Responses Of Mesenteric Arteries Research Articles

Pregnancy Enhances the Prejunctional Vasodilator Response to Adrenomedullin in Selective Regions of the Arterial Bed of Wistar Rats

The objective of this study is to determine whether the vascular response to adrenomedullin is modulated by pregnancy. To this end, the authors study the effect of adrenomedullin on different contractile responses of mesenteric, uterine, renal, and saphenous arteries of 10-day pregnant and nonpregnant rats in myographs. Adrenomedullin inhibited contractile responses induced by electrical field stimulation in only the mesenteric and uterine arteries. This effect was more pronounced during pregnancy than in the nonpregnant state. Adrenomedullin did not modify concentration response curves to noradrenaline. The reduction of contractile responses to 40 mmol/L K(+) by adrenomedullin was similar in arteries of pregnant and nonpregnant rats. However, after incubation with capsaicin, this effect was significantly increased in mesenteric arteries of the pregnant group. The authors conclude that pregnancy is associated with a rise in the prejunctional inhibitory effect of adrenomedullin in some regions of the arterial system.

Prenatal stress in the rat results in increased blood pressure responsiveness to stress and enhanced arterial reactivity to neuropeptide Y in adulthood

We have shown previously that stress in the pregnant rat leads to a heightened cardiovascular response to restraint in adult offspring. The present study was undertaken to explore further the persistent cardiovascular effects of prenatal stress, with a focus on peripheral vascular function. Sprague-Dawley female rats were exposed to restraint/bright light three times daily in the last week of pregnancy. Litters from stressed and control females were cross-fostered to control dams to eliminate possible effects of maternal stress on nursing behaviour. At 120 days, offspring cardiovascular variables were measured by radiotelemetry. Reactivity of mesenteric small arteries was assessed by myography, and responses to electrical field stimulation determined. Resting cardiovascular parameters in prenatally stressed (PS) offspring were similar to controls but PS rats showed a greater increase in systolic blood pressure following restraint stress (P<0.05). Recovery was also prolonged in PS animals compared with controls and was of longer duration in PS females than in PS males (P<0.05). Adult PS females, but not males, also had elevated basal plasma corticosterone levels in comparison with controls (P<0.05). Vascular reactivity to neuropeptide Y (P<0.05) and electrical field stimulation (P<0.05) in mesenteric arteries was also significantly increased in PS animals. Vascular responses to adrenergic agonists as well as endothelial dilator function did not differ between PS and controls. We conclude that prenatal stress during late gestation has long-lasting effects on cardiovascular responsiveness and vascular reactivity to neuropeptide Y in the offspring.

Guanylate Cyclase Activators Influence Reactivity of Human Mesenteric Superior Arteries Retrieved and Preserved in the Same Conditions as Transplanted Kidneys

Introduction This study sought to investigate the mechanisms of relaxation induced by the (nitric oxide (NO)-independent soluble guanylyl cyclase (sGC) stimulators 3-[5′-hydroxymethyl-2′-furyl]-1-benzylindazole (YC-1) in human mesenteric arteries relaxed and precontracted with 1 μmol/L 5-hydroxytryptamine (serotonin). Material and methods Human mesenteric arteries obtained during kidney retrieval were preserved in the same conditions as transplanted kidneys. All experiments were performed after reperfusion with Krebs buffer in 37°C and 100% oxygen exposure. Results In endothelium-intact rings, YC-1 (0.001 to 30 mmol/L) caused concentration-dependent relaxation (pEC 50: 6.59 ± 0.12), which shifted to the right in endothelium-denuded rings. The sGC inhibitor 1 H- [1,2,4]oxadiazolo[4,3- a]quinoxalin-1-one (ODQ 10 mmol/L) partially attenuated the maximal responses to YC-1 (E max = 51.30% ± 3.70%; n = 6) and displaced its curve to the right in intact and denuded vessels. Both, the NO synthesis inhibitor N-nitro-L-arginine methyl ester (100 mmol/L) and the NO scavenger carboxy-2-[4-carboxyphenyl]-4,4,5,5-tetramethylimidazoline-1-oxyl-3-oxide (100 mmol/L) significantly reduced YC-1 relaxation. The sodium pump inhibitor ouabain (1μmol/L) produced a greater decrease in the vasodilator response of YC-1 (E max = 18.7% ± 4.55%; n = 9). ODQ (10 μmol/L) plus 1 μmol/L ouabain abolished the relaxant response of YC-1 (E max = 9.4% ± 2.94%, n = 9). Conclusions This study demonstrated that sodium pump stimulation by YC-1 as an additional mechanism of sGC activation independent of cGMP relaxed human mesenteric artery, including blockade of Ca 2+ influx. Furthermore, this study suggested an ability of NO to mediate relaxation of resistance-like arteries through the activation of soluble guanylate cyclase and K + channels.

