Small-diameter Arteries Research Articles

In situ tissue regeneration using a novel tissue-engineered, small-caliber vascular graft without cell seeding

Various types of natural and synthetic scaffolds with arterial tissue cells or differentiated stem cells have recently attracted interest as potential small-caliber vascular grafts. It was thought that the synthetic graft with the potential to promote autologous tissue regeneration without any seeding would be more practical than a seeded graft. In this study, we investigated in situ tissue regeneration in small-diameter arteries using a novel tissue-engineered biodegradable vascular graft that did not require ex vivo cell seeding. Small-caliber vascular grafts (4 mm in diameter) were fabricated by compounding a collagen microsponge with a biodegradable woven polymer tube that was constructed in a plain weave pattern with a double layer of polyglycolic acid (core) and poly-L-lactic acid (sheath) fibers. We implanted these tissue-engineered vascular grafts bilaterally into the carotid arteries of mongrel dogs (body weight, 20-25 kg). No anticoagulation regimen was used after implantation. We sacrificed the dogs 2, 4, 6, and 12 months (n = 4 in each group) after implantation and evaluated the explants histologically and biochemically. All of the tissue-engineered vascular grafts were patent with no signs of thrombosis or aneurysm at any time. Histologic and biochemical examinations showed excellent in situ tissue regeneration with an endothelial cell monolayer, smooth muscle cells, and a reconstructed vessel wall with elastin and collagen fibers. Our study indicated that this novel tissue-engineered vascular graft promoted in situ tissue regeneration and did not require ex vivo cell seeding, thereby conferring better patency on small-caliber vascular prostheses.

Preliminary Study of the Use of Drug-eluting Stents in Atherosclerotic Renal Artery Stenoses 4 mm in Diameter or Smaller

To describe restenosis and clinical outcomes with drug-eluting stents (DESs) and compare them to those of bare metal stents (BMSs) in the treatment of symptomatic atherosclerotic renal artery stenosis (RAS) in the same patients. A retrospective study was performed of all patients with RAS treated with a DES (Taxus Express 2 or Cypher). DESs were used for RASs with luminal vessel diameters of 4 mm or smaller and BMSs were used for those larger than 4 mm. Sixteen patients (eight women; mean age, 72 years +/- 8) underwent treatment of 27 RASs for worsening renal function (n = 10) and uncontrolled hypertension (n = 6). Eighteen RASs were treated with 23 DESs (Cypher, n = 12; Taxus, n = 11) and nine were treated with BMSs. The average follow-up was 22 months +/- 10. After the procedure, the mean systolic blood pressure decreased significantly (P < .05), with no change in the mean diastolic pressure, serum creatinine, or number of antihypertensive medications. By Kaplan-Meier estimates, the 1- and 2-year patency rates for DESs were 78% and 68%, respectively; and for BMSs, the respective rates were 58% and 47% (P = NS). The average diameters of RASs were 3.4 mm +/- 0.6 in the DES group and 5.3 mm +/- 0.6 in the BMS group (P < .05). There were two technical failures (7.7%) in the DES group. There was one minor complication and a non-flow-limiting dissection. DESs were used to treat RASs with good technical results and low restenosis rates compared with BMSs despite the smaller artery diameters in the DES group.

Decellularized, xenogeneic small‐diameter arteries: Transition from a muscular to an elastic phenotype in vivo

Reports regarding the biocompatibility of xenogeneic, decellularized bioprosthetic implants differ between bioinertness and complete graft degradation. We investigated heparin-crosslinked and nonheparinized, xenogeneic vascular substitutes in a rat model. Porcine arteries (15 x 1.5 mm) were decellularized by multistep detergent and enzymatic techniques, which were followed by heparin-crosslinking in 50% of the implants. Prostheses were implanted into the abdominal aorta of 76 rats for 1 day and up to 6 months. Retrieved specimens were evaluated by histology, immunohistochemistry, laser scanning, and scanning electron microscopy. Graft patency did not differ between groups (97.3%). Heparinized grafts showed a statistically significant lower rate of aneurysm formation (p = 0.04 %). Implants revealed infiltration with granulocytes and macrophages up to 3 months. Recellularization with endothelial cells and myofibroblasts was detectable within 1 month. After 6 months elastin biosynthesis and complete graft remodeling toward an elastic vessel was evident. These results indicate that temporary inflammation does not interfere with long-term vascular remodeling.

