The effects of potassium channel openers on saphenous vein exposed to arterial flow

In Vitro Effects of Eicosapentaenoic and Docosahexaenoic Acid on the Vascular Tone of a Human Saphenous Vein: Influence of Precontractile Agents

BACKGROUND

Cardiovascular effects of omega-3 polyunsaturated fatty acids including eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA) have been widely reported. However, there are limited studies concerning their effects on human blood vessels. Therefore, the aim of this study was to investigate the direct vascular effects of EPA and DHA on the human saphenous vein (SV) precontracted with either prostaglandin F2α (PGF2α), or thromboxane A2 analogue (U46619), or norepinephrine (NE). Moreover, we aimed to investigate the protein expression of free fatty acid receptor 4 (FFAR4) in human SV.

METHODS

Pretreatment of human SV rings with EPA and DHA (100 μM, 30 min) was tested on vascular reactivity induced by PGF2α (10 nM to 5 μM), NE (10 nM to 100 μM), and U46619 (1 nM to 100 nM). In addition, direct relaxant effects of EPA/DHA (1-100 μM) were tested in human SV rings precontracted by PGF2α, NE, and U46619. Furthermore, the involvement of potassium channels on their vascular effects was investigated in the presence of the nonselective K⁺ channel inhibitor tetraethylammonium chloride.

RESULTS

Pretreatment with EPA and DHA resulted in a significant decrease in vascular reactivity induced by U46619 and PGF2α compared to NE. In the presence of TEA, the relaxant effects of EPA and DHA were significantly decreased in SV preparations precontracted by U46619 and PGF2α for DHA. Furthermore, FFAR-4 protein was expressed in tissue extracts of human SV.

CONCLUSIONS

Our study demonstrates that both EPA and DHA reduce the increased vascular tone elicited by contractile agents on the human SV and that the direct vasorelaxant effect is likely to involve potassium channels.

Varicose veins: role of mechanotransduction of venous hypertension

Varicose veins affect approximately one-third of the adult population and result in significant psychological, physical, and financial burden. Nevertheless, the molecular pathogenesis of varicose vein formation remains unidentified. Venous hypertension exerted on veins of the lower extremity is considered the principal factor in varicose vein formation. The role of mechanotransduction of the high venous pressure in the pathogenesis of varicose vein formation has not been adequately investigated despite a good progress in understanding the mechanomolecular mechanisms involved in transduction of high blood pressure in the arterial wall. Understanding the nature of the mechanical forces, the mechanosensors and mechanotransducers in the vein wall, and the downstream signaling pathways will provide new molecular targets for the prevention and treatment of varicose veins. This paper summarized the current understanding of mechano-molecular pathways involved in transduction of hemodynamic forces induced by blood pressure and tries to relate this information to setting of venous hypertension in varicose veins.

Endothelium-dependent nitric oxide and hyperpolarization-mediated venous relaxation pathways in rat inferior vena cava

INTRODUCTION

The vascular endothelium plays a major role in the control of arterial tone; however, its role in venous tissues is less clear. The purpose of this study was to determine the role of endothelium in the control of venous function and the relaxation pathways involved.

METHODS

Circular segments of inferior vena cava (IVC) from male Sprague-Dawley rats were suspended between two wires and isometric contraction to phenylephrine (Phe; 10(-5)M) and 96 mM KCl was measured. Acetylcholine (Ach; 10(-10) to 10(-5)M) was added and the percentage of venous relaxation was measured. To determine the role of nitric oxide (NO) and prostacyclin (PGI(2)), vein relaxation was measured in the presence of the nitric oxide synthase inhibitor N(ω)-nitro-L-arginine methyl ester (L-NAME; 3 × 10(-4) M) and the cyclooxygenase inhibitor indomethacin (10(-5) M). To measure the role of hyperpolarization, vein relaxation was measured in the presence of K(+) channel activator cromakalim (10(-11) to 10(-6) M), and the nonselective K(+) channel blocker tetraethylammonium (TEA; 10(-3) M). To test for the contribution of a specific K(+) channel, the effects of K(+) channel blockers: glibenclamide (adenosine triphosphate [ATP]-sensitive K(ATP), 10(-5) M), 4-aminopyridine (4-AP; voltage-dependent K(v), 10(-3) M), apamin (small conductance Ca(2+)-dependent SK(Ca), 10(-7) M), and iberiotoxin (large conductance Ca(2+)-dependent BK(Ca), 10(-8) M) on Ach-induced relaxation were tested.

