Human Subcutaneous Resistance Arteries Research Articles

Microstructural Characterization of Resistance Artery Remodelling in Diabetes Mellitus

Introduction: Microvascular remodelling is a symptom of cardiovascular disease. Despite the mechanical environment being recognized as a major contributor to the remodelling process, it is currently only understood in a rudimentary way. Objective: A morphological and mechanical evaluation of the resistance vasculature in health and diabetes mellitus. Methods: The cells and extracellular matrix of human subcutaneous resistance arteries from abdominal fat biopsies were imaged using two-photon fluorescence and second harmonic generation at varying transmural pressure. The results informed a two-layer mechanical model. Results: Diabetic resistance arteries reduced in wall area as pressure was increased. This was attributed to the presence of thick, straight collagen fibre bundles that braced the outer wall. The abnormal mechanical environment caused the internal elastic lamina and endothelial and vascular smooth muscle cell arrangements to twist. Conclusions: Our results suggest diabetic microvascular remodelling is likely to be stress-driven, comprising at least 2 stages: (1) Laying down of adventitial bracing fibres that limit outward distension, and (2) Deposition of additional collagen in the media, likely due to the significantly altered mechanical environment. This work represents a step towards elucidating the local stress environment of cells, which is crucial to build accurate models of mechanotransduction in disease.

Transglutaminase 2 Inhibitor LDN 27219 Age-Dependently Lowers Blood Pressure and Improves Endothelium-Dependent Vasodilation in Resistance Arteries.

Transglutaminase 2 (TG2) is an enzyme which in the open conformation exerts transamidase activity, leading to protein cross-linking and fibrosis. In the closed conformation, TG2 participates in transmembrane signaling as a G protein. The unspecific transglutaminase inhibitor cystamine causes vasorelaxation in rat resistance arteries. However, the role of TG2 conformation in vascular function is unknown. We investigated the vascular effects of selective TG2 inhibitors by myography in isolated rat mesenteric and human subcutaneous resistance arteries, patch-clamp studies on vascular smooth muscle cells, and blood pressure measurements in rats and mice. LDN 27219 promoted the closed TG2 conformation and inhibited transamidase activity in mesenteric arteries. In contrast to TG2 inhibitors promoting the open conformation (Z-DON, VA5), LDN 27219 concentration-dependently relaxed rat and resistance human arteries by a mechanism dependent on nitric oxide, large-conductance calcium-activated and voltage-gated potassium channels 7, lowering blood pressure. LDN 27219 also potentiated acetylcholine-induced relaxation by opening potassium channels in the smooth muscle; these effects were abolished by membrane-permeable TG2 inhibitors promoting the open conformation. In isolated arteries from 35- to 40-week-old rats, transamidase activity was increased, and LDN 27219 improved acetylcholine-induced relaxation more than in younger rats. Infusion of LDN 27219 decreased blood pressure more effectively in 35- to 40-week than 12- to 14-week-old anesthetized rats. In summary, pharmacological modulation of TG2 to the closed conformation age-dependently lowers blood pressure and, by opening potassium channels, potentiates endothelium-dependent vasorelaxation. Our findings suggest that promoting the closed conformation of TG2 is a potential strategy to treat age-related vascular dysfunction and lowers blood pressure.