Coronary Flow Reserve and Small Artery Remodelling in Hypertensive Patients

Objective: To evaluate the relationship between subcutaneous small arteries structure and coronary flow reserve in patients with essential hypertension. Methods: Eighteen patients with mild to moderate essential hypertension and normal epicardial coronary arteries were included in the study and underwent a biopsy of the subcutaneous fat from the gluteal region. Subcutaneous small arteries were dissected and mounted on a micromyograph. The media thickness, the normalized internal diameter and the media: lumen ratio were then calculated. Measurement of coronary flow velocity in the left anterior descending artery before and during maximal pharmacological vasodilatation was performed by transesophageal Doppler echocardiography. Results: Mean and peak coronary flow reserve (CFR) as well as minimum coronary resistance were significantly correlated to both media: lumen ratio and normalized internal diameter of subcutaneous small arteries. Those patients with a mean CFR (mCFR) of <2.69 (i.e. 1 SD below the mean of values in our normal reference subjects) had a greater media: lumen ratio and a smaller internal diameter compared with those with a mCFR of ≥2.69. Similarly, patients with a media: lumen ratio of ≥0.109 had a smaller mCFR compared with those with a media: lumen ratio of <0.109. Conclusions: These results are in keeping with the hypothesis of a generalized remodelling of small arteries, that may also be responsible for a reduced coronary vasodilator capacity even in patients with mild to moderate essential hypertension.

Reconstruction of Small Diameter Arteries with Using Bio-scaffold

Reconstruction of Small Diameter Arteries with Using Bio-scaffold

Physiological roles of K&lt;sup&gt;+&lt;/sup&gt; channels in vascular smooth muscle cells

In this review, we present the basic properties, physiological functions, regulation, and pathological alterations of four major classes of K+ channels that have been detected in vascular smooth muscle cells. Voltage-dependent K+ (Kv) channels open upon depolarization of the plasma membrane in vascular smooth muscle cells. The subsequent efflux of K+ through the channels induces repolarization to the resting membrane potential. Changes in the intracellular Ca2+ concentration and membrane depolarization stimulate large-conductance Ca2+-activated K+ (BKCa) channels, which are thought to play an important role in maintaining the membrane potential. ATP-sensitive K+ (K(ATP)) channels underscore the functional bond between cellular metabolism and membrane excitability. The blockade of KATP channel function results in vasoconstriction and depolarization in various types of vascular smooth muscle. Inward rectifier K+ (Kir) channels, which are expressed in smooth muscle of the small-diameter arteries, contribute to the resting membrane potential and basal tone. Kir channel activation has been shown to raise the extracellular K+ concentration to 10-15 mM, resulting in vasodilation. Each of K+ channels listed above is responsive to a number of vasoconstrictors and vasodilators, which act through protein kinase C (PKC) and protein kinase A (PKA), respectively. Impaired Kv, KATP, and Kir channel functions has been linked to a number of pathological conditions, which may lead to vasoconstriction.

Abstract 3057: Tissue Engineered Three-layered Artery Using Hemodynamically-equivalent Pulsatile Bioreactor

Introduction: Due to lack of anti-thrombogenicity in small diameter vascular prostheses, there is a strong demand for tissue-engineering autologous small-diameter graft, which can function in arterial high pressure and flow environments. Here, we investigated a potential to engineer a robust and three-layered artery. Methods: Endothelial cells (ECs), smooth muscle cells (SMCs) and fibroblasts were isolated with collagenase from bovine artery, then each cell were primary cultured. A polyglycolic acid (PGA) sheet seeded with SMCs was wrapped on silicon tube with a diameter of 6 mm. After 4 days of incubation, a polycaprolactone sheet seeded with SMCs was over-wrapped. Then, after incubation for another 3 weeks, a PGA sheet seeded with fibroblasts was over-wrapped and incubated for another 2 days. The luminal tube was then removed out, and the tubular tissue was mounted on a specially designed chamber. The luminal areas of engineered tubular structure were seeded with half a million of ECs and incubated for another 2days. Finally, bioreactor culture was performed for more 2weeks (n=10). Pulsatile flow and pressure were gradually increased from mean flow rate of 0.2(0.5/0) L/min and mean pressure of 20(40/15) mmHg to 0.6(1.4/0.2) L/min and 100(120/80) mmHg. Results: Scanning electron microscopic examination and von Willebrand factor stain revealed that ECs lined over the lumen and oriented by flow stimulation. Stain of Elastica Van Gieson and Masson Tricrome showed that a lot of elastin components and collagen were produced in the tissue engineered grafts. Stain for α-SMA, calponin, and vimentin showed that engineered arteries were constructed from SMC and fibroblasts regions and morphologically organized as three layered. Tensile tests demonstrated that ultimate strength of engineered arteries were increased by 6.7 times as compared with that of biodegradable polymers (n=8) and become equivalent to that of the native arteries (Native: 882±133kPa, Engineered: 827±155kPa) (each n=8). Conclusion: A robust and three-layered small-diameter elastic artery was engineered from vascular cells and biodegradable polymers using the hemodynamically-equivalent pulsatile bioreactor.

