Contribution of chondroitin-dermatan sulfate-containing proteoglycans to the function of rat mesenteric arteries

Glycosaminoglycans modulate compressive stiffness and circumferential residual stress in the porcine thoracic aorta

The mechanical properties of the aorta are influenced by the extracellular matrix, a network mainly comprised of fibers and glycosaminoglycans (GAG). In this work, we demonstrate that GAG contribute to the opening angle (a marker of circumferential residual stresses) in intact and glycated aortic tissue. Enzymatic GAG depletion was associated with a decrease in the opening angle, by approximately 25% (p = 0.009) in the ascending (AS) region, 32% (p = 0.003) in the aortic arch (AR), and 42% (p = 0.001) in the lower descending thoracic (LDT) region. A similar effect of GAG depletion on aortic ring opening angle was also observed in previously glycated tissues. Using indentation testing, we found that the radial compressive stiffness significantly increased in the AS region following GAG depletion, compared to fresh (p = 0.006) and control samples (p = 0.021), and that the compressive properties are heterogeneous along the aortic tree. A small loss of water content was also detected after GAG depletion, which was most prominent under hypotonic conditions. Finally, the AS region was also associated with a significant loss of compressive deformation (circumferential stretch that is < 1) in the inner layer of the aorta following GAG depletion, suggesting that GAG interact with ECM fibers in their effect on aortic mechanics. The importance of this work lies in its identification of the role of GAG in modulating the mechanical properties of the aorta, namely the circumferential residual stresses and the radial compressive stiffness, as well as contributing to the swelling state and the level of circumferential prestretch in the tissue. STATEMENT OF SIGNIFICANCE: The mechanical properties of the aorta are influenced by the composition and organization of its extracellular matrix (ECM) and are highly relevant to medical conditions affecting the structural integrity of the aorta. The extent of contribution of glycosaminoglycans (GAG), a relatively minor ECM component, to the mechanical properties of the aorta, remains poorly characterized. This works shows that GAG contribute on average 30% to the opening angle (an indicator of circumferential residual stresses) of porcine aortas, and that GAG-depletion is associated with an increased radial compressive stiffness of the aorta. GAG-depletion was also associated with a loss of water content and compressive deformation in the inner layers of the aortic wall providing insight into potential mechanisms for their biomechanical role.

The mechanical and morphological properties of systemic and pulmonary arteries differ in the earth boa, a snake without ventricular pressure separation

The walls of the mammalian aorta and pulmonary artery are characterized by diverging morphologies and mechanical properties, which has been correlated with high systemic and low pulmonary blood pressures, as a result of intraventricular pressure separation. However, the relation between intraventricular pressure separation and diverging aortic and pulmonary artery wall morphologies and mechanical characteristics is not understood. The snake cardiovascular system poses a unique model for the study of this question, since representatives both with and without intraventricular pressure separation exist. In this study we perform uniaxial tensile testing on vessel samples taken from the aortas and pulmonary arteries of the earth boa, Acrantophis madagascariensis, a species without intraventricular pressure separation. We then compare these morphological and mechanical characteristics with samples from the ball python, Python regius, and the yellow anaconda, Eunectes notaeus, species with and without intraventricular pressure separation, respectively. Our data suggest that although the aortas and pulmonary arteries of A. madagascariensis respond similarly to the same intramural blood pressures, they diverge in morphology, and that this attribute extends to E. notaeus. In contrast, P. regius aortas and pulmonary arteries diverge both morphologically and in terms of their mechanical properties. Our data indicate that intraventricular pressure separation cannot fully explain diverging aortic and pulmonary artery morphologies. Following the Law of Laplace, we propose that pulmonary arteries of small luminal diameter represent a mechanism to protect the fragile pulmonary vasculature by reducing the blood volume that passes through, to which genetic factors may contribute more strongly than physiological parameters.