Vascular bed-specific endothelium-dependent vasomomotor relaxation in the hagfish,Myxine glutinosa

The last common ancestor of hagfish and gnathostomes was also the last common ancestor of all extant vertebrates that lived some time more than 500 million years ago. Features that are shared between hagfish and gnathostomes can be inferred to have already been present in this ancestral vertebrate. We recently reported that hagfish endothelium displays phenotypic heterogeneity in ultrastructure, lectin binding, and mechanisms of leukocyte adhesion. Thus, phenotypic cell heterogeneity evolved as an early feature of the endothelium. In the present study, we wanted to extend these observations by determining whether hagfish endothelium plays a role in mediating vasomotor tone. Response of mesenteric and skeletal muscle arteries to a variety of mediators was assayed by videomicroscopy. Phenylephrine and acetylcholine induced vasoconstriction of mesenteric and skeletal muscle arteries. Bradykinin (BK) and ADP promoted vasorelaxation in precontracted mesenteric arteries but not those from skeletal muscle. BK- and ADP-mediated vasorelaxation of the mesenteric artery was abrogated by mechanical denudation of the endothelium but was unaffected by N(G)-nitro-L-arginine methyl ester. Indomethacin significantly inhibited the vasodilatory response to ADP but not BK. The nitric oxide donor sodium nitroprusside resulted in endothelium-independent relaxation of both mesenteric and skeletal muscle arteries. Together, these data suggest that site-specific endothelium-dependent vasorelaxation is an evolutionarily conserved property of this cell lineage.

Bimodal effects of chronically administered neurokinin B (NKB) on in vivo and in vitro cardiovascular responses in female rats

The in vivo cardiovascular effects of acutely administered neurokinin B (NKB) have been attributed both to direct effects on vascular tone and to indirect effects on central neuroendocrine control of the circulation. We proposed: 1) that a modest long-term increase in plasma NKB levels would decrease mean arterial pressure (MAP) due to attenuated peripheral vascular tone, and 2) that chronic high-dose NKB would increase MAP, due to increased sympathetic outflow which would override the peripheral vasodilation. We examined the in vivo and in vitro cardiovascular effects of chronic peripheral NKB. Low- (1.8 nmol/h) or high- (20 nmol/h) dose NKB was infused into conscious female rats bearing telemetric pressure transducers. MAP, heart rate (HR) and the pressor responses to I.V. phenylephrine (PE, 8 μg) and angiotensin II (Ang II, 150 ng) were measured. Concentration–response curves of small mesenteric arteries were constructed to PE using wire myography. Low-dose NKB reduced basal MAP (88 ± 2 mm Hg to 83 ± 2 mm Hg), did not affect resting HR, reduced the pressor responses to PE, and attenuated the maximal constriction of mesenteric arteries to PE and KCl. By contrast, high-dose NKB increased basal MAP (86 ± 1 mm Hg to 89 ± 1 mm Hg), increased HR (350 ± 3 beats/min to 371 ± 3 beats/min), increased the pressor responses to Ang II and, contrary to our hypothesis, increased the maximum contractile responses of mesenteric arteries to PE and KCl. The cardiovascular effects of NKB are thus dose-dependent: whereas chronic low-dose NKB directly modulates vascular tone to reduce blood pressure, chronic high-dose NKB induces an increase in blood pressure through both central (indirect) and peripheral (direct) pathways.

