Acute, pro-contractile effects of prorenin on rat mesenteric arteries

Prorenin and the prorenin receptor ((P)RR) are important, yet controversial, members of the renin-angiotensin-aldosterone system. The ((P)RR) is expressed throughout the body, including the vasculature, however, the direct effect of prorenin on arterial contractility is yet to be determined. Within rat mesenteric arteries, immunostaining and proximity ligation assays were used to determine the interacting partners of (P)RR in freshly isolated vascular smooth muscle cells (VSMCs). Wire myography examined the functional effect of prorenin. Simultaneous changes in [Ca2+ ]i and force were recorded in arteries loaded with Fura-2AM. Spontaneously transient outward currents were recorded via perforated whole-cell patch-clamp configuration in freshly isolated VSMCs. We found that the (P)RR is located within a distance of less than 40 nm from the V-ATPase, caveolin-1, ryanodine receptors, and large conductance Ca2+ -activated K+ channels (BKCa ) in VSMCs. [Ca2+ ]i imaging and isometric tension recordings indicate that 1 nM prorenin enhanced α1-adrenoreceptor-mediated contraction, associated with an increased number of Ca2+ waves, independent of voltage-gated Ca2+ channels activation. Incubation of VSMCs with 1 nM prorenin decreased the amplitude and frequency of spontaneously transient outward currents and attenuated BKCa -mediated relaxation. Inhibition of the V-ATPase with 100 nM bafilomycin prevented prorenin-mediated inhibition of BKCa -derived relaxation. Renin (1 nM) had no effect on BKCa -mediated relaxation. In conclusion, prorenin enhances arterial contractility by inhibition of BKCa and increasing intracellular Ca2+ release. It is likely that this effect is mediated through a local shift in pH upon activation of the (P)RR and stimulation of the V-ATPase.

The vascular Na,K-ATPase: clinical implications in stroke, migraine, and hypertension

In the vascular wall, the Na,K-ATPase plays an important role in the control of arterial tone. Through cSrc signaling, it contributes to the modulation of Ca2+ sensitivity in vascular smooth muscle cells. This review focuses on the potential implication of Na,K-ATPase-dependent intracellular signaling pathways in severe vascular disorders; ischemic stroke, familial migraine, and arterial hypertension. We propose similarity in the detrimental Na,K-ATPase-dependent signaling seen in these pathological conditions. The review includes a retrospective proteomics analysis investigating temporal changes after ischemic stroke. The analysis revealed that the expression of Na,K-ATPase α isoforms is down-regulated in the days and weeks following reperfusion, while downstream Na,K-ATPase-dependent cSrc kinase is up-regulated. These results are important since previous studies have linked the Na,K-ATPase-dependent cSrc signaling to futile recanalization and vasospasm after stroke. The review also explores a link between the Na,K-ATPase and migraine with aura, as reduced expression or pharmacological inhibition of the Na,K-ATPase leads to cSrc kinase signaling up-regulation and cerebral hypoperfusion. The review discusses the role of an endogenous cardiotonic steroid-like compound, ouabain, which binds to the Na,K-ATPase and initiates the intracellular cSrc signaling, in the pathophysiology of arterial hypertension. Currently, our understanding of the precise control mechanisms governing the Na,K-ATPase/cSrc kinase regulation in the vascular wall is limited. Understanding the role of vascular Na,K-ATPase signaling is essential for developing targeted treatments for cerebrovascular disorders and hypertension, as the Na,K-ATPase is implicated in the pathogenesis of these conditions and may contribute to their comorbidity.

Colchicine enhances β adrenoceptor-mediated vasodilation in men with essential hypertension

AIMS

The aim of this study is to examine whether colchicine improves β adrenoceptor-mediated vasodilation in humans by conducting a double-blinded, placebo-controlled intervention study. Colchicine treatment has known beneficial effects on cardiovascular health and reduces the incidence of cardiovascular disease. Studies in isolated rodent arteries have shown that colchicine can enhance β adrenoceptor-mediated vasodilation, but this has not been determined in humans.

METHODS

Middle-aged men with essential hypertension were randomly assigned firstly to acute treatment with either 0.5 mg colchicine (n = 19) or placebo (n = 12). They were subsequently re-randomized for 3 weeks of treatment with either colchicine 0.5 mg twice daily (n = 16) or placebo (n = 15) followed by a washout period of 48-72 h. The vasodilator responses to isoprenaline, acetylcholine and sodium nitroprusside were determined as well as arterial pressure, arterial compliance and plasma inflammatory markers.