PP.33.12

Objective: Obesity is a strong risk factor for hypertension but the mechanism linking obesity to hypertension is not fully elucidated. In obesity, circulating concentrations of adiponectin are decreased and hypoadiponectinamia has in some but not all studies been associated with increased risk of hypertension. Due to this inconsistency, we decided to study adiponectin from 2 aspects in a cross-sectional in vivo study and in an experimental in vitro study. Design and method: In the cross-sectional study, 103 men with body mass index (BMI) > = 30.0 kg/m2 were studied; 63 had 24-hr ambulatory blood pressure (AMBP) > = 130/80 mmHg (OHT) and 40 had 24-hr AMBP < 130/80 mmHg (ONT). As controls, we studied 27 men with BMI between 20.0 and 24.9 kg/m2 and 24-hr AMBP < 130/80 mmHg (LNT). Serum concentrations of adiponectin and body composition using dual energy X-ray absorptiometry scanning were determined. In the in vitro study, the direct vasomotor response of adiponectin was tested on subcutaneous resistance arteries from human abdominal adipose tissue. Results: The 2 obese groups had lower serum adiponectin concentrations compared with LNT (P < 0.01) (median (interquartile range): OHT 6.5 (5.1–8.3) mg/L; ONT 6.6 (5.2–7.8) mg/L; and LNT 9.4 (6.7–12.4) mg/L, but there was no significant difference in adiponectin concentrations (or body composition) between OHT and ONT (P = 0.67), although OHT had much higher 24-hr systolic AMBP (mean ± SD: 137 ± 11 mm Hg vs. 117 ± 6 mm Hg, P < 0.001) and much higher 24-hr diastolic AMBP (mean ± SD: 83 ± 6 mm Hg vs. 73 ± 4 mm Hg, P < 0.001) compared with ONT. In the in vitro study, adiponectin neither had any direct vasodilatory effect nor did adiponectin affect angiotensin II stimulated vasoconstriction. Conclusions: Despite large differences in 24-hr AMBP (and comparable body compostion), obese hypertensive men had similar serum concentrations of adiponectin as obese normotensive men, and furthermore adiponectin did not show any vasodilatory properties in human subcutaneous resistance arteries. Thus overall, our findings do not support a major role, if any, for adiponectin in human hypertension.

CNP, but not ANP or BNP, Relax Human Isolated Subcutaneous Resistance Arteries by an Action Involving Cyclic GMP and BKCa Channels

Natriuretic peptides play an important role in sodium regulation and blood pressure (BP) control. We examined the effects of atrial natriuetic peptide (ANP), brain natriuretic peptide (BNP) and C-type natriuretic peptide (CNP) on human isolated resistance arteries and the mechanisms involved in vasorelaxation. Human subcutaneous resistance arteries were mounted in an isometric myograph and contracted with phenylephrine. CNP, but not ANP or BNP, relaxed arteries in a concentration dependent manner. The action of CNP was unaffected by removal of the endothelium, inhibition of nitric oxide synthase by NG-monomethyl-Larginine or inhibition of soluble guanylate cyclase by 1H-[1,2,4] oxadiazolo [4,3-alpha] quinoxalin-1-one. Blockade of cyclic GMPdependent kinase by 8- bromoguanosine- 3, 5- cyclic monophosphorothioate, Rp-isomer (Rp-8-Br-cGMPS) inhibited CNP relaxation. CNP relaxation was also inhibited by high potassium or iberiotoxin, indicating that it was due to opening of BKCa channels. Omapatrilat, a vasopeptidase inhibitor of neutral endopeptidase and angiotensin-converting enzyme, enhanced the effect of CNP and inhibited responses to Ang I. In summary, CNP, but not ANP or BNP, relaxes human resistance arteries by activating cyclic GMP-dependent kinase and BKCa. The effects of CNP are enhanced by vasopeptidase inhibition and this may contribute to the vasodilator effects of these agents in vivo. Since CNP is widely present in endothelium it may play a role in the regulation of peripheral resistance in man in physiological and pathological circumstances.

Making Up and Breaking Up

The concept that blood pressure influences vessel wall composition and structure is well established. Any alterations in stretch (caused by changes in blood pressure) solicit counteracting radial and tangential forces, driving transformations in the vessel wall that aim to accommodate the new conditions and to ultimately restore basal levels of tensile stress.1–3 Thus, when tensile stress increases because of a rise in arterial pressure, smooth muscle cell hypertrophy and collagen and elastin contents augment. Inversely, when the circumferential stress falls, the wall undergoes atrophy.4,5 See page 957 Understanding how vessel wall remodeling occurs is valuable because it relies on the delicate balance between restructuralization and overcompensation. For instance, vascular accumulation of type I, III, and IV collagen in hypertensive patients6 and in animal models of hypertension7,8 effectively counteracts the distending force of blood pressure, but at length it also results in increased vascular stiffness. Clinically, vascular stiffness translates to greater pulse wave velocity, which is an independent predictor of mortality in patients with end-stage renal failure, hypertension, and diabetes, as well as in older individuals.9 In this issue of Atherosclerosis, Thrombosis and Vascular Biology , Jackson et al10 show that the opposite holds true for vessels in which axial tension is off-loaded. In rabbit carotid arteries elongated by interposition of the contralateral carotid artery, the authors found that tortuosity caused by loss of axial strain was not rectified in the months that followed the intervention. Smooth muscle cell proliferation was observed, but this was offset by equivalent cell apoptosis. Likewise, elastin and collagen accumulation was countered by matrix metalloproteinase (MMP) upregulation and activation. But on close observation of the extracellular matrix (ECM), it became apparent that the fenestrae of the internal elastic laminae were greatly enlarged, and indeed the authors found that …