Is It Good to Have a Stiff Aorta with Aging? Causes and Consequences

Aortic stiffness increases with advancing age, more than doubling during the human life span, and is a robust predictor of cardiovascular disease (CVD) clinical events independent of traditional risk factors. The aorta increases in diameter and length to accommodate growing body size and cardiac output in youth, but in middle and older age the aorta continues to remodel to a larger diameter, thinning the pool of permanent elastin fibers, increasing intramural wall stress and resulting in the transfer of load bearing onto stiffer collagen fibers. Whereas aortic stiffening in early middle age may be a compensatory mechanism to normalize intramural wall stress and therefore theoretically "good" early in the life span, the negative clinical consequences of accelerated aortic stiffening beyond middle age far outweigh any earlier physiological benefit. Indeed, aortic stiffness and the loss of the "windkessel effect" with advancing age result in elevated pulsatile pressure and flow in downstream microvasculature that is associated with subclinical damage to high-flow, low-resistance organs such as brain, kidney, retina, and heart. The mechanisms of aortic stiffness include alterations in extracellular matrix proteins (collagen deposition, elastin fragmentation), increased arterial tone (oxidative stress and inflammation-related reduced vasodilators and augmented vasoconstrictors; enhanced sympathetic activity), arterial calcification, vascular smooth muscle cell stiffness, and extracellular matrix glycosaminoglycans. Given the rapidly aging population of the United States, aortic stiffening will likely contribute to substantial CVD burden over the next 2-3 decades unless new therapeutic targets and interventions are identified to prevent the potential avalanche of clinical sequelae related to age-related aortic stiffness.

The Contribution of Glycosaminoglycans/Proteoglycans to Aortic Mechanics in Health and Disease: A Critical Review

While elastin and collagen have received a lot of attention as major contributors to aortic biomechanics, glycosaminoglycans (GAGs) and proteoglycans (PGs) recently emerged as additional key players whose roles must be better elucidated if one hopes to predict aortic ruptures caused by aneurysms and dissections more reliably. GAGs are highly negatively charged polysaccharide molecules that exist in the extracellular matrix (ECM) of the arterial wall. In this critical review, we summarize the current understanding of the contributions of GAGs/PGs to the biomechanics of the normal aortic wall, as well as in the case of aortic diseases such as aneurysms and dissections. Specifically, we describe the fundamental swelling behavior of GAGs/PGs and discuss their contributions to residual stresses and aortic stiffness, thereby highlighting the importance of taking these polyanionic molecules into account in mathematical and numerical models of the aorta. We suggest specific lines of investigation to further the acquisition of experimental data to complement simulations and solidify our current understanding. We underscore different potential roles of GAGs/PGs in thoracic aortic aneurysm (TAAD) and abdominal aortic aneurysm (AAA). Namely, we report findings according to which the accumulation of GAGs/PGs in TAAD causes stress concentrations which may be sufficient to initiate and propagate delamination. On the other hand, there seems to be no clear indication of a relationship between the marked reduction in GAG/PG content and the stiffening and weakening of the aortic wall in AAA.

Analysis of vascular mechanical properties of the yellow anaconda reveals increased elasticity and distensibility of the pulmonary artery during digestion

In animals with functional division of blood systemic and pulmonary pressures, such as mammals, birds, crocodilians and a few non-crocodilian reptiles, the vessel walls of systemic and pulmonary arteries are exquisitely adapted to endure different pressures during the cardiac cycle, systemic arteries being stronger and stiffer than pulmonary arteries. However, the typical non-crocodilian reptile heart possesses an undivided ventricle that provides similar systolic blood pressure to both circuits. This raises the question whether in these species the systemic and pulmonary mechanical vascular properties are similar. Snakes also display large organ plasticity and increased cardiac output in response to digestion, and we speculate how the vascular circuit would respond to this further stress. We addressed these questions by testing the mechanical vascular properties of the dorsal aorta and the right pulmonary artery of fasted and fed yellow anacondas, Eunectes notaeus, a snake without functional ventricular separation that also exhibits large metabolic and cardiovascular responses to digestion. Similar to previous studies, the dorsal aorta was thicker, stronger, stiffer and more elastic than the pulmonary artery. However, unlike any other species studied so far, the vascular distensibility (i.e. the relative volume change given a pressure change) was similar for the two circuits. Most striking, the pulmonary artery elasticity (i.e. its capacity to resume its original form after being stretched) and distensibility increased during digestion, which suggests that this circuit is remodeled to accommodate the larger stroke volume and enhance the Windkessel effect, thus providing a more constant blood perfusion during digestion.