RESULTS

Acute colchicine treatment increased isoprenaline (by 38% for the highest dose) as well as sodium nitroprusside (by 29% main effect) -induced vasodilation but had no effect on the response to acetylcholine. The 3-week colchicine treatment followed by a washout period did not induce an accumulated or sustained effect on the β adrenoceptor response, and there was no effect on arterial pressure, arterial compliance or the level of measured inflammatory markers.

CONCLUSION

Colchicine acutely enhances β adrenoceptor- and nitric oxide-mediated changes in vascular conductance in humans, supporting that the mechanism previously demonstrated in rodents, translates to humans. The results provide novel translational evidence for a transient enhancing effect of colchicine on β adrenoceptor-mediated vasodilation in humans with essential hypertension.

CONDENSED ABSTRACT

Preclinical studies in isolated rodent arteries have shown that colchicine can enhance β adrenoceptor-mediated vasodilation. Here we show that this effect of colchicine can be translated to humans. Acute colchicine treatment was found to increase both isoprenaline- and sodium nitroprusside-induced vasodilation. The study provides the first translational evidence for a transient β adrenoceptor-mediated vasodilatory effect of colchicine in humans. The finding of an acute effect suggests that it may be clinically important to maintain an adequate bioavailability of colchicine.

Vasomotion in Retinal Arterioles Is Modified by Exercise and Flicker Stimulation

PURPOSE

Vasomotion is spontaneous oscillations in the diameter of resistance vessels with derived effects on blood flow, and it has been proposed that disturbances in vasomotion may be involved in retinal vascular disease. The purpose of this study was to investigate whether retinal vasomotion shows regional variation and is modified by activated autoregulation.

METHODS

Video recordings of the diameter of retinal arterioles previously obtained from 55 normal persons were subjected to Fourier analysis to characterize the frequencies and propagation of spontaneous diameter changes in retinal arterioles. The analyses were performed on peripapillary temporal retinal arterioles, on arteriolar branches toward the macular area and the retinal periphery, and were performed during rest, during an increase in the arterial blood pressure induced by isometric exercise, and during increased retinal metabolism induced by flickering light.

RESULTS

There was no propagation of diameter changes along the studied vascular segments. Isometric exercise constricted the arterioles significantly by (mean ± SD) 1.76% ± 3.56% (P = 0.02) and increased the power of diameter oscillations at very low frequencies (0.1-1.4 c/min). Flicker stimulation dilated the arterioles significantly by (mean ± SD) 5.10% ± 2.91% (P < 0.0001) and reduced the power of diameter oscillations at all but the very low frequencies (P < 0.006 for all comparisons). Flicker-induced dilation and changes in hydraulic conductance were lower in peripheral than in macular arterioles.

CONCLUSIONS

Retinal vasomotion in normal persons increases during increased arterial blood pressure and decreases during flicker stimulation. The findings may act as a basis for the study of vasomotion in retinal vascular disease.

The microtubule network enables Src kinase interaction with the Na,K-ATPase to generate Ca2+ flashes in smooth muscle cells

Background: Several local Ca²⁺ events are characterized in smooth muscle cells. We have previously shown that an inhibitor of the Na,K-ATPase, ouabain induces spatially restricted intracellular Ca²⁺ transients near the plasma membrane, and suggested the importance of this signaling for regulation of intercellular coupling and smooth muscle cell contraction. The mechanism behind these Na,K-ATPase-dependent “Ca²⁺ flashes” remains to be elucidated. In addition to its conventional ion transport function, the Na,K-ATPase is proposed to contribute to intracellular pathways, including Src kinase activation. The microtubule network is important for intracellular signaling, but its role in the Na,K-ATPase-Src kinase interaction is not known. We hypothesized the microtubule network was responsible for maintaining the Na,K-ATPase-Src kinase interaction, which enables Ca²⁺ flashes.

Methods: We characterized Ca²⁺ flashes in cultured smooth muscle cells, A7r5, and freshly isolated smooth muscle cells from rat mesenteric artery. Cells were loaded with Ca²⁺-sensitive fluorescent dyes, Calcium Green-1/AM and Fura Red/AM, for ratiometric measurements of intracellular Ca²⁺. The Na,K-ATPase α2 isoform was knocked down with siRNA and the microtubule network was disrupted with nocodazole. An involvement of the Src signaling was tested pharmacologically and with Western blot. Protein interactions were validated with proximity ligation assays.