Morning rise of blood pressure and subcutaneous small resistance artery structure

It has been previously demonstrated that the morning rise (MoR) of blood pressure (BP) may predict major cardiovascular events in hypertensive patients. Structural alterations of small resistance arteries, as evaluated by the tunica media to internal lumen ratio (M/L) of subcutaneous small resistance arteries, may also predict cardiovascular events. Because an increased M/L may amplify the effect of hypertensive stimuli, the present study aimed to evaluate the possible relationships between MoR and M/L in a population of hypertensive patients. Sixty-four patients with essential hypertension were included in the present study. All patients were submitted to a biopsy of subcutaneous fat. Small resistance arteries were dissected and mounted on an isometric myograph, and the M/L was measured. In addition, MoR was calculated from ambulatory blood pressure monitoring (ABPM) according to four previously published different methods (MoR1 to MoR4). A statistically significant correlation was observed between M/L and MoR1 (r = 0.52, P < 0.001), MoR2 (r = 0.32, P < 0.01), MoR3 (r = 0.25, P < 0.05) and MoR4 (r = 0.27, P < 0.05), as well as between internal diameter of subcutaneous small arteries and MoR1 (r = -0.45, P < 0.001) and MoR2 (r = -0.28, P < 0.05). Our results indicate that subcutaneous small artery structure is related to MoR, possibly because an altered vascular structure may amplify BP changes or, vice versa, because a greater MoR may further damage peripheral vasculature.

Increased Inhibition of Inward Rectifier K + Channels by Angiotensin II in Small-Diameter Coronary Artery of Isoproterenol-Induced Hypertrophied Model

We investigated the effects of angiotensin II (Ang II) on inward rectifier K+ (Kir) channels in small-diameter coronary arterial smooth muscle cells (SCASMCs) of control and isoproterenol (Iso)-induced hypertrophied rabbits. Kir current amplitude and Kir channel protein expression were definitely lower in the Iso-induced hypertrophied model than in the control. In a pressurized arterial experiment, 15 mmol/L K+-induced vasodilation was greater in the control arteries than in the arteries of Iso-induced hypertrophied model. Ang II reduced the Kir current in a concentration-dependent manner, and this inhibition was greater in SCASMCs from Iso-induced hypertrophied model than from control. Although, there was no difference in the expression of Ang II type 2 (AT2) receptor between SCASMCs of control and Iso-induced hypertrophied model, the expression of Ang II type 1 (AT1) receptor and phosphorylated PKC alpha were greater in SCASMCs of Iso-induced hypertrophied model than of control. Ang II inhibits Kir channels more prominently in SCASMCs of Iso-induced hypertrophied model owing to increases in the expression of AT1 receptor and the activation of PKC alpha. Our findings about the differential expression of Kir channels and different modulation of Kir channels by a vasoconstrictor (Ang II) in a hypertrophy model are important for better understanding the responsiveness of small-diameter arteries during hypertrophy.