Multiscale Mechanical Evaluation of Human Supraspinatus Tendon Under Shear Loading After Glycosaminoglycan Reduction

Proteoglycans (PGs) are broadly distributed within many soft tissues and, among other roles, often contribute to mechanical properties. Although PGs, consisting of a core protein and glycosaminoglycan (GAG) sidechains, were once hypothesized to regulate stress/strain transfer between collagen fibrils and help support load in tendon, several studies have reported no changes to tensile mechanics after GAG depletion. Since GAGs are known to help sustain nontensile loading in other tissues, we hypothesized that GAGs might help support shear loading in human supraspinatus tendon (SST), a commonly injured tendon which functions in a complex multiaxial loading environment. Therefore, the objective of this study was to determine whether GAGs contribute to the response of SST to shear, specifically in terms of multiscale mechanical properties and mechanisms of microscale matrix deformation. Results showed that chondroitinase ABC (ChABC) treatment digested GAGs in SST while not disrupting collagen fibers. Peak and equilibrium shear stresses decreased only slightly after ChABC treatment and were not significantly different from pretreatment values. Reduced stress ratios were computed and shown to be slightly greater after ChABC treatment compared to phosphate-buffered saline (PBS) incubation without enzyme, suggesting that these relatively small changes in stress values were not due strictly to tissue swelling. Microscale deformations were also not different after ChABC treatment. This study demonstrates that GAGs possibly play a minor role in contributing to the mechanical behavior of SST in shear, but are not a key tissue constituent to regulate shear mechanics.

The contribution of glycosaminoglycans to the mechanical behaviour of the posterior human sclera

We characterized the structural and mechanical changes after experimental digestion of sulfated glycosaminoglycans (s-GAGs) in the human posterior sclera, using ultrasound thickness measurements and an inflation test with three-dimensional digital image correlation (3D-DIC). Each scleral specimen was first incubated in a buffer solution to return to full hydration, inflation tested, treated in a buffer solution with chondroitinase ABC (ChABC), then inflation tested again. After each test series, the thickness of eight locations was measured. After enzymatic treatment, the average scleral thickness decreased by 13.3% (p < 0.001) and there was a stiffer overall stress-strain response (p < 0.05). The stress-strain response showed a statistically significant increase in the low-pressure stiffness, high-pressure stiffness and hysteresis. Thus, s-GAGs play a measurable role in the mechanical behaviour of the posterior human sclera.

The effects of glycosaminoglycan degradation on the mechanical behavior of the posterior porcine sclera

Pathological changes in scleral glycosaminoglycan (GAG) content and in scleral mechanical properties have been observed in eyes with glaucoma and myopia. The purpose of this study is to investigate the effect of GAG removal on the scleral mechanical properties to better understand the impact of GAG content variations in the pathophysiology of glaucoma and myopia. We measured how the removal of sulphated GAG (s-GAG) affected the hydration, thickness and mechanical properties of the posterior sclera in enucleated eyes of 6-9 month-old pigs. Measurements were made in 4 regions centered on the optic nerve head (ONH) and evaluated under 3 conditions: no treatment (control), after treatment in buffer solution alone, and after treatment in buffer containing chondroitinase ABC (ChABC) to remove s-GAGs. The specimens were mechanically tested by pressure-controlled inflation with full-field deformation mapping using digital image correlation (DIC). The mechanical outcomes described the tissue tensile and viscoelastic behavior. Treatment with buffer alone increased the hydration of the posterior sclera compared to controls, while s-GAG removal caused a further increase in hydration compared to buffer-treated scleras. Buffer-treatment significantly changed the scleral mechanical behavior compared to the control condition, in a manner consistent with an increase in hydration. Specifically, buffer-treatment led to an increase in low-pressure stiffness, hysteresis, and creep rate, and a decrease in high-pressure stiffness. ChABC-treatment on buffer-treated scleras had opposite mechanical effects than buffer-treatment on controls, leading to a decrease in low-pressure stiffness, hysteresis, and creep rate, and an increase in high-pressure stiffness and transition strain. Furthermore, s-GAG digestion dramatically reduced the differences in the mechanical behavior among the 4 quadrants surrounding the ONH as well as the differences between the circumferential and meridional responses compared to the buffer-treated condition. These findings demonstrate a significant effect of s-GAGs on both the stiffness and time-dependent behavior of the sclera. Alterations in s-GAG content may contribute to the altered creep and stiffness of the sclera of myopic and glaucoma eyes.