Results: The Ca²⁺ flashes were induced by micromolar concentrations of ouabain. Knockdown of the α2 isoform Na,K-ATPase abolished Ca²⁺ flashes, as did inhibition of tyrosine phosphorylation with genistein and PP2, and the inhibitor of the Na,K-ATPase-dependent Src activation, pNaKtide. Ouabain-induced Ca²⁺ flashes were associated with Src kinase activation by phosphorylation. The α2 isoform Na,K-ATPase and Src kinase colocalized in the cells. Disruption of microtubule with nocodazole inhibited Ca²⁺ flashes, reduced Na,K-ATPase/Src interaction and Src activation.

Conclusion: We demonstrate that the Na,K-ATPase-dependent Ca²⁺ flashes in smooth muscle cells require an interaction between the α2 isoform Na, K-ATPase and Src kinase, which is maintained by the microtubule network.

Migraine-Associated Mutation in the Na,K-ATPase Leads to Disturbances in Cardiac Metabolism and Reduced Cardiac Function

Background

Mutations in ATP1A2 gene encoding the Na,K‐ATPase α₂ isoform are associated with familial hemiplegic migraine type 2. Migraine with aura is a known risk factor for heart disease. The Na,K‐ATPase is important for cardiac function, but its role for heart disease remains unknown. We hypothesized that ATP1A2 is a susceptibility gene for heart disease and aimed to assess the underlying disease mechanism.

Methods and Results

Mice heterozygous for the familial hemiplegic migraine type 2–associated G301R mutation in the Atp1a2 gene (α₂^+/G301R mice) and matching wild‐type controls were compared. Reduced expression of the Na,K‐ATPase α₂ isoform and increased expression of the α₁ isoform were observed in hearts from α₂^+/G301R mice (Western blot). Left ventricular dilation and reduced ejection fraction were shown in hearts from 8‐month‐old α₂^+/G301R mice (cardiac magnetic resonance imaging), and this was associated with reduced nocturnal blood pressure (radiotelemetry). Cardiac function and blood pressure of 3‐month‐old α₂^+/G301R mice were similar to wild‐type mice. Amplified Na,K‐ATPase–dependent Src kinase/Ras/Erk1/2 (p44/42 mitogen‐activated protein kinase) signaling was observed in hearts from 8‐month‐old α₂^+/G301R mice, and this was associated with mitochondrial uncoupling (respirometry), increased oxidative stress (malondialdehyde measurements), and a heart failure–associated metabolic shift (hyperpolarized magnetic resonance). Mitochondrial membrane potential (5,5´,6,6´‐tetrachloro‐1,1´,3,3´‐tetraethylbenzimidazolocarbocyanine iodide dye assay) and mitochondrial ultrastructure (transmission electron microscopy) were similar between the groups. Proteomics of heart tissue further suggested amplified Src/Ras/Erk1/2 signaling and increased oxidative stress and provided the molecular basis for systolic dysfunction in 8‐month‐old α₂^+/G301R mice.

Conclusions

Our findings suggest that ATP1A2 mutation leads to disturbed cardiac metabolism and reduced cardiac function mediated via Na,K‐ATPase–dependent reactive oxygen species signaling through the Src/Ras/Erk1/2 pathway.

Differential Effects of Intra- and Extravascular ATP on the Diameter of Porcine Vessels at Different Branching Levels Ex Vivo

Purpose

Adenosine triphosphate (ATP) is involved in the diameter regulation of retinal vessels. The compound has been shown to induce both constriction and dilatation, but the detailed mechanisms underlying these effects and the site of action of the compound are not known in detail. Therefore, the purpose of the present study was to investigate whether the vasoactive effects of ATP on retinal vessels depend on intra- and extravascular application, and to study whether the effects differ at different vascular branching levels.

Methods

Diameter changes in arterioles, pre-capillary arterioles, and capillaries were studied in perfused porcine hemiretinas (n = 48) ex vivo after intra- and extravascular application of the nondegradable ATP analogue ATP-γ-S or ATP in the presence or not of antagonists to the CD73/ecto-5′-nucleotidase (AOPCP), the P2-purinergic receptor (PPADS), the A3-adenosine receptor (MRS1523), and the synthesis of cyclooxygenase products (ibuprofen).