Primary Aldosteronism May Induce Pronounced Changes in the Extracellular Matrix in Human Subcutaneous Small Resistance Arteries

Primary Aldosteronism May Induce Pronounced Changes in the Extracellular Matrix in Human Subcutaneous Small Resistance Arteries

Only weak vasorelaxant properties of loop diuretics in isolated resistance arteries from man, rat and guinea pig

Besides their diuretic action, loop diuretics may induce a rapid vasodilator effect that contribute to their short-term therapeutic properties. We examined the effects of furosemide (10 −6–10 −3 mol l −1) in comparison with bumetanide (10 −6–10 −4 mol l −1) on isolated resistance arteries from rat and guinea pig mesentery and human subcutaneous fat, and investigated the mechanism of the acute direct vasorelaxant action on an isometric microvascular myograph. Both loop diuretics induced concentration-dependent relaxation of resistance vessels irrespective of membrane potential. The maximal effect of furosemide was greatest in rat and least in human arteries. Both diuretics caused a rightward shift in the concentration–response curve to extracellular Ca 2+. Incubation with indomethacin (2×10 −5 mol l −1) or mechanical removal of the endothelium did not inhibit the loop diuretic-induced relaxation. At high concentrations (10 −4–10 −3 mol l −1) loop diuretics exert only weak direct relaxant effects on isolated human subcutaneous resistance arteries compared to the vasorelaxant effects in rat and guinea pig mesenteric vessels.

Endothelial function is preserved in pregnant women with well-controlled type 1 diabetes.

Pregnant women with diabetes mellitus have a higher incidence of adverse pregnancy outcomes. Vascular, and in particular, endothelial function may be significantly modified in diabetes resulting in impaired endothelium-dependent relaxation. This study aims to investigate endothelium-dependent relaxation in pregnant women with pre-existing type I diabetes mellitus. Small arteries (mean luminal diameter approximately 295 microm) were isolated from biopsies of subcutaneous fat from pregnant women with pre-existing type I diabetes mellitus, non-diabetic pregnant women, and non-diabetic non-pregnant women. Endothelial and smooth muscle function were determined using wire myography, and the contributions of nitric oxide, vasodilator prostanoid and endothelial hyperpolarisation were studied using specific inhibitors. Arteries obtained from the diabetic pregnant women did not demonstrate any difference in either endothelial or smooth muscle function when compared with non-diabetic pregnant women. The contribution of nitric oxide to endothelium-dependent relaxation was approximately 20% in the pregnant women regardless of whether they were diabetic, and approximately 11% in the non-pregnant women. Endothelial hyperpolarisation appeared to contribute largely to vasorelaxation in human subcutaneous arteries, and was at least twice that of nitric oxide in pregnant women and fivefold greater in non-pregnant women. This study provides evidence that pregnant women with well-controlled pre-existing type 1 diabetes mellitus have both normal endothelial and smooth muscle function. Endothelium-dependent hyperpolarisation appears to play a large role in vascular relaxation in human subcutaneous resistance arteries. This study suggests that the problems associated with diabetic pregnancies are unlikely to be due to vascular dysfunction.

Endothelial function is preserved in pregnant women with well-controlled type 1 diabetes