Vascular dysfunction precedes hypertension associated with a blood pressure locus on rat chromosome 12

We previously isolated a 6.1-Mb region of SS/Mcwi (Dahl salt-sensitive) rat chromosome 12 (13.4-19.5 Mb) that significantly elevated blood pressure (BP) (Δ+34 mmHg, P < 0.001) compared with the SS-12(BN) consomic control. In the present study, we examined the role of vascular dysfunction and remodeling in hypertension risk associated with the 6.1-Mb (13.4-19.5 Mb) locus on rat chromosome 12 by reducing dietary salt, which lowered BP levels so that there were no substantial differences in BP between strains. Consequently, any observed differences in the vasculature were considered BP-independent. We also reduced the candidate region from 6.1 Mb with 133 genes to 2 Mb with 23 genes by congenic mapping. Both the 2 Mb and 6.1 Mb congenic intervals were associated with hypercontractility and decreased elasticity of resistance vasculature prior to elevations of BP, suggesting that the vascular remodeling and dysfunction likely contribute to the pathogenesis of hypertension in these congenic models. Of the 23 genes within the narrowed congenic interval, 12 were differentially expressed between the resistance vasculature of the 2 Mb congenic and SS-12(BN) consomic strains. Among these, Grifin was consistently upregulated 2.7 ± 0.6-fold (P < 0.05) and 2.0 ± 0.3-fold (P < 0.01), and Chst12 was consistently downregulated -2.8 ± 0.3-fold (P < 0.01) and -4.4 ± 0.4-fold (P < 0.00001) in the 2 Mb congenic compared with SS-12(BN) consomic under normotensive and hypertensive conditions, respectively. A syntenic region on human chromosome 7 has also been associated with BP regulation, suggesting that identification of the genetic mechanism(s) underlying cardiovascular phenotypes in this congenic strain will likely be translated to a better understanding of human hypertension.

The effect of static stretch on elastin degradation in arteries

Previously we have shown that gradual changes in the structure of elastin during an elastase treatment can lead to important transition stages in the mechanical behavior of arteries. However, in vivo arteries are constantly being loaded due to systolic and diastolic pressures and so understanding the effects of loading on the enzymatic degradation of elastin in arteries is important. With biaxial tensile testing, we measured the mechanical behavior of porcine thoracic aortas digested with a mild solution of purified elastase (5 U/mL) in the presence of a static stretch. Arterial mechanical properties and biochemical composition were analyzed to assess the effects of mechanical stretch on elastin degradation. As elastin is being removed, the dimensions of the artery increase by more than 20% in both the longitude and circumference directions. Elastin assays indicate a faster rate of degradation when stretch was present during the digestion. A simple exponential decay fitting confirms the time constant for digestion with stretch (0.11 ± 0.04 h(-1)) is almost twice that of digestion without stretch (0.069 ± 0.028 h(-1)). The transition from J-shaped to S-shaped stress vs. strain behavior in the longitudinal direction generally occurs when elastin content is reduced by about 60%. Multiphoton image analysis confirms the removal/fragmentation of elastin and also shows that the collagen fibers are closely intertwined with the elastin lamellae in the medial layer. After removal of elastin, the collagen fibers are no longer constrained and become disordered. Release of amorphous elastin during the fragmentation of the lamellae layers is observed and provides insights into the process of elastin degradation. Overall this study reveals several interesting microstructural changes in the extracellular matrix that could explain the resulting mechanical behavior of arteries with elastin degradation.

Early hypercholesterolemia contributes to vasomotor dysfunction and injury associated atherogenesis that can be inhibited by nitric oxide

OBJECTIVE

Atherosclerosis results in vasomotor dysfunction, in part, through impairment of nitric oxide (NO) dependent vasodilation. It is unclear whether blood vessels are dysfunctional in an early environment of hypercholesterolemia alone and if this contributes to the vascular injury response. We hypothesize that early hypercholesterolemia, prior to gross vascular changes, contributes to vasomotor dysfunction and the vascular injury response. The efficacy of NO therapy to protect against the injury response in this setting was also assessed.

METHODS

The effect of oxidized low density lipoprotein (oxLDL) and inducible NO synthase (iNOS) gene transfer on rat aortic smooth muscle cell (SMC) proliferation was measured with (3)H-thymidine incorporation. Common carotid arteries (CCA) from wild-type C57BL6 (WT or C57) and apolipoprotein E deficient (ApoE KO) mice fed normal or Western diets for 6 to 8 weeks were tested for vasomotor function using an arteriograph system. Studies were repeated after CCA injury. The effect of iNOS gene transfer on morphometry by histology and vasomotor responses in injured CCAs in ApoE KO was examined.