Results

Intravascular ATP-induced constriction and extravascular ATP-induced dilatation of retinal arterioles, pre-capillary arterioles and capillaries, and dilatation was inhibited by ibuprofen. Both constriction and dilatation of arterioles were inhibited by antagonizing ATP degradation. Furthermore, constriction at all three branching levels was antagonized by blocking the A3 purinoceptor, whereas constriction in arterioles and pre-capillary arterioles was antagonized by blocking the P2 purinoceptor.

Conclusions

ATP affects the diameter of retinal arterioles, pre-capillary arterioles, and capillaries through different pathways, and the effects depend on whether the compound is administered intravascularly or extravascularly. This may form the basis for selective interventions on retinal vascular disease with differential involvement of vessels at different branching levels.

Sodium bicarbonate cotransporter NBCn1/Slc4a7 affects locomotor activity and hearing in mice

Despite a widespread expression pattern in the central nervous system, the role of the sodium bicarbonate cotransporter NBCn1/Slc4a7 has not been investigated for locomotor activity, emotion and cognition. Here, we addressed the behavioral consequences of NBCn1 knockout and evaluated hearing and vision that are reportedly impaired in an earlier line of NBCn1 knockout mice and may contribute to behavioral changes. In a circular open field, the knockout mice traveled a shorter distance, especially in the periphery of the chamber, than wildtype littermates. The knockout mice also traveled a shorter total distance in a home cage-like open field. Rearing and grooming behaviors were reduced. The knockout and control mice displayed similar time spent and number of open and closed arms in the elevated plus maze test, indicating negligible change in anxiety. In the Morris water maze test, both groups of mice learned the location of an escape platform within comparable time on the training trials and showed similar platform identification on the probe trial. The knockout mice maintained normal visual responses in the optokinetic drum and produced evoked potentials in response to light stimuli. However, these mice failed to produce auditory evoked potentials. qPCR revealed a robust expression of an alternatively transcribed NBCn1 variant in the knockout mouse retina. These results indicate that NBCn1 deletion leads to reduced locomotor activity in mice by affecting their exploratory behaviors or emotionality. The deletion also causes hearing loss, but its effect on vision varies between different lines of knockout mice.

A paradoxical increase of force development in saphenous and tail arteries from heterozygous ANO1 knockout mice

A Ca²⁺‐activated Cl⁻ channel protein, ANO1, is expressed in vascular smooth muscle cells where Cl⁻ current is thought to potentiate contraction by contributing to membrane depolarization. However, there is an inconsistency between previous knockout and knockdown studies on ANO1’s role in small arteries. In this study, we assessed cardiovascular function of heterozygous mice with global deletion of exon 7 in the ANO1 gene. We found decreased expression of ANO1 in aorta, saphenous and tail arteries from heterozygous ANO1 knockout mice in comparison with wild type. Accordingly, ANO1 knockdown reduced the Ca²⁺‐activated Cl⁻ current in smooth muscle cells. Consistent with conventional hypothesis, the contractility of aorta from ANO1 heterozygous mice was reduced. Surprisingly, we found an enhanced contractility of tail and saphenous arteries from ANO1 heterozygous mice when stimulated with noradrenaline, vasopressin, and K⁺‐induced depolarization. This difference was endothelium‐independent. The increased contractility of ANO1 downregulated small arteries was due to increased Ca²⁺ influx. The expression of L‐type Ca²⁺ channels was not affected but expression of the plasma membrane Ca²⁺ ATPase 1 and the Piezo1 channel was increased. Expressional analysis of tail arteries further suggested changes of ANO1 knockdown smooth muscle cells toward a pro‐contractile phenotype. We did not find any difference between genotypes in blood pressure, heart rate, pressor response, and vasorelaxation in vivo. Our findings in tail and saphenous arteries contrast with the conventional hypothesis and suggest additional roles for ANO1 as a multifunctional protein in the vascular wall that regulates Ca²⁺ homeostasis and smooth muscle cell phenotype.