Persistence of PAR-2 vasodilation despite endothelial dysfunction in BPH/2 hypertensive mice

This study investigated relaxation of vascular smooth muscle by acetylcholine, bradykinin and protease-activated receptor 2 (PAR-2) to characterise endothelial dysfunction in spontaneously hypertensive mice (BPH/2). We hypothesised that PAR-2 induced vasodilation would be preserved in BPH/2 despite the presence of hypertension and impaired vasodilator responses to acetylcholine and bradykinin. Mean arterial blood pressure (MAP), heart rate and locomotor activity were assessed in conscious mice over 24-h periods by radiotelemetry. Relaxation responses of small mesenteric arteries to acetylcholine, bradykinin and the PAR-2 agonist, 2-furoyl-LIGRLO-amide (2fly), were assessed using wire myographs. MAP and heart rate of BPH/2 were 15 and 18%, respectively, higher than in controls (BPN/3). BPH/2 also exhibited increased locomotor activity. Maximal relaxations of arteries by acetylcholine and bradykinin in BPH/2 were reduced by 25-50% relative to BPN/3. In contrast, relaxation responses to 2fly were only slightly (6%), albeit significantly, reduced. Sodium nitroprusside-induced relaxations were not different between strains. Treatment of BPH/2 arteries with inhibitors of calcium-activated K(+) channels was sufficient to block persistent 2fly- and residual ACh- and bradykinin-induced relaxations, whereas NO synthase inhibitor failed to inhibit these relaxations. In BPH/2 mice, vascular smooth muscle relaxation by PAR-2 is well preserved despite the presence of hypertension and impaired vasodilation responses to acetylcholine and bradykinin.

Adrenomedullin-2, a Novel Calcitonin/Calcitonin-Gene-Related Peptide Family Peptide, Relaxes Rat Mesenteric Artery: Influence of Pregnancy

Adrenomedullin-2 (ADM2), a novel calcitonin/calcitonin-gene-related peptide family peptide, is reported to reduce blood pressure in both normal and hypertensive rats. This study demonstrates gestational regulation of circulatory ADM2 in rat plasma. ADM2 dose-dependently reduces the mean arterial pressure in rats, whereas the hypotensive effect of ADM2 is significantly higher during pregnancy. In addition, immunoreactive ADM2 protein is distributed in perivascular fibers of rat mesenteric artery, and levels of pre-pro-ADM2 are significantly (P<0.05) elevated in pregnant compared with nonpregnant rat mesenteric artery. Furthermore, incubation of endothelium intact arterial tissue from pregnant rats with ADM217-47, an ADM2 antagonist, shifted the dose-dependent relaxation curve to the right in wire myography. Inhibition of soluble guanylate cyclase with 1H-[1,2,4]oxadiazolo[4,3-a]quinoxalin-1-one (10 microM) or endothelial nitric oxide synthase with N-nitro-L-arginine methyl ester (100 microM) reduced the relaxation of mesenteric artery induced by ADM2. Inhibition of adenylate cyclase with SQ22536 (10 microM) or protein kinase A with the Rp diastereomer of cyclic adenosine 3',5'-phosphorothioate (10 microM) also reduced the maximal relaxation responses induced by ADM2. Blockade of calcium-activated potassium channels with tetraethylammonium chloride (1 mM) inhibited the ADM2-induced relaxation, whereas blockade of ATP-sensitive potassium channels with glybenclamide (10 microM) did not affect the relaxation response. Hence the mechanism of ADM2-induced vasorelaxation is nitric oxide and receptor mediated and cGMP and cAMP dependent and occurs through activation of calcium-activated potassium channels. In conclusion, rat pregnancy is associated with increased levels of circulatory and vascular tissue ADM2 with concomitant increase in the in vivo hypotensive effect of ADM2 and vascular reactivity of mesenteric artery to ADM2, thus suggesting involvement of ADM2 in vascular adaptations during pregnancy.

Blood flow responses in celiac and superior mesenteric arteries to meal

Blood flow responses in celiac and superior mesenteric arteries to meal

CHANGES OF RHO KINASE ACTIVITY AFTER HEMORRHAGIC SHOCK AND ITS ROLE IN SHOCK-INDUCED BIPHASIC RESPONSE OF VASCULAR REACTIVITY AND CALCIUM SENSITIVITY