Objective Pregnant women with diabetes mellitus have a higher incidence of adverse pregnancy outcomes. Vascular, and in particular, endothelial function may be significantly modified in diabetes resulting in impaired endothelium-dependent relaxation. This study aims to investigate endothelium-dependent relaxation in pregnant women with pre-existing type 1 diabetes mellitus. Methods Small arteries (mean luminal diameter ∼295 μm) were isolated from biopsies of subcutaneous fat from pregnant women with pre-existing type 1 diabetes mellitus, non-diabetic pregnant women, and non-diabetic non-pregnant women. Endothelial and smooth muscle function were determined using wire myography, and the contributions of nitric oxide, vasodilator prostanoid and endothelial hyperpolarisation were studied using specific inhibitors. Results Arteries obtained from the diabetic pregnant women did not demonstrate any difference in either endothelial or smooth muscle function when compared with non-diabetic pregnant women. The contribution of nitric oxide to endothelium-dependent relaxation was ∼20% in the pregnant women regardless of whether they were diabetic, and ∼11% in the non-pregnant women. Endothelial hyperpolarisation appeared to contribute largely to vasorelaxation in human subcutaneous arteries, and was at least twice that of nitric oxide in pregnant women and fivefold greater in non-pregnant women. Conclusions This study provides evidence that pregnant women with well-controlled pre-existing type 1 diabetes mellitus have both normal endothelial and smooth muscle function. Endothelium-dependent hyperpolarisation appears to play a large role in vascular relaxation in human subcutaneous resistance arteries. This study suggests that the problems associated with diabetic pregnancies are unlikely to be due to vascular dysfunction.

Mechanism of action of angiotensin II in human isolated subcutaneous resistance arteries

1. Human isolated subcutaneous arteries were mounted in a myograph and isometric tension measured. In some experiments, intracellular calcium [Ca(2+)]i was also measured using fura-2. 2. Angiotensin II (100 pM - 1 microM) increased [Ca(2+)]i and tone in a concentration-dependent manner. The effects of angiotensin II (100 nM) were inhibited by an AT1-receptor antagonist, candesartan (100 pM). 3. Ryanodine (10 microM), had no effect on angiotensin II-induced responses, but removal of extracellular Ca(2+) abolished angiotensin II-induced rise in [Ca(2+)]i and tone. Inhibition of Ca(2+) entry by Ni(2+) (2 mM), also inhibited angiotensin II responses. The dihydropyridine, L-type calcium channel antagonist, amlodipine (10 microM), only partially attenuated angiotensin II responses. 4. Inhibition of protein kinase C (PKC) by chelerythrine (1 microM), or by overnight exposure to a phorbol ester (PDBu; 500 nM) had no effect on angiotensin II-induced contraction. 5. Genistein (10 microM), a tyrosine kinase inhibitor, inhibited angiotensin II-induced contraction, but did not inhibit the rise in [Ca(2+)]i, suggesting that at this concentration it affected the calcium sensitivity of the contractile apparatus. Genistein did not affect responses to norepinephrine (NE) or high potassium (KPSS). 6. A selective MEK inhibitor, PD98059 (30 microM), inhibited both the angiotensin II-induced contraction and rise in [Ca(2+)]i, but had no effect on responses to NE or KPSS. 7. AT1 activation causes Ca(2+) influx via L-type calcium channels and a dihydropyridine-insensitive route, but does not release Ca(2+) from intracellular sites. Activation of tyrosine kinase(s) and the ERK 1/2 pathway, but not classical or novel PKC, also play a role in angiotensin II-induced contraction in human subcutaneous resistance arteries.

Endothelium‐derived hyperpolarizing factor and potassium use different mechanisms to induce relaxation of human subcutaneous resistance arteries

This investigation examined the hypothesis that release of K(+) accounts for EDHF activity by comparing relaxant responses produced by ACh and KCl in human subcutaneous resistance arteries. Resistance arteries (internal diameter 244+/-12 microm, n=48) from human subcutaneous fat biopsies were suspended in a wire myograph. Cumulative concentration-response curves were obtained for ACh (10(-9) - 3x10(-5) M) and KCl (2.5 - 25 mM) following contraction with noradrenaline (NA; 0.1 - 3 microM). ACh (E(max) 99.07+/-9.61%; -LogIC(50) 7.03+/-0.22; n=9) and KCl (E(max) 74.14+/-5.61%; -LogIC(50) 2.12+/-0.07; n=10)-induced relaxations were attenuated (P<0.0001) by removal of the endothelium (E(max) 8.21+/-5.39% and 11.56+/-8.49%, respectively; n=6 - 7). Indomethacin (10 microM) did not alter ACh-induced relaxation whereas L-NOARG (100 microM) reduced this response (E(max) 61.7+/-3.4%, P<0.0001; n=6). The combination of ChTx (50 nM) and apamin (30 nM) attenuated the L-NOARG-insensitive component of ACh-induced relaxation (E(max): 15.2+/-10.5%, P<0.002, n=6) although these arteries retained the ability to relax in response to 100 microM SIN-1 (E(max) 127.6+/-13.0%, n=3). Exposure to BaCl(2) (30 microM) and Ouabain (1 mM) did not attenuate the L-NOARG resistant component of ACh-mediated relaxation (E(max), 76.09+/-8.92, P=0.16; n=5). KCl-mediated relaxation was unaffected by L-NOARG+indomethacin (E(max); 68.1+/-5.6%, P=0.33; n=5) or the combination of L-NOARG/indomethacin/ChTx/apamin (E(max); 86.61+/-14.02%, P=0.35; n=6). In contrast, the combination of L-NOARG, indomethacin, ouabain and BaCl(2) abolished this response (E(max), 5.67+/-2.59%, P<0.0001, n=6). The characteristics of KCl-mediated relaxation differed from those of the nitric oxide/prostaglandin-independent component of the response to ACh, and were endothelium-dependent, indicating that K(+) does not act as an EDHF in human subcutaneous resistance arteries.