RESULTS

Ach caused concentration-dependent relaxation of Phe contraction (maximum 49.9 ± 4.9%). Removal of endothelium abolished Ach-induced relaxation. IVC treatment with L-NAME partially reduced Ach relaxation (32.8 ± 4.9%). In IVC treated with L-NAME plus indomethacin, significant Ach-induced relaxation (33.6 ± 3.2%) could still be observed, suggesting a role of endothelium-derived hyperpolarizing factor (EDHF). In IVC treated with L-NAME, indomethacin and TEA, Ach relaxation was abolished, supporting a role of EDHF. In veins stimulated with high KCl, Ach caused relaxation (maximum 59.5 ± 3.5%) that was abolished in the presence of L-NAME and indomethacin suggesting that any Ach-induced EDHF is blocked in the presence of high KCl depolarizing solution, which does not favor outward movement of K(+) ion and membrane hyperpolarization. Cromakalim, an activator of K(ATP), caused significant IVC relaxation when applied alone or on top of maximal Ach-induced relaxation, suggesting that the Ach response may not involve K(ATP). Ach-induced relaxation was not inhibited by glibenclamide, 4-AP, or apamin, suggesting little role of K(ATP), K(v) or SK(Ca), respectively. In contrast, iberiotoxin significantly inhibited Ach-induced relaxation, suggesting a role of BK(Ca).

CONCLUSIONS

Thus, endothelium-dependent venous relaxation plays a major role in the control of venous function. In addition to NO, an EDHF pathway involving BK(Ca) may play a role in endothelium-dependent venous relaxation.

Calf venous compliance in multiple system atrophy

In multiple system atrophy (MSA), increased venous compliance with excessive venous pooling is assumed to be a major contributor to orthostatic hypotension (OH); however, venous compliance has never been assessed in MSA patients. We evaluated the severity and distribution of adrenergic, cardiovagal, and sudomotor failure in 11 patients with probable MSA, 14 age- and sex-matched control subjects, and 8 patients with Parkinson's disease (PD) but not OH. Calf venous compliance, venous filling, and capillary filtration were measured using calf plethysmography. The response to the directly acting α-adrenergic stimulation (10 mg midodrine) on calf venous compliance was additionally evaluated. Contrary to our hypothesis, pressure-volume curves in the legs of MSA patients were flatter than in PD patients ( P < 0.05) or controls ( P < 0.001); this indicated reduced calf venous compliance in MSA. The MSA group had reduced venous filling compared with control ( P < 0.001) or PD subjects ( P < 0.001) but had a normal capillary filtration rate ( P = 0.73). Direct α-adrenergic stimulation resulted in a slight but significant reduction of calf venous compliance in controls ( P = 0.001) and PD subjects ( P < 0.001) but not in the MSA group. The compliance change in MSA significantly regressed with autonomic failure (composite autonomic severity scale, r2 = 0.56) but not with parkinsonism (Unified MSA Rating Scale, r2 = 0.12). Our data indicate that MSA patients with chronic OH have reduced, rather than increased, venous compliance in the lower leg. We postulate that chronic venous distension that is associated with OH results in structural remodeling of veins, leading to reduced compliance, a change which may protect patients against orthostatic stress.

Matrix metalloproteinase 2-induced venous dilation via hyperpolarization and activation of K+ channels: relevance to varicose vein formation

BACKGROUND

Varicose veins are a common disorder of extensive venous dilation and remodeling with an as-yet unclear mechanism. Studies have shown increased plasma and tissue levels of matrix metalloproteinases (MMPs) in human varicose veins and animal models of venous hypertension. Although the effects of MMPs are generally attributed to extracellular matrix degradation, their effects on the mechanisms of venous contraction/relaxation are unclear. Our preliminary experiments have demonstrated that MMP-2 causes inhibition of phenylephrine-induced venous contraction. The purpose of this study was to determine whether MMP-induced inhibition of venous contraction involves an endothelium-dependent and/or -independent pathway.

METHODS

Circular segments of the inferior vena cava (IVC) were isolated from male Sprague-Dawley rats and suspended between two wire hooks in a tissue bath, and the effects of MMP-2 on phenylephrine- and KCl-induced contraction were measured. To study the role of endothelium-derived vasodilators, experiments were performed in the presence and absence of endothelium; N(G)-l-nitro-arginine methyl ester (L-NAME), an inhibitor of nitric oxide synthesis; indomethacin, an inhibitor of prostacyclin synthesis; cromakalim, an activator of adenosine triphosphate-sensitive K+ channels (K(ATP)); and iberiotoxin, a blocker of large-conductance Ca2+-dependent K+ channels (BK(Ca)) and smooth muscle hyperpolarization.