The purpose of the present study is to investigate the changes of Rho kinase activity and its role in biphasic response of vascular reactivity and calcium sensitivity after hemorrhagic shock. The vascular reactivity and calcium sensitivity of superior mesenteric artery (SMA) from hemorrhagic shock rats were determined via observing the contraction initiated by norepinephrine (NE) and Ca under depolarizing conditions (120 mmol/L K) with isolated organ perfusion system. At same time, Rho kinase activity in mesenteric artery was measured, and the effects of Rho kinase activity-regulating agents, angiotensin II (Ang-II), insulin, and Y-27632, on vascular reactivity and calcium sensitivity were also observed. The results indicated that the vascular reactivity and calcium sensitivity were increased at early shock (immediate and 30 min after shock) and decreased at late shock (1 and 2 h after shock). The maximal contractions of NE and Ca were significantly increased (P < 0.05 or P < 0.01) at early shock. But they were significantly decreased at late shock (P < 0.05 or P < 0.01). Rho kinase activity was significantly increased at early shock (immediate after shock) (P < 0.05) but significantly decreased at 1 and 2 h after shock (P < 0.05 or P < 0.01). It was positively correlated with the changes of vascular reactivity and calcium sensitivity. Insulin decreased the increased contractile response of SMA to NE and Caat early shock (P < 0.05 or P < 0.01). Angiotensin II increased the decreased contractile response of SMA to NE and Ca at 2-h shock (P < 0.05 or P < 0.01); Y-27632, Rho kinase-specific antagonist, decreased the contractile response of SMA to NE and Ca at 2-h shock, and abolished Ang-II induced the increase of vascular reactivity and calcium sensitivity. The results suggest that Rho kinase may be involved in the biphasic change of vascular reactivity and calcium sensitivity after hemorrhagic shock. Rho kinase may regulate vascular reactivity through the regulation of calcium sensitivity. Rho kinase-regulating agents may have some beneficial effects on shock-induced vascular hyporeactivity.

Mesenteric Vascular Responsiveness in a Rat Model of Pregnancy-Induced Hypertension

Reduced perfusion to the placenta in early pregnancy is believed to be the initiating factor in the development of preeclampsia, triggering local ischemia and systemic vascular hyperresponsiveness. This sequence of events creates a predisposition to the development of altered vascular function and hypertension. This study was designed to determine the influence of placental insufficiency on the responsiveness of mesenteric resistance arteries in an animal model of preeclampsia. Placental insufficiency was induced by reduction in uteroplacental perfusion pressure (RUPP) in experimental Sprague-Dawley rat dams. The uterine branches of the ovarian arteries and the abdominal aortae of pregnant rats were surgically constricted on gestational Day 14. Dams in the control group underwent a sham procedure. Rats were euthanized on gestational Day 20, followed by removal of the small intestine and adjacent mesentery. First-order mesenteric resistance arteries were mounted on a small vessel wire myograph and challenged with incremental concentrations of vasoconstrictors and vasorelaxants. Mesenteric arteries in dams with placental insufficiency demonstrated an increased maximal tension to phenylephrine (7.15 +/- 0.15 vs. 5.4 +/- 0.27 mN/mm, P < 0.001); potassium chloride at 60 mM (3.43 +/- 0.11 vs. 2.77 +/- 0.14 mN/mm, P < 0.01) and 120 mM (3.92 +/- 0.18 vs. 2.97 +/- 0.16 mN/mm, P < 0.01); and angiotensin II (2.59 +/- 0.42 vs. 1.51 +/- 0.22 mN/mm, P < 0.05). Maximal relaxation to endothelium-dependent relaxants acetylcholine and calcium ionophore (A23187) was not significantly reduced. Data suggest that placental insufficiency leads to hyperresponsiveness to vasoconstrictor stimuli in mesenteric arteries.

In Vitro Continuous Amperometry with a Diamond Microelectrode Coupled with Video Microscopy for Simultaneously Monitoring Endogenous Norepinephrine and Its Effect on the Contractile Response of a Rat Mesenteric Artery