Modifications of Arterial Phenotype in Response to Amine Oxidase Inhibition by Semicarbazide

Semicarbazide-sensitive amine oxidase (SSAO)-deficient mice present no alteration in elastin cross-linking processes and carotid mechanical properties. In contrast, previous studies have shown that SSAO inhibitors induced marked anomalies in arterial structure and function. The aim of the present study was to examine the effect of semicarbazide (SCZ), an efficient SSAO inhibitor, on the arterial phenotype of the carotid artery in relation to modulation of SSAO and lysyl oxidase activities in growing rats. We first show that after 6 weeks of SCZ treatment (100 mg/kg per day), SSAO activity was reduced by 90%, whereas lysyl oxidase activity was only partially inhibited (<60%) in carotid artery, compared with controls. There was significant growth inhibition and no difference in mean arterial pressure but an increase in pulse pressure with a smaller arterial diameter in SCZ-treated rats. SCZ decreased aortic insoluble elastin without a change in total collagen. In addition, extracellular proteins other than insoluble elastin and collagen were increased in SCZ-treated rats. All of the elastic lamellae presented globular masses along their periphery, and focal disorganization was observed in the ascending aorta. Carotid artery mechanical strength was lower in SCZ-treated rats, and the elastic modulus-wall stress curve was shifted leftward compared with controls, indicating increased stiffness. Thus, SCZ modifies arterial geometry and mechanical properties, alters elastic fiber structure, and reduces the content of cross-linked elastin. Because these abnormalities are essentially absent in SSAO-deficient mice, our results suggest that lysyl oxidase inhibition is responsible for the major part of the vascular phenotype of SCZ-treated rats.

Magnetic Guide-Wire Navigation in Pulmonary and Systemic Arterial Catheterization: Initial Experience in Pigs

Cardiovascular catheterization can be challenging whenever a stenosis or an abnormal vascular course interferes with probing the target vessel. This study addresses the feasibility of navigating a guide wire with a magnetic tip by an external magnetic field through pulmonary and systemic arteries in an experimental porcine model. We investigated six piglets using magnetic guide-wire navigation. Two pulmonary arteriograms were taken from different angles in order to reconstruct the three-dimensional vessel anatomy. A computer interface then calculated three-dimensional coordinates for the vessel in space. Using these coordinates, two external magnets were positioned to create magnetic vectors along the expected vessel course. Magnetically enabled guide wires were then navigated into the vessels using the magnetic field to orient the guide-wire tips. Aortic and renal branches were addressed in a similar fashion. Difficulty in reaching the target vessel was reflected by the number of attempts that were necessary. After 10 failed attempts, the maneuver was recorded to have failed. Thirty-five of 37 (94.6%) arteries with branches at acute angles were reached successfully using magnetic navigation. In two pigs, the left upper lobe artery could not be probed. Peripheral arteries of small diameter were easier to reach than large central arteries, requiring less attempts. Magnetic guide-wire navigation is feasible in the arteries of the lungs, the head and neck, and the kidneys. It is particularly useful in entering small arterial branches at acute angles and may facilitate interventional therapy in a variety of vascular diseases in children and adults.

Development of Tissue Engineered Vascular Grafts

Vascular bypass grafting is a commonly performed procedure for ischemic heart disease and peripheral vascular disease. However, approximately one in fourteen patients do not have suitable autologous arteries or veins available for grafting. Synthetic vascular grafts were introduced in the 1960s to overcome these problems, but while they perform adequately in high-flow, large-diameter vessel settings they are generally not suited to low-flow, small-diameter vessels. Tissue engineering is a relatively new discipline that offers the potential to create replacement structures from autologous cells and biodegradable polymer scaffolds. Because tissue engineering constructs contain living cells, they may have the potential to grow, self-repair, and self-remodel. Therefore, recently there has been much interest in the use of this technique to produce low-flow small-diameter arteries. The latest and most exciting developments in this area involve the use of multipotent stem cells as a cell source for tissue engineering of vascular grafts (both in vivo and in vitro).

215 脱細胞血管スキャフォールドによる小口径血管の再建(OS11-3:マイクロ・ナノバイオメカニクス,オーガナイズドセッション11:マイクロ・ナノバイオメカニクス)

215 脱細胞血管スキャフォールドによる小口径血管の再建(OS11-3:マイクロ・ナノバイオメカニクス,オーガナイズドセッション11:マイクロ・ナノバイオメカニクス)

TISSUE ENGINEERING OF SMALL-DIAMETER ARTERIES USING HUMAN ADIPOSE STEM CELLS ON AN ELECTROSPUN SCAFFOLD.