Increased Alcohol Consumption in Mice Lacking Sodium Bicarbonate Transporter NBCn1

The previous reports on an addiction vulnerability marker in the human SLC4A7 gene encoding the Na/HCO3 transporter NBCn1 suggest that this pH-regulating protein may affect alcohol-related behavior and response. Here, we examined alcohol consumption and sensitivity to the sedative effects of alcohol in male NBCn1 knockout mice. These mice displayed lower pH in neurons than wildtype controls, determined by intracellular pH in hippocampal neuronal cultures. Neurons from knockout mice had a higher action potential threshold and a more depolarized membrane potential, thus reducing membrane excitability. In a two-bottle free choice procedure, knockout mice consumed more alcohol than controls and consistently increased alcohol consumption after repeated alcohol deprivation periods. Quinine and sucrose preference was similar between genotypes. Knockout mice showed increased propensity for alcohol-induced conditioned place preference. In loss of righting reflex assessment, knockout mice revealed increased sensitivity to alcohol-induced sedation and developed tolerance to the sedation after repeated alcohol administrations. Furthermore, chronic alcohol consumption caused NBCn1 downregulation in the hippocampus and striatum of mice and humans. These results demonstrate an important role of NBCn1 in regulation of alcohol consumption and sensitivity to alcohol-induced sedation.

Smooth muscle Ca2+ sensitization causes hypercontractility of middle cerebral arteries in mice bearing the familial hemiplegic migraine type 2 associated mutation

Familial hemiplegic migraine type 2 (FHM2) is associated with inherited point-mutations in the Na,K-ATPase α2 isoform, including G301R mutation. We hypothesized that this mutation affects specific aspects of vascular function, and thus compared cerebral and systemic arteries from heterozygote mice bearing the G301R mutation (Atp1a2^+/-G301R) with wild type (WT). Middle cerebral (MCA) and mesenteric small artery (MSA) function was compared in an isometric myograph. Cerebral blood flow was assessed with Laser speckle analysis. Intracellular Ca²⁺ and membrane potential were measured simultaneously. Protein expression was semi-quantified by immunohistochemistry. Protein phosphorylation was analysed by Western blot. MSA from Atp1a2^+/-G301R and WT showed similar contractile responses. The Atp1a2^+/-G301R MCA constricted stronger to U46619, endothelin and potassium compared to WT. This was associated with an increased depolarization, although the Ca²⁺ change was smaller than in WT. The enhanced constriction of Atp1a2^+/-G301R MCA was associated with increased cSrc activation, stronger sensitization to [Ca²⁺]_i and increased MYPT1 phosphorylation. These differences were abolished by cSrc inhibition. Atp1a2^+/-G301R mice had reduced resting blood flow through MCA in comparison with WT mice. FHM2-associated mutation leads to elevated contractility of MCA due to sensitization of the contractile machinery to Ca²⁺, which is mediated via Na,K-ATPase/Src-kinase/MYPT1 signalling.

Rat mesenteric small artery neurogenic dilatation is predominantly mediated by β1 -adrenoceptors in vivo

KEY POINTS

The prevailing dogma about neurogenic regulation of vascular tone consists of major vasodilatation caused by CGRP (and possibly substance P) released from sensory-motor nerves and vasoconstriction caused by noradrenaline, ATP and neuropeptode Y release from sympathetic nerves. Most studies on perivascular nerve-mediated vasodilatation are made in vitro. In the present study, we provide evidence indicating that in vivo electrical perivascular nerve stimulation in rat mesenteric small arteries causes a large β1-adrenoceptor-mediated vasodilatation, which contrasts with a smaller vasodilatation caused by endogenous CGRP that is only visible after inhibition of Y1 NPY receptors.