Continuous amperometry with a diamond microelectrode and video microscopy were used to record (in vitro) endogenous norepinephrine release simultaneously with the evoked contractile response of a mesenteric artery from a healthy Sprague Dawley rat. Norepinephrine (NE) is a vasoconstricting neurotransmitter released from sympathetic nerves that innervate the smooth muscle cell layers surrounding arteries and veins. Using these two techniques along with several drugs, the NE released at sympathetic neuroeffector junctions nearby the microelectrode was measured as an oxidation current. Key to the amperometric measurement was the use of a diamond microelectrode because of the response sensitivity, reproducibility, and stability it provided. NE release was elicited by electrical stimulation at frequencies between 1 and 60 Hz, with a maximum response seen at 20 Hz. Confirmation that the oxidation current was, in fact, associated with endogenous NE came from the results of several drugs. Tetrodotoxin (TTX, 0.3 microM), a voltage-dependent sodium channel antagonist that blocks nerve conduction, abolished both the oxidation current and the arterial constriction. The alpha(2)-adrenergic autoreceptor antagonist, yohimbine (1.0 microM), caused an increase in the oxidation current and the corresponding constriction. The addition of cocaine (10 microM), an antagonist that inhibits neuronal NE reuptake, caused both the oxidation current and the contractile response to increase. These results, combined with the fact that the hydrodynamic voltammetric E(1/2) for endogenous NE was identical to that for a standard solution, confirmed that the oxidation current was due to NE and that this compound caused, at least in part, the contractile response. The results demonstrate that continuous amperometric monitoring of NE with a diamond microelectrode and video imaging of vascular tone allow real time local measurement of the temporal relationship between nerve-stimulated NE release and arterial constriction.

Enhanced vascular contractility and diminished coronary artery flow in rats made hypertensive from diet-induced obesity

To determine whether obesity-induced hypertension was associated with alterations in vascular contractility and/or cardiac function. Male Sprague-Dawley rats were fed either a low fat (LF; 11% kcal as fat) or a moderately high fat (MHF; 32% kcal as fat) diet for 11 weeks. Body weight; mean arterial pressure; angiotensin peptides; mesenteric contractile response to phenylephrine (PE), potassium chloride (KCl), serotonin, angiotensin II (AngII), calcium chloride; baseline and isoproterenol-induced cardiac contractility; baseline and isoproterenol-induced coronary artery blood flow. Rats fed the MHF diet segregated into obesity-prone (OP) and obesity-resistant (OR) groups. OP rats exhibited elevations in mean arterial pressure (MAP) and elevations in systemic concentrations of angiotensin peptides. Mesenteric arteries from OP rats exhibited a greater contractile response to PE, KCl and serotonin (5-HT). Heightened responses to PE persisted in arteries from OP rats even after normalization of the response to KCl. In contrast, the response of permeabilized mesenteric arteries to a maximal concentration of calcium was similar in rats from each group. Isolated perfused hearts exhibited similar baseline and isoproterenol-induced contractility in rats from each group. However, isoproterenol was unable to increase coronary artery blood flow in hearts from OP rats. Enhanced vascular reactivity may contribute to obesity-induced hypertension, while reductions in coronary artery relaxation would impair the ability of the heart to respond to increased myocardial demand.

Impaired small‐conductance Ca2+‐activated K+ channel‐dependent EDHF responses in Type II diabetic ZDF rats

We have examined the relative contributions of small- and intermediate-conductance Ca(2+)-activated K(+) channels (SK(Ca) and IK(Ca)) to the endothelium-derived hyperpolarizing factor (EDHF) pathway response in small mesenteric arteries of Zucker Diabetic Fatty (ZDF) rats, before and after the development of Type II diabetes, together with Lean controls. Smooth muscle membrane potential was recorded using sharp microelectrodes in the presence of 10 microM indomethacin plus 100 microM N(omega)-nitro-L-arginine. SK(Ca) was selectively inhibited with 100 nM apamin, whereas IK(Ca) was blocked with 10 microM TRAM-39 (2-(2-chlorophenyl)-2,2-diphenylacetonitrile). Resting membrane potentials were similar in arteries from 17- to 20-week-old control and diabetic rats (approximately -54 mV). Responses elicited by 1 and 10 microM acetylcholine (ACh) were significantly smaller in the diabetic group (e.g. hyperpolarizations to -69.5 +/- 0.8 mV (ZDF; n = 12) and -73.2 +/- 0.6 mV (Lean; n = 12; P < 0.05) evoked by 10 microM ACh). The IK(Ca)-mediated components of the ACh responses were comparable between groups (hyperpolarizations to approximately -65 mV on exposure to 10 microM ACh). However, SK(Ca)-mediated responses were significantly reduced in the diabetic group (hyperpolarizations to -63.1 +/- 1.0 mV (ZDF; n = 6) and -71.5 +/- 1.2 mV (Lean; n = 6; P < 0.05) on exposure to 10 microM ACh. Impaired ACh responses were not observed in arteries from 5- to 6-week-old (pre-diabetic) animals. SK(Ca) subunit mRNA expression was increased in the diabetic group. The EDHF pathway, especially the SK(Ca)-mediated response, is impaired in Type II diabetic ZDF rats without a reduction in channel gene expression. These results may be particularly relevant to the microvascular complications of diabetes. The functional separation of SK(Ca) and IK(Ca) pathways is discussed.