Signalling mechanisms underlying the myogenic response in human subcutaneous resistance arteries.

In this study we have examined for the first time the signal transduction mechanisms involved in the generation of pressure-dependent myogenic tone in human small resistance arteries from the subcutaneous vascular bed. Myogenic responses and the subcellular mechanisms involved in the generation of this response were studied on a pressure myograph. Human subcutaneous resistance arteries constricted 14.1+/-1.1% in response to an increases in intraluminal pressure from 40 to 80 mmHg and a further 3.5+/-1.7% in response to the 80-120-mmHg pressure step. Ca(2+) depletion or nifedipine abolished this response, whereas BAY K 8644 increased this response to 20.6+/-2.1% (P<0.05, response vs. control). The phospholipase C inhibitor U-73122 reduced the myogenic response to 2.5+/-1.0% at 80 mmHg (P<0.01, response vs. control) and abolished it at 120 mmHg. Diacylglycerol lipase inhibition with RHC-80267 abolished all myogenic responses to pressure. The protein kinase C (PKC) activator phorbol 12,13-dibutyrate increased the maximal myogenic response to 20.9+/-1.8% (P<0.05, response vs. control), whereas the PKC inhibitor calphostin C abolished myogenic responses. These data show that the generation of pressure-dependent myogenic tone in human subcutaneous arteries is dependent on Ca(2+) influx via voltage operated Ca(2+) channels (VOCCs) and a concomitant requirement for the activation of phospholipase C (PLC), diacylglycerol, and PKC.

Characterisation of angiotensin II receptors in isolated human subcutaneous resistance arteries.

Subcutaneous arteries have been used as a model for resistance arteries, which are potentially involved in enhanced blood pressure (BP) regulation in man. Angiotensin II (Ang II) is an important regulator of tone, acting via type 1 (AT1-) and type 2 (AT2-) receptor subtypes. The aim of this study was to characterise the Ang II receptors in isolated human subcutaneous arteries, using pharmacological and molecular methods. Subcutaneous arteries were obtained from patients undergoing elective gut surgery and were carefully dissected from the abdominal wall. Cylindrical segments were mounted on two L-shaped metal prongs, one of which was connected to a force-displacement transducer for continuous recording of isometric tension. Concentration-response curves to Ang II were constructed in the presence and absence of various selective AT1-receptor antagonists, candesartan, EXP3174, irbesartan and losartan, and the AT2-receptor antagonist, PD 123319. Responses to Ang II were measured as increases in force (mN) and expressed as a percentage of the response to 60 mM of KCl. Ang II caused a concentration-dependent contraction (pEC50=9.45±0.48, Emax=120±13%). Candesartan and EXP3174 caused concentration-dependent depression of the Emax of Ang II without any major shift of pEC50. Losartan and irbesartan caused a significant, dose-dependent rightward shift of the Ang II contraction-response curve in human subcutaneous arteries. The results show that contractile responses of human subcutaneous arteries are mediated via the AT1-receptor. The AT1-receptor antagonists, candesartan and EXP3174, acted in an insurmountable manner, while losartan and irbesartan were surmountable AT1-receptor antagonists. The AT2-receptor antagonist, PD 123319, (10, 100 nM) had no effect on Ang II-induced contraction. This is supported by the positive identification of mRNA for the human AT 1-receptor by RT-PCR.