Introduction Cardiovascular disease is the leading cause of mortality in the United States. Over half of these patients die from coronary artery disease. This study uses novel bioengineering methods toward the development of functional, biocompatible small-diameter arteries grown in vitro for use as bypass grafts in treating coronary artery disease. Methods The scaffold for the artificial artery was made by electrospinning collagen (50 mg/mL) and elastin (25 mg/mL) dissolved in 1,1,1,3,3,3-hexafluoro-2-propanol (HFP). Gluteraldehyde was added to the solution to mediate fiber cross-linking. Human adipose stem cells (hASCs) were isolated from lipoaspirate samples and subjected to various differentiation protocols to produce smooth muscle cells (SMCs) and endothelial cells (ECs), the two cellular components of native arteries. The electrospun scaffold was seeded with hASCs to determine cell viability. Results Scanning electron micrographs (SEMs) showed electrospun fibers. Mechanical testing of the sheet of fibers showed a tensile strength of 6.23 MPa. Immunofluorescence staining demonstrated that hASCs were positive for SMC markers: SM-α-actin, h-caldesmon, calponin, and SM-myosin. At confluence, hASCs formed a network of branched tube-like structures and were positive for EC markers: CD31, CD144, and vWF. SEMs showed that hASCs populated the electrospun scaffold, produced their own extracellular matrix, and migrated into the fiber structures. Conclusion and Future Directions This work has shown that the electrospun collagen and elastin can be used as a durable scaffold, the hASCs can differentiate into SMCs and ECs, and the hASCs can attach to and proliferate within the electrospun scaffold. Future studies will consist of purifying the SMC and EC population from the hASCs, seeding the purified SMCs on a tubular electrospun scaffold under pulsatile flow conditions for 4 to 6 weeks, seeding the purified ECs on the luminal side of the tubular electrospun scaffold for 3 to 7 days under the same conditions, testing the mechanical properties of this cellular construct, and performing in vivo studies by implanting and immunostaining the construct to determine functionality and biocompatibility.

In vivo guided regeneration of small diameter arteries

In vivo guided regeneration of small diameter arteries

Development of a Tissue-Engineered Bypass Graft Seeded with Stem Cells

The gold standard conduit for bypass of diseased small-diameter arteries remains autologous vascular tissue. In the absence of such tissue, patients are offered bypass with prosthetic material, with far less durable results. Vascular tissue engineering, the creation of a vascular conduit by seeding a tubular scaffold with various cells, may offer an alternative approach to this difficult situation. Herein we review some of the significant challenges that remain in designing an ideal vascular conduit and outline potential solutions offered by a graft created by seeding natural vascular tissue (decellularized vein allograft) with readily available autologous cells (adipose-derived stem cells).

MECHANISM OF INCREASED Α1‐ADRENOCEPTOR‐MEDIATED CONTRACTION IN SMALL RESISTANCE ARTERIES OF RATS WITH HEART FAILURE

1. Alterations in a(1)-adrenoceptor signalling that result in enhanced contraction in resistance arteries in heart failure are not well characterized. To clarify whether this enhanced constriction is due to Ca(2+)-dependent or -independent effects, we measured the phenylephrine-induced changes in [Ca(2+)](i) in the presence of a Rho kinase inhibitor or an inositol 1,4,5-trisphosphate (IP(3)) receptor inhibitor. 2. Heart failure was induced in rats by ligation of the left coronary artery. Changes in the internal diameter of pressurized small femoral arteries were examined using videomicroscopy. Phenylephrine concentration-response curves, constructed in the presence of the Rho kinase inhibitor Y27632 (0.3 micromol/L) or the IP(3) receptor inhibitor xestospongin C (0.3 micromol/L), were compared in heart failure rats and sham-operated (control) rats; fura-2 Ca(2+) signals were measured in the arteries of both groups. 3. The heart : bodyweight ratio, lung : bodyweight ratio, left ventricular end-diastolic pressure and plasma B-type natriuretic peptide were significantly higher in heart failure rats compared with control rats. Phenylephrine-induced contractile responses and increases in [Ca(2+)](i) were significantly greater in arteries from heart failure rats compared with arteries from control rats. At 0.3 micromol/L, Y27632 selectively inhibited phenylephrine-induced constrictions of heart failure arteries, but had no effect on the increase in [Ca(2+)](i). 4. Immunohistochemical staining for Rho kinase was greater in heart failure rats compared with control rats. 5. The degree of inhibition of both the phenylephrine-induced constriction and the increase in [Ca(2+)](i) by xestospongin C (0.3 micromol/L) was greater in arteries from heart failure rats than in those from control rats. 6. The increased contractile response to phenylephrine in arteries of heart failure rats results from IP(3)-dependent increases in [Ca(2+)](i) and from an enhanced Ca(2+) sensitivity via a Rho kinase-dependent mechanism.