Mechanisms underlying the impaired EDHF-type relaxation response in mesenteric arteries from Otsuka Long-Evans Tokushima Fatty (OLETF) rats

We previously reported that in mesenteric arteries from streptozotocin-induced diabetic rats, the endothelium-derived hyperpolarizing factor (EDHF)-type relaxation is impaired, possibly due to a reduced action of cAMP. Here, we observed an impairment of acetylcholine-induced EDHF-type relaxation in mesenteric arteries from a type 2 diabetic model, Otsuka Long-Evans Tokushima Fatty (OLETF) rats [vs. age-matched control Long-Evans Tokushima Otsuka (LETO) rats], and we investigated the mechanism underlying this impairment. In the LETO group, this EDHF-type relaxation was attenuated by 18α-glycyrrhetinic acid (a gap-junction inhibitor) and by a protein kinase A (PKA) inhibitor. In both groups (OLETF and LETO), it was enhanced by 3-isobutyl-1-methylxanthine, a cAMP-phosphodiesterase (PDE) inhibitor, but following these enhancements it was still weaker in OLETF rats than in LETO rats. The relaxations induced by cilostamide (a selective PDE3 inhibitor) and 8-bromo-cAMP (a cell-permeant cAMP analog) were reduced in OLETF rats, as was PKA activity. The relaxations induced by two activators of Ca 2+-activated K + channels (K Ca) [1-ethyl-2-benzimidazolinone (1-EBIO), intermediate-conductance K Ca channel (IK Ca) activator, and riluzole, small-conductance K Ca channel (SK Ca) activator] were also impaired in OLETF rats. We conclude that the impairment of EDHF-type relaxation seen in OLETF rats may be attributable not only to a reduction in cAMP/PKA signaling, but also to reduced endothelial K Ca channel activities.

Augmented Endothelium-Derived Hyperpolarizing Factor-Mediated Relaxations Attenuate Endothelial Dysfunction in Femoral and Mesenteric, but Not in Carotid Arteries from Type I Diabetic Rats

Individual vascular beds exhibit differences in vascular reactivity. The present study investigates the effects of streptozotocin-induced type I diabetes on endothelium-dependent responses of rat carotid, femoral, and mesenteric arteries. Rings with and without endothelium, suspended in organ chambers for isometric tension recording, were contracted with phenylephrine and exposed to increasing concentrations of acetylcholine. In carotid and femoral arteries, acetylcholine produced concentration- and endothelium-dependent relaxations that were abolished by Nomega-nitro-L-arginine methyl ester (L-NAME; specific nitric-oxide synthase inhibitor) and were impaired slightly in preparations from streptozotocin-treated rats (STZ-rats). This impairment could be prevented by L-arginine. In femoral arteries incubated with L-NAME, acetylcholine caused endothelium-dependent contractions that were abolished by 3-[(6-amino-(4-chlorobenzensulfonyl)-2-methyl-5,6,7,8-tetrahydronapht]-1-yl) propionic acid (S18886) (antagonist of thromboxane A2/prostaglandins H2-receptors) and reversed to relaxation by indomethacin (inhibitor of cyclooxygenase). The latter relaxation was inhibited by charybdotoxin plus apamin, suggesting a role of endothelium-dependent hyperpolarizing factor (EDHF). This EDHF-mediated component was augmented slightly in arteries from STZ-rats. In mesenteric arteries, relaxations to acetylcholine were only partially inhibited by L-NAME, and the L-NAME-resistant component was abolished by charybdotoxin plus apamin. In the mesenteric arteries from STZ-rats, L-NAME-sensitive relaxations to acetylcholine were reduced and the EDHF-component was augmented. These findings demonstrate a marked heterogeneity in endothelium-dependent responses in rat arteries and their differential adaptation in the course of type I diabetes. In particular, the EDHF-mediated component not only compensates for the reduced bioavailability of nitric oxide in the femoral and mesenteric artery but also counteracts the augmented endothelium-dependent contractions in the former.