RESULTS

OxLDL increased SMC proliferation by >50%. In SMC expressing iNOS, NO production was unaffected by oxLDL and reduced oxLDL and still inhibited SMC proliferation. Endothelium dependent vasorelaxation was reduced in uninjured CCAs from ApoE KO and C57 mice on the Western vs normal diet (ApoE 39% ± 2% vs 55% ± 13%; C57 50% ± 13% vs 76% ± 5%, P < .001) and was increased with longer durations of hypercholesterolemia. Endothelium-dependent and independent vasodilator responses were severely disrupted in C57 and ApoE KO mice 2 weeks following CCA injury but both recovered by 4 weeks. CCA injury in ApoE KO mice resulted in the formation of atheromatous lesions while C57 mice showed no change (intima 27,795 ± 1829 vs 237 ± 28 μm(2); media 46,306 ± 2448 vs 11,714 ± 392 μm(2), respectively; P < .001). This structural change in the ApoE KO reduced distensibility and increased stiffness. Finally, iNOS gene transfer to injured CCA in ApoE KO mice dramatically reduced atheromatous neointimal lesion formation.

CONCLUSIONS

Early hypercholesterolemia impairs endothelial function, with severity being related to duration and magnitude of hypercholesterolemia. Severe hypercholesterolemia leads to atheromatous lesion formation following injury and stresses the role of vascular injury in atherogenesis and suggests different mechanisms are involved in endothelial dysfunction and the injury response. Despite these changes, iNOS gene transfer still effectively inhibits atheroma formation. These findings support early correction of hypercholesterolemia and emphasize the potential role for NO based therapies in disease states.

Contribution of glycosaminoglycans to viscoelastic tensile behavior of human ligament

The viscoelastic properties of human ligament potentially guard against structural failure, yet the microstructural origins of these transient behaviors are unknown. Glycosaminoglycans (GAGs) are widely suspected to affect ligament viscoelasticity by forming molecular bridges between neighboring collagen fibrils. This study investigated whether GAGs directly affect viscoelastic material behavior in human medial collateral ligament (MCL) by using nondestructive tensile tests before and after degradation of GAGs with chondroitinase ABC (ChABC). Control and ChABC treatment (83% GAG removal) produced similar alterations to ligament viscoelasticity. This finding was consistent at different levels of collagen fiber stretch and tissue hydration. On average, stress relaxation increased after incubation by 2.2% (control) and 2.1% (ChABC), dynamic modulus increased after incubation by 3.6% (control) and 3.8% (ChABC), and phase shift increased after incubation by 8.5% (control) and 8.4% (ChABC). The changes in viscoelastic behavior after treatment were significantly more pronounced at lower clamp-to-clamp strain levels. A 10% difference in the water content of tested specimens had minor influence on ligament viscoelastic properties. The major finding of this study is that mechanical interactions between collagen fibrils and GAGs are unrelated to tissue-level viscoelastic mechanics in mature human MCL. These findings narrow the possible number of extracellular matrix molecules that have a direct contribution to ligament viscoelasticity.

Disturbance of macro- and microcirculation: relations with pulse pressure and cardiac organ damage

Whereas large arteries dampen oscillations resulting from intermittent ventricular ejection, small arteries steadily deliver optimal blood flow to various organs as the heart. The transition from pulsatile to steady pressure is influenced by several factors as wave travel, damping, and reflections, which are mainly determined by the impedance mismatch between large vessels and arteriolar bifurcations. The mechanism(s) behind the dampening of pressure wave in the periphery and the links between central and peripheral pulsatile pressure (PP) may determine cardiac damage. Active pathways participate to pulse widening and changes in pulse amplitude in microvessels. Steady and cyclic stresses operate through different transduction mechanisms, the former being focal adhesion kinase and the latter being free radicals and oxidative stress. Independently of mechanics, calcifications and attachment molecules contribute to enhance vessel wall stiffness through changes in collagen cross-links, proteoglycans, integrins, and fibronectin. Enhanced PP transmission may thus occur and precipitate organ damage at each time that autoregulatory mechanisms, normally protecting the heart from vascular injury, are blunted. Such circumstances, observed in old subjects with systolic hypertension and/or Type 2 diabetes mellitus, particularly under high-sodium diet, cause cardiac damage and explain why increased PP and arterial stiffness are significant predictors of morbidity and mortality in the elderly.