Small-Diameter Artificial Arteries Engineered In Vitro

Although the need for a functional arterial replacement is clear, the lower blood flow velocities of small-diameter arteries like the coronary artery have led to the failure of synthetic materials that are successful for large-diameter grafts. Although autologous vessels remain the standard for small diameter grafts, many patients do not have a vessel suitable for use because of vascular disease, amputation, or previous harvest. As a result, tissue engineering has emerged as a promising approach to address the shortcomings of current therapies. Investigators have explored the use of arterial tissue cells or differentiated stem cells combined with various types of natural and synthetic scaffolds to make tubular constructs and subject them to chemical and/or mechanical stimulation in an attempt to develop a functional small-diameter arterial replacement graft with varying degrees of success. Here, we review the progress in all these major facets of the field.

Vascular reactivity of mesenteric arteries and veins to endothelin-1 in a murine model of high blood pressure

We characterized vascular reactivity to endothelin-1 (ET-1) in mesenteric vessels from DOCA-salt hypertensive and SHAM control mice and assessed the effect that endothelial-derived vasodilators have on ET-1-induced vasoconstriction. Changes in the diameter of unpressurized small mesenteric arteries and veins (100- to 300-μm outside diameter) were measured in vitro using computer-assisted video microscopy. Veins were more sensitive than arteries to the contractile effects of ET-1. There was a decrease in arterial maximal responses ( E max) compared to veins, this effect was larger in DOCA-salt arteries. The selective ET B receptor agonist, sarafotoxin 6c (S6c), contracted DOCA-salt and SHAM veins but did not contract arteries. The ET B receptor antagonist, BQ-788 (100 nM), but not the ET A receptor antagonist, BQ-610 (100 nM), blocked S6c responses. BQ-610 partially inhibited responses to ET-1 in mesenteric veins from DOCA-salt and SHAM mice while BQ-788 did not affect responses to ET-1. Co-administration of both antagonists inhibited responses to ET-1 to a greater extent than BQ-610 alone suggesting a possible functional interaction between ET A and ET B receptors. Responses to ET-1 in mesenteric arteries were completely inhibited by BQ-610 while BQ-788 did not affect arterial responses. Nitric oxide synthase inhibition potentiated ET-1 responses in veins from SHAM but not DOCA-salt mice. There was a prominent role for ET-mediated nitric oxide release in DOCA-salt but not SHAM arteries. In summary, these studies showed a differential regulation of ET-1 contractile mechanisms between murine mesenteric arteries and veins.

Contribution of Na+-K+ pump and KIR currents to extracellular pH-dependent changes of contractility in rat superior mesenteric artery

We compared the branches and trunk of rat superior mesenteric artery (SMA) with respect to extracellular pH (pHo)-dependent changes in vascular contractility. Decreases in pHo from 7.8 to 6.4 significantly reduced apparent affinity (pD2) to norepinephrine (NE) and maximal contraction by NE, which were more prominent in larger-diameter arteries. On the other hand, decreases in pHo significantly reduced Ba2+-sensitive K+-induced relaxation (which was evoked by elevation of extracellular K+ concentration from 6 to 12 mM) in the first branch and inhibited inwardly rectifying K+ (K(IR)) currents in cultured smooth muscle cells (SMCs) of SMA. RT-PCR revealed transcripts for Kir2.1 in the SMCs. Real-time PCR analysis revealed 6.1-, 3.3-, and 2.2-fold increases in the Kir2.1 mRNA-to-beta-actin mRNA ratios of SMCs of the third, second, and first branches, respectively, vs. the corresponding relative levels of trunk SMCs. The magnitudes of K+-induced relaxation were significantly greater in smaller-diameter arteries, and there was a strong correlation between the transcript levels of Kir2.1 and K+-induced relaxation. A decrease in pHo reduced ouabain-sensitive K+-induced relaxation and ouabain-induced contraction. A decrease in pHo from 7.4 to 6.4 depolarized membrane potential of the cultured SMCs. From these results, we conclude that an increase in pHo activates K(IR) currents and the Na+ -K+ pump, which then reduces vascular contractility. Inasmuch as K(IR) channel densities are significantly greater in smaller-diameter arteries, the reduction in vascular contractility on increasing pHo is more pronounced in smaller-diameter arteries.