The Role of Potassium Channels in Relaxant Effect of Levosimendan in Rat Small Mesenteric Arteries

We investigated both the effect of levosimendan and the role of various potassium channels in KCl-precontracted rat small mesenteric arteries. Levosimendan (10(-6)-10(-3) M) or cromakalim (CRO, 10(-7)-10(-4) M) produced concentration-dependent relaxation responses in small mesenteric arteries precontracted by 30 mM KCl. The relaxant responses to levosimendan in KCl-precontracted arteries did not differ significantly between endothelium-intact and endothelium-denuded preparations. Incubation of rat small mesenteric arterial segments with ATP-dependent potassium channel (KATP) blocker glibenclamide (GLI, 10(-6) M) for 30 min significantly inhibited the relaxant responses to both levosimendan and CRO. Neither the Ca(2+)-activated potassium channel (KCa) blocker iberiotoxin (10(-7) M) nor the voltage-dependent potassium channel (KV) blocker 4-aminopyridine (5 mM) incubation for 30 min caused significant alterations in relaxant responses to levosimendan in KCl-precontracted small mesenteric arteries. These findings suggested that levosimendan-induced relaxation responses in isolated rat small mesenteric arteries were neither depended on endothelium nor inhibited by the blockers of KV or KCa but, they rather seem to depend on the activation of KATP.

Differential roles for IKCa and SKCa channels in EDHF‐type responses in mouse mesenteric arteries

Activation of intermediate- and small-conductance Ca2+-activated K+ channels (IKCa and SKCa) by the endothelium-dependent agonist SLIGRL, evokes smooth muscle cell hyperpolarization and relaxation in mouse mesenteric arteries. The aim of this study was to establish whether differential activation of the KCa channels occurred in the presence and absence of pre-contraction. Isolated mesenteric arteries were mounted isometrically, enabling simultaneous recording of smooth muscle cell membrane potential and tension. In the presence of the NO-synthase inhibitor L-NAME (100 μM), SLIGRL (1–30μM) evoked both relaxation and hyperpolarization. In un-stimulated arteries the maximal increase in membrane potential (18.1 ± 1.0 mV, n = 14) was almost fully blocked by the selective inhibitor of IKCa, TRAM-34 (1μM) alone. However, when arteries were pre-contracted and depolarized with phenylephrine (0.2–3μM), similar levels of inhibition were achieved only in the presence of both TRAM-34 (1μM) and the selective SKCa inhibitor, apamin (50nM). These data suggest that IKCa are the predominant endothelial channels associated with SLIGRL-induced smooth muscle hyperpolarization in mouse mesenteric arteries, whilst SKCa contribute to SLIGRL-evoked repolarization and relaxation subsequent to depolarization. These findings are in direct contrast to those obtained in the rat mesenteric artery, and appear to reflect the sub-cellular profile of KCa channel expression. These data reiterate the importance of using selective inhibitors at different levels of pre-contraction when studying EDHF responses. Supported by the Wellcome Trust

Role of Vascular Heme Oxygenase in Reduced Myogenic Reactivity Following Chronic Hypoxia

Exposure to chronic hypoxia (CH) results in a persistent endothelium-dependent vascular smooth muscle hyperpolarization that diminishes vasoconstrictor reactivity. Experiments were performed to test the hypothesis that products of both cytochrome P450 epoxygenase (CYP) and heme oxygenase (HO) are required for the persistent diminished myogenic reactivity following CH. The authors examined myogenic responses of mesenteric arteries isolated from control and CH (48 h; P(B) = 380 mmHg) rats in the presence of a HO inhibitor (zinc protoporphyrin IX; ZnPPIX) or combined HO and CYP epoxygenase inhibition (sulfaphenazole). Arteries were isolated and cannulated and the vascular smooth muscle was loaded with the Ca2+ indicator Fura-2. Control vessels maintained their internal diameter in response to step increases in intraluminal pressure, whereas arteries from CH animals passively distended. ZnPPIX augmented myogenic reactivity and [Ca2+] in arteries from CH animals. Combined administration of sulfaphenazole and ZnPPIX did not have an additional effect compared to ZnPPIX alone. Myogenic reactivity in control vessels was not altered by ZnPPIX or ZnPPIX + sulfaphenazole. HO appears to play a role in regulating myogenic reactivity following CH. Furthermore, these data suggest that products of HO and CYP are both required for the observed attenuation in vasoreactivity following CH.