RESULTS

In endothelium-intact IVC segments, phenylephrine (10(-5) mol/L) caused significant contraction that slowly declined to 82.0% in 30 minutes. The addition of MMP-2 (1 microg/mL) caused a gradual decrease of phenylephrine contraction to 39.5% at 30 minutes. In endothelium-denuded IVC, MMP-2 induced a greater reduction of phenylephrine contraction, to 7.6%. In the presence of L-NAME (10(-4) mol/L), MMP-2 caused a marked decrease in phenylephrine contraction, to 4.4%. Large MMP-2-induced inhibition of phenylephrine contraction was also observed in IVC treated with L-NAME plus indomethacin. MMP-2 caused relaxation of phenylephrine contraction in IVC pretreated with cromakalim (10(-7) mol/L), an activator of K(ATP) channels. MMP-2-induced inhibition of phenylephrine contraction was abrogated in the presence of iberiotoxin (10(-8) mol/L), a blocker of BK(Ca). MMP-2 did not inhibit venous contraction during membrane depolarization by 96 mmol/L KCl, a condition that prevents outward K+ conductance and cell hyperpolarization.

CONCLUSIONS

MMP-2 causes significant IVC relaxation that is potentiated in the absence of endothelium or during blockade of endothelium-mediated nitric oxide and prostacyclin synthesis. The lack of effects of MMP-2 on KCl contraction and in iberiotoxin-treated veins suggests MMP-2-induced smooth muscle hyperpolarization and activation of BK(Ca) channels--a novel effect of MMP that may play a role in the early stages of venous dilation and varicose vein formation.

The potassium channel opener levcromakalim causes expansive remodelling of experimental vein grafts

BACKGROUND

Maintenance of luminal area is essential for the optimal performance of venous bypass grafts. However, injury and response to the arterial circulation evoke vascular remodelling that favors intimal hyperplasia, with luminal encroachment and inward remodelling. Potassium channel-opening drugs reduce tissue workload and peripheral vascular resistance and through these mechanisms could favor outward or expansive remodelling of vein grafts. We tested the hypothesis that levcromakalim, a potassium channel opener, would enhance expansive remodelling in vein grafts.

METHODS

A randomized, double-blind, placebo-controlled trial was conducted in 33 rats with vena cava-to-aorta bypass grafts. Drugs were administered via osmotic pump for 7 days after surgery. Half the cohort had bromodeoxyuridine (BrdU) infused at day 6. Morphometric analysis was conducted of pressure perfusion-fixed grafts harvested at 1 week and 4 weeks.

RESULTS

At 1 week, lumen area was similar in both groups (1.82 +/- 0.39 mm(2) placebo vs 1.85 +/- 0.36 mm(2) levcromakalim), although medial cell density and BrdU staining were significantly increased in the placebo group. At 4 weeks, lumen area was unchanged in the placebo group (1.88 +/- 0.51 mm(2)) but had increased to 2.32 +/- 0.46 mm(2) in the levcromakalim group (P = .039 vs 1 week), with a very significant reduction in the intimal area (levcromakalim, 0.06 +/- 0.02 mm(2) vs placebo, 0.33 +/- 0.17 mm(2); P = .001).

CONCLUSIONS

Early, short-term treatment with levcromakalim favors expansive remodelling of experimental vein grafts to mimic the effect of external stenting. This expansive remodelling was associated with a reduction in medial cell proliferation at 1 week.

CLINICAL RELEVANCE

Critical limb ischemia can be treated by bypass surgery or angioplasty, but inward remodelling with restenosis is a common problem. There has been little previous experimental work to identify treatments associated with expansive remodelling, which would increase the chances of vessel patency. Here, in a randomized trial, we show that short-term treatment with a potassium channel opener (a class of drug that can be used to treat hypertension) results in strong, expansive remodelling, with increases the lumen area and graft size of experimental vein grafts by >25%.

F-actin capping (CapZ) and other contractile saphenous vein smooth muscle proteins are altered by hemodynamic stress: a proteonomic approach