Functional heterogeneity of large and small resistance arteries isolated from biopsies of subcutaneous fat: Implications for investigation of vascular pathophysiology

Few studies using human subcutaneous resistance arteries acknowledge the possibility of functional heterogeneity in these vessels. Large (∼500 μm) and small (≥200 μm) resistance arteries ( n=11) and veins ( n=5) were identified using physical, structural and functional criteria in 14 biopsies of human gluteal fat. Endothelium-dependent relaxation was not evident in veins, while, unlike small resistance arteries ( E max 95.74±1.86%; −log IC 50 7.28±0.09), large resistance arteries with an intact endothelium failed to respond to acetylcholine. These results suggest that large resistance arteries may lack muscarinic receptors on the endothelium and emphasise the importance of careful vessel selection and characterisation in studies using human resistance arteries.

Differential Responses in Human Subcutaneous and Skeletal Muscle Vascular Beds to Critical Limb Ischaemia

Objectives to investigate the effects of chronic ischaemia on the subcutaneous and the skeletal muscle resistance vasculature. To understand the redistribution of available blood in the ischaemic limb. Methods human subcutaneous and skeletal muscle resistance arteries were obtained from limbs amputated for critical limb ischaemia and studied under isobaric conditions using pressure myography. Morphological measurements of wall and lumen were analysed using light microscopy and image analysis. Vasoconstrictor responses to potassium and adrenoceptor agonists were used to measure functional status. Noradrenaline re-uptake mechanisms and α1-selectivity were investigated. Results both human skeletal muscle and subcutaneous resistance arteries undergo a severe atrophy of the arterial wall in ischaemic conditions. However, whereas subcutaneous resistance arteries become less able to vasoconstrict to adrenoceptor stimulation, the response of skeletal muscle resistance arteries becomes exaggerated and significantly augmented. This is true in response to both the endogenous vasoconstrictor noradraline and the α1-selective adrenoceptor agonist phenylephrine. Conclusions hypersensitivity to circulating catecholamines in the skeletal muscle vascular resistance bed may contribute to the progression of ischaemic disease by differentially diverting available blood to the subcutaneous tissue to the detriment of skeletal muscle perfusion.

Determination of an Optimal Axial‐Length Tension for the Study of Isolated Resistance Arteries on a Pressure Myograph

The effects of longitudinal stretch on vasoreactivity were assessed in isolated, pressurised arteries from the rat mesentery, rat mid-cerebral and human subcutaneous vascular beds. A stretch-dependent increase in reactivity was observed only in rat third order mesenteric arteries. Longitudinal stretch >> 20 % (force equal to 0.23 +/- 0.04 mN) optimises vasoconstrictor responses to noradrenaline and phenylephrine in rat third order mesenteric arteries. Stretch did not affect the vasoconstriction response to depolarisation by 30 mM K+ PSS in any of the arteries studied. Similarly, stretch had no affect on pressure-dependent myogenic responses in rat mid-cerebral arteries. Endothelium-independent and endothelium-dependent mechanisms of vasorelaxation were unaffected by stretch in rat third order mesenteric arteries. Likewise, stretch did not affect vasoreactivity in rat mid-cerebral and human subcutaneous resistance arteries. Our results show that longitudinal stretch in isobaric-mounted rat third order mesenteric arteries is an important methodological consideration. Considering our results, we recommend that isobaric-mounted rat third order mesenteric arteries are stretched >> 20 % to provide optimal experimental conditions for pharmacological studies.

Role of the endothelium in the vasorelaxing influence of histamine on isolated human subcutaneous resistance arteries.

Marked species and regional differences have been reported in the effects of histamine on vascular tone. The ultimate effect is the balance between contractile and relaxing components, originating directly from the smooth muscle cells and indirectly from endothelial cells [1, 2]. Studies on isolated blood vessels analysing the effects of histamine in the human circulation have mainly been performed on largeand medium-calibre conduit arteries [1]. In this study, we analysed the relaxation effect of histamine on small human arteries isolated from subcutaneous fat tissue and the contribution of the endothelial relaxing factors nitric oxide (NO) and prostanoids to this effect.

Mechanics and composition of human subcutaneous resistance arteries in essential hypertension.