Changes in proteoglycans and lung tissue mechanics during excessive mechanical ventilation in rats

Excessive mechanical ventilation results in changes in lung tissue mechanics. We hypothesized that changes in tissue properties might be related to changes in the extracellular matrix component proteoglycans (PGs). The effect of different ventilation regimens on lung tissue mechanics and PGs was examined in an in vivo rat model. Animals were anesthetized, tracheostomized, and ventilated at a tidal volume of 8 (VT(8)), 20, or 30 (VT(30)) ml/kg, positive end-expiratory pressure of 0 (PEEP(0)) or 1.5 (PEEP(1.5)) cmH(2)O, and frequency of 1.5 Hz for 2 h. The constant-phase model was used to derive airway resistance, tissue elastance, and tissue damping. After physiological measurements, one lung was frozen for immunohistochemistry and the other was reserved for PG extraction and Western blotting. After 2 h of mechanical ventilation, tissue elastance and damping were significantly increased in rats ventilated at VT(30)PEEP(0) compared with control rats (ventilated at VT(8)PEEP(1.5)). Versican, basement membrane heparan sulfate PG, and biglycan were all increased in rat lungs ventilated at VT(30)PEEP(0) compared with control rats. At VT(30)PEEP(0), heparan sulfate PG and versican staining became prominent in the alveolar wall and airspace; biglycan was mostly localized in the airway wall. These data demonstrate that alterations in lung tissue mechanics with excessive mechanical ventilation are accompanied by changes in all classes of extracellular matrix PG.

Effect of glycosaminoglycan degradation on lung tissue viscoelasticity

We tested the hypothesis that matrix glycosaminoglycans contribute to lung tissue viscoelasticity. We exposed lung parenchymal strips to specific degradative enzymes (chondroitinase ABC, heparitinase I, and hyaluronidase) and determined whether the mechanical properties of the tissue were affected. Subpleural parenchymal strips were obtained from Sprague-Dawley rats and suspended in a Krebs-filled organ bath. One end of the strip was attached to a force transducer and the other to a servo-controlled lever arm that effected sinusoidal oscillations. Recordings of tension and length at different amplitudes and frequencies of oscillation were recorded before and after enzyme exposure. Resistance, dynamic elastance, and hysteresivity were estimated by fitting the equation of motion to changes in tension and length. Quasi-static stress-strain curves were also obtained. Exposure to chondroitinase and heparitinase I caused significant increases in hysteresivity, no decrement in resistance, and similar decreases in dynamic elastance relative to control strips exposed to Krebs solution only. Conversely, measures of static elastance were different in treated versus control strips. Hyaluronidase treatment did not alter any of the mechanical measures. These data demonstrate that digestion of chondroitin sulfate and heparan sulfate alters the mechanical behavior of lung parenchymal tissues.

Endothelin and nitric oxide mediate reduced myogenic reactivity of small renal arteries from pregnant rats

We tested the hypothesis that endothelin acting through the endothelial ET(B) receptor subtype and the nitric oxide (NO) pathway accounts for reduced myogenic reactivity of the renal resistance vasculature during pregnancy. Small renal arteries (100-200 microm) were isolated from virgin and midterm pregnant rats when gestational renal hyperfiltration and vasodilation are maximal in this species. Myogenic reactivity (the adjustment of arterial diameter in response to a change in transmural pressure) was assessed with a pressurized myograph system. A rapid increase in transmural pressure from 60 to 80 mmHg resulted in a 2.4% diameter increase in vessels from virgin compared with an 8.1% increase in arteries from midgestation rats (n = 8 each, P < 0.05). Thus myogenic reactivity is markedly reduced during pregnancy. Incubation with the NO synthase inhibitors, an ET(B) receptor subtype antagonist (RES-701-1), the nonselective ET(A/B) receptor blocker (SB-209670), or endothelial removal abrogated the reduced myogenic reactivity of vessels from gravid rats without affecting myogenic reactivity in arteries from virgin animals. Thus the endothelium mediates the reduced myogenic reactivity of small renal arteries of midgestation rats most likely through the ET(B) receptor subtype and NO pathway.