Increased force generation and smooth muscle remodeling follow the implantation of saphenous vein as an arterial bypass graft. Previously, we characterized and mapped 129 proteins in human saphenous vein medial smooth muscle using two-dimensional (2-D) PAGE and matrix-assisted laser desorption/ionization time-of-flight mass spectrometry. Here, we focus on actin filament remodeling in response to simulated arterial flow. Human saphenous vein was exposed to simulated venous or arterial flow for 90 min in vitro, and the contractile medial smooth muscle was dissected out and subjected to 2-D gel electrophoresis using a non-linear immobilized pH 3-10 gradient in the first dimension. Proteins were analyzed quantitatively using PDQuest 2-D software. The actin polymerization inhibitor cytochalasin B (1 microm) prevented increases in force generation after 90 min of simulated arterial flow. At this time point, there were several consistent changes in actin filament-associated protein expression (seven paired vein samples). The heat shock protein HSP27, identified as a three-spot charge train, showed a 1.6-fold increase in abundance (p = 0.01), but with reduced representation of the phosphorylated Ser(82) and Ser(15)Ser(82) isoforms (p = 0.018). The abundance of actin-capping protein alpha2 subunit CapZ had decreased 3-fold, p = 0.04. A 19-kDa proteolytic fragment of actin increased 2-fold, p = 0.04. For the four-spot charge train of gelsolin, there was reduced representation of the more acidic isoforms, p = 0.022. The abundance of other proteins associated with actin filaments, including cofilin and destrin, remained unchanged after arterial flow. Actin filament remodeling with differential expression and/or post-translational modification of proteins involved in capping the barbed end of actin filaments, HSP27 and CapZ, is an early response of contractile saphenous vein smooth muscle cells to hemodynamic stress. The observed changes would favor the generation of contractile stress fibers.

The mechanism of excitation-contraction coupling in phenylephrine-stimulated human saphenous vein

The human saphenous vein (HSV) is the most widely used graft in coronary artery revascularization procedures and is susceptible to spasm perioperatively. The aim of this study is to elucidate the mechanism(s) of agonist-induced excitation-contraction coupling in this vessel. Isometric contraction experiments were combined with in situ smooth muscle intracellular Ca(2+) concentration ([Ca(2+)](i)) imaging by confocal microscopy of intact undistended HSV segments during activation with phenylephrine (PE; 50 microM). Stimulation with PE produced a sustained contraction. Preincubation with 5 microM nifedipine, a blocker of the L-type voltage-operated Ca(2+) channel, or 50 microM SKF-96365, a blocker of both the voltage- and receptor-operated channels, reduced force generation by 25-30%. Ca(2+) imaging revealed that PE elicited only a transient rise in [Ca(2+)](i), suggesting that Ca(2+) plays only a minor role. However, a requirement for basal Ca(2+) levels was demonstrated when PE contractions could not be maintained in Ca(2+)-free medium. In light of the transient Ca(2+) response, it appears that signals other than Ca(2+) must maintain the tonic contraction elicited by PE, such as those that sensitize the myofilaments to Ca(2+). Application of HA-1077 (a Rho kinase inhibitor) at the peak of the contraction completely abolished the plateau phase of the response, whereas application of genistein (a tyrosine kinase inhibitor) reduced this phase by approximately 50%. The foregoing results suggest that, whereas the transient Ca(2+) signal can contribute to the development of force, maintenance of the plateau phase of the PE contraction in the HSV is the result of myofilament Ca(2+) sensitization by Rho kinase and tyrosine phosphorylation. The elucidation of the mechanisms of excitation-contraction coupling in the HSV may be useful for the development of therapeutic strategies for the alleviation of vein graft spasm.

A new perfusion culture system used to study human vein

BACKGROUND

Cell culture studies, ring studies, and indirect physiologic studies are the predominant models used to study human vascular tissue. Such studies are limited in their capacity to permit physiologic single-factor changes or to provide the proper mechanical stress or extracellular matrix present in normal tissues. We present a newly devised organ culture system that addresses these issues and permits survival of intact segments of human vascular tissue in a perfused environment. Our experience culturing human saphenous vein with this system is detailed.

METHODS

Perfusion culture chambers were designed and constructed in our laboratory. Excess saphenous vein segments were collected from coronary artery bypass graft cases at our hospital and then mounted into our perfusion culture system for 0, 24, 48, 72, or 96 h. Vasomotor assays, hematoxylin and eosin staining, bromodeoxyuridine staining, and factor VIII staining were performed to assess tissue survival.

RESULTS

A total of 24 veins were cultured. Average vessel length was 5 cm. The vessels contracted and relaxed the following amounts: time 0 (6.7% contraction, 5.0% relaxation), 24 h (5.7%, 5.3%), 48 h (5.2%, 2.8%), 72 h (4.8%, 5.3%), 96 h (4.8%, 3.8%). Hematoxylin and eosin staining, bromodeoxyuridine staining, and factor VIII staining support the viability of the tissue segments.

CONCLUSION

A new perfusion organ culture system has been devised that permits survival of intact human venous tissue for periods up to 96 h. Studies that permit physiologic single-factor changes along with precise control of the hemodynamic environment are possible with this system.