Mechanical properties of arteries are altered in some rat models of hypertension, and this may influence peripheral resistance and blood pressure as well as some of the complications of hypertension. It has usually been assumed that arterial wall stiffness is increased in hypertension, although recent studies suggest that this may not necessarily be the case in large arteries. We determined whether the mechanics of human resistance arteries are altered in hypertension. Subcutaneous resistance arteries (lumen diameter<300 microm) were isolated from hypertensive and normotensive subjects of similar ages (46+/-3 and 43+/-4 years, respectively). Vessels were mounted in a pressurized myograph, deactivated, and exposed to intraluminal pressures ranging from 3 to 140 mm Hg. At each pressure, lumen and media dimensions were measured. Media-to-lumen ratio and media width were greater in hypertensive vessels, reducing wall stress (P<0.01), whereas media cross section was similar in vessels from both groups. Isobaric elastic modulus (which is influenced by vessel geometry and by wall component stiffness) was lower in hypertensive vessels (P<0. 01). Stiffness of wall components (slope of incremental elastic modulus versus stress, which is geometry-independent) was significantly lower in hypertensive vessels (8.2+/-0.7) versus normotensive vessels (11.0+/-1.0, P<0.05), whereas distensibility was unchanged. Electron microscopic analysis of the media of the small arteries showed a greater collagen to elastin ratio (P<0.05) in the media of vessels from hypertensive patients. In conclusion, the stiffness of wall components (slope of elastic modulus versus stress) is not increased but is in fact decreased in subcutaneous resistance arteries from patients with mild essential hypertension. Reduced stiffness of resistance arteries from hypertensive patients does not appear to relate to changes in volume density of extracellular matrix components but may be the result of changes in extracellular matrix architecture or cell-matrix attachment, which remains to be established.

Mechanisms involved in the vasorelaxing influence of histamine on isolated human subcutaneous resistance arteries

The effects of histamine were analysed on human subcutaneous small arteries. No effect was seen on non-precontracted preparations. After precontraction (norepinephrine 1 μM and K + 30 mM) histamine potently relaxed the arteries (EC 50=0.3 μM; max. effect=95% relaxation). The histamine H 1 receptor antagonist, pyrilamine (10 μM), had only a small, non-significant inhibitory influence on histamine-induced relaxation while the histamine H 2 receptor antagonist, cimetidine (0.1 mM), had a significant inhibitory influence. Relaxation was completely blocked in the presence of both antagonists. Both 2-pyridylethylamine (histamine H 1 receptor agonist) and dimaprit (histamine H 2 receptor agonist) elicited relaxation. Removal of endothelium reduced the relaxation effects of histamine and 2-pyridylethylamine, but not of dimaprit. Inhibition of nitric oxide synthesis by nitro- l-arginine significantly inhibited histamine-induced relaxation and even more clearly the cimetidine-resistant component. We conclude that histamine potently relaxes human subcutaneous arterioles, and that most probably both muscular histamine H 2 receptors and endothelial histamine H 1 receptors, thus activating nitric oxide release, contribute to the relaxation.

The influence of pregnancy on endothelium-derived nitric oxide mediated relaxations in isolated human resistance vessels.

Pregnancy is associated with drastic hemodynamic adaptations, including a decrease in peripheral resistance. Vascular resistance is substantially influenced by endothelium-derived nitric oxide (NO). This study was designed to investigate whether pregnancy might influence endothelium-derived NO-mediated relaxations in human resistance arteries. Reactivity of isolated human subcutaneous arteries, dissected out of abdominal fat from women who underwent a laparotomy or cesarean section, was studied using a small vessel myograph. Addition of acetylcholine (1 nM-10 microM) or bradykinin (1 nM-10 microM) to precontracted preparations elicited concentration-dependent relaxation responses that were dependent on the presence of the endothelium and were partially inhibited by the NO-synthase inhibitor nitro-L-arginine (0.1 mM). The relaxations to acetylcholine and bradykinin were similar in vessels isolated from pregnant and non-pregnant women. Nitro-L-arginine (0.1 mM) had no influence on basal tone and had a similar inhibitory influence on the endothelium-mediated relaxations in vessels from non-pregnant and pregnant women. These results indicate that the influence of endothelium-derived NO in human subcutaneous resistance arteries is not altered at the end of pregnancy.