively to increases in TMP. Preexisting tone present in 5.5 mmol/L glucose was eliminated on exposure to 44 mmol/L glucose, which decreased tone from 30 +/- 5% to 5 +/- 4% (P < .01). Glucose-induced dilations were concentration dependent such that half-maximal responses were obtained at 25 +/- 2 mmol/L. Endothelial removal abolished this effect, and the amount of tone was similar in 5.5 versus 44 mmol/L glucose (percent tone, 46 +/- 6% versus 49 +/- 5%; P > .05), as did inhibition of nitric oxide production with 0.3 mmol/L nitro-L-arginine (percent tone, 52 +/- 4% versus 46 +/- 3%; P > .05); however, blockade of the cyclooxygenase pathway with indomethacin (10(-5) mmol/L) only partially inhibited the dilation to glucose (percent tone, 32 +/- 3% in 5.5 mmol/L versus 12.4 +/- 3% in 44 mmol/L; P < .01). CONCLUSIONS: Acute glucose exposure dilates arteries with intrinsic tone and impairs cerebrovascular reactivity to TMP via an endothelium-mediated mechanism that involves nitric oxide and prostaglandins.

Estrogen replacement increases beta-adrenoceptor-mediated relaxation of rat mesenteric arteries.

The purpose of the present study was to determine whether estrogen replacement in ovariectomized rats could modulate arterial diameter responses to beta-adrenoceptor activation. Under relaxed conditions (0.1 mM papaverine) there were no differences in the lumen diameter of isolated, pressurized (50 mm Hg) mesenteric arteries from nontreated (191.7 +/- 13.8 microns; n = 19) versus those from estrogen-treated (190.1 +/- 11 microns; n = 14) ovariectomized Sprague-Dawley rats. In arteries precontracted with noradrenaline (0.3-1 microM), isoprenaline (0.01-10 microM)-induced relaxation was significantly increased in arteries from ovariectomized estrogen-treated rats (52.4 +/- 2% of the maximal relaxation induced by 0.1 mM papaverine, vs. 33.3 +/- 6.5%; p < 0.01). The half-maximal concentration value was 0.04 +/- 0.05 microM in estrogen-treated rats and 0.4 +/- 0.1 microM in nontreated rats (p < 0.01). This response was inhibited by propranolol (1 microM) in both groups to a comparable extent (61.5%), and was unaffected by endothelial removal. Forskolin (0.01-10 microM) induced similar concentration-dependent vasodilation in arteries of both groups of rats with no differences in sensitivity or maximal response. These results suggest that isoprenaline acts through beta-adrenoceptors present on vascular smooth muscle and that estrogen replacement enhances the relaxant responses induced by beta-adrenoceptor activation by an endothelium-independent mechanism.

alpha-Toxin perfusion: a new method for selective impairment of endothelial function in isolated vessels or intact vascular beds.

We describe a method for selectively permeabilizing endothelial cells, using the membrane pore forming exoprotein Staphylococcus aureus alpha-toxin. Experiments were performed in rabbit central ear artery or its main side branch under isometric conditions, on the isolated perfused kidney, or in cannulated pressurized renal arteries. In presence of alpha-toxin, endothelial-dependent vasodilator responses elicited by acetylcholine or A23187 were abolished, whereas the sensitivity of smooth muscle cells to constrictors (norepinephrine, phenylephrine, or KCl) or dilators (sodium nitroprusside) was not affected. The results indicate that restricting the alpha-toxin to the luminal surface induces selective impairment of vascular endothelial function. This method of eliminating endothelium-dependent vasodilator responses may prove to be useful in the study of endothelial-smooth muscle interactions of isolated small arteries and intact vascular beds.

Mechanotransduction by vascular smooth muscle.

Mechanotransduction by vascular smooth muscle (VSM) is defined as a cellular response (contraction, secretion, growth, division) to transmural pressure or stretch. This review includes an overview of the physical forces VSM cells experience in vivo, consideration of experimental techniques used to study VSM mechanotransduction, and a discussion of the scientific literature pertinent to the individual cellular components that have been implicated in the transduction of physical forces. These include: the extracellular matrix, integrins, ion channels, the sarcoplasmic reticulum, second messenger systems, contractile proteins, and the cytoskeleton.