Segmental Differences in the Roles of Rho-Kinase and Protein Kinase C in Mediating Vasoconstriction

Rho-kinase and protein kinase C (PKC) have each been reported to mediate vasoconstriction via calcium sensitization. However, the relative contributions of these two kinases to vascular contraction, and whether their roles vary between large and small arteries, are largely unknown. We therefore assessed the relative roles of rho-kinase and PKC in mediating vasoconstriction in arteries from three segments of the aortic and mesenteric vasculature. We studied contractile responses of rat isolated thoracic aorta (diameter approximately 2 mm), superior mesenteric artery (SMA; approximately 1.5 mm), and second order branches of the superior mesenteric artery (BMA; approximately 300 mum). The roles of rho-kinase and PKC in mediating contractile responses to phenylephrine, 9,11-dideoxy-9,11-methanoepoxy prostaglandin F(2alpha) (U46619), and KCl were assessed by using the rho-kinase inhibitor R-[+]-trans-N-[4-pyridyl]-4-[1-aminoethyl]-cycloheaxanecarboxamide (Y-27632) (1 and 10 muM) and the PKC inhibitor 3-[1-[3-(amidinothio)propyl-1H-indol-3-yl]-3-(1-methyl-1H-indol-3-yl) maleimide (Ro 31-8220) (5 muM). Contractile responses of aorta and SMA were reduced by either 1 or 10 muM Y-27632 (P < 0.05), whereas responses of BMA were reduced by 10 muM (P < 0.05) but not 1 muM Y-27632. In contrast, Ro 31-8220 partly reduced contractile responses in aorta and SMA (P < 0.05), but it abolished responses of BMA (P < 0.05). Cotreatment with Y-27632 and Ro 31-8220 markedly attenuated contractile responses to phenylephrine and KCl in all vessels, but it had only a moderate inhibitory effect on responses to U46619 in aorta and SMA. Thus, contractile responses of the larger arteries can involve both rho-kinase and PKC to varying degrees. Conversely, contractile responses of small mesenteric resistance arteries seem to be mediated exclusively by PKC, with no apparent role for rho-kinase.

Effects of idarubicin and idarubicinol on rat coronary resistance and vasoconstrictor responsiveness of isolated aorta and mesentery

It has been hypothesized that coronary vasoconstriction is involved in the cardiotoxic action of anthracyclines. The purpose of this study was to determine whether an increase in coronary resistance induced by idarubicin (IDA) or its primary circulating metabolite idarubicinol (IDOL) is correlated with a decrease in vascular sensitivity to vasoconstrictor agonists. Coronary vasoconstriction was studied in single-pass perfused rat hearts after a 10-min infusion of 0.5 mg IDA or IDOL. In the endothelium-intact rat thoracic aorta and mesentery we measured the inhibition of phenylephrine (PE)- and KCl-induced contraction in the presence of IDA and IDOL, respectively. The increase in coronary vascular resistance evoked by IDOL (121%) exceeded that of IDA (75%). IDA (10-100 micromol/l) concentration-dependently diminished vascular sensitivity to PE and KCl due to a reduction in maximal contractile response (Emax), i.e. the antagonism by IDA of PE- or KCl-induced vasoconstriction was non-competitive, indicating a post-receptor cellular mechanism. These reductions of PE or KCl efficacy elicited by IDOL were significantly larger than those elicited by the corresponding doses of IDA. The decrease in efficacy of PE in the presence of IDA and IDOL was characterized by IC50 estimates of 44.3 and 30.7 mumol/l, respectively. With a 10-fold lower IC50, IDA inhibited the reactivity of small mesenteric arteries to noradrenaline with 10-fold higher potency. The correlation between the increase in coronary resistance and the decrease in vasoconstrictor responsiveness may suggest that these anthracyclines act through a common cellular mechanism.