ABSTRACT

Mesenteric arteries are densely innervated and the nerves are important regulators of vascular tone and hence blood pressure and blood flow. Perivascular sensory-motor nerves have been shown to cause vasodilatation in vitro. However, less is known about their function in vivo. Male Wistar rats (10-12 weeks old; n = 72) were anaesthetized with ketamine (3 mg kg^-1 ) and xylazine (0.75 mg kg^-1 ) or pentobarbital (60 mg kg^-1 ). After a laparotomy, a section of second-order mesenteric artery was visualized in an organ bath after minimal removal of perivascular adipose tissue. The effects of electrical field stimulation (EFS) and drugs on artery diameter and blood flow were recorded with intravital microscopy and laser speckle imaging. EFS caused vasodilatation in arteries constricted with 1 μm U46619 in the presence of 140 μm suramin and 1 μm prazosin. The vasodilatation was inhibited by 1 μm tetrodotoxin and 5 μm guanethidine, although not by the 1 μm of the CGRP receptor antagonist BIBN4096bs. In the presence of 0.3 μm Y1 receptor antagonist BIBP3226, BIBN4096bs partly inhibited the vasodilatation. Atenolol at a concentration 1 μm inhibited the vasodilatation, whereas 0.1 μm of the β₂ -adrenoceptor selective antagonist ICI-118,551 had no effect. Increasing the extracellular [K⁺ ] to 20 mm caused vasodilatation but was converted to vasoconstriction in the presence of 1 μm BIBN4096bs, and constriction to 30 mm potassium was potentiated by BIBN4096bs. Atenolol but not BIBN4096bs increased contraction to EFS in the absence of suramin and prazosin. In mesenteric small arteries of anaesthetized rats, EFS failed to stimulate major dilatation via sensory-motor nerves but induced sympathetic β₁ -adrenoceptor-mediated dilatation.

The vasodilating effect of glucose differs among vessels at different branching level in the porcine retina ex vivo

Diabetic retinopathy is characterized by retinal lesions related to disturbances in retinal blood flow. The metabolic dysregulation in diabetes involves hyperglycemia which in both clinical and experimental studies has been shown to induce dilatation of larger retinal vessels, which has been suggested to be mediated by nitric oxide (NO). However, the effects of glucose on the diameter of smaller retinal vessels that are the site of development of diabetic retinopathy are unknown. Diameter changes in porcine retinal arterioles, pre-capillary arterioles and capillaries were studied ex vivo during acute changes in intraluminal glucose concentrations that mimicked changes in plasma glucose in diabetic patients. The experiments were repeated during blocking of NO-synthesis. Intravascular application of 2 mM glucose dilated arterioles and capillaries significantly, while 20 mM glucose dilated precapillary arterioles significantly. Intravascular application of 20 mM glucose dilated precapillary arterioles previously exposed to 2 mM glucose, while no significant diameter changes were observed after application of 2 mM glucose in vessels previously exposed to 20 mM glucose. No diameter changes were observed after application of 5.5 mM glucose in vessels previously exposed to both 2 mM and 20 mM glucose in either order. There was no significant difference between the diameter responses in the absence and presence of NO-synthesis blocker. Glucose induced dilatation of porcine precapillary arterioles ex vivo differs from the response in larger arterioles and capillaries, and the response is unaffected by the blocking of NO-synthesis. This may have implications for understanding the pathophysiology of diseases in the retinal microcirculation, such as diabetic retinopathy.

The α2 isoform Na,K-ATPase modulates contraction of rat mesenteric small artery via cSrc-dependent Ca2+ sensitization

AIMS

The Na,K-ATPase is involved in a large number of regulatory activities including cSrc-dependent signalling. Upon inhibition of the Na,K-ATPase with ouabain, cSrc activation is shown to occur in many cell types. This study tests the hypothesis that acute potentiation of agonist-induced contraction by ouabain is mediated through Na,K-ATPase-cSrc signalling-dependent sensitization of vascular smooth muscle cells to Ca²⁺ .

METHODS

Agonist-induced rat mesenteric small artery contraction was examined in vitro under isometric conditions and in vivo in anaesthetized rats. Arterial wall tension and [Ca²⁺ ]_i in vascular smooth muscle cells were measured simultaneously. Changes in cSrc and myosin phosphatase targeting protein 1 (MYPT1) phosphorylation were analysed by Western blot. Protein expression was examined with immunohistochemistry. The α1 and α2 isoforms of the Na,K-ATPase were transiently downregulated by siRNA transfection in vivo.

RESULTS

Ten micromolar ouabain, but not digoxin, potentiated contraction to noradrenaline. This effect was not endothelium-dependent. Ouabain sensitized smooth muscle cells to Ca²⁺ , and this was associated with increased phosphorylation of cSrc and MYPT1. Inhibition of tyrosine kinase by genistein, PP2 or pNaKtide abolished the potentiating effect of ouabain on arterial contraction and Ca²⁺ sensitization. Downregulation of the Na,K-ATPase α2 isoform made arterial contraction insensitive to ouabain and tyrosine kinase inhibition.

CONCLUSION

Data suggest that micromolar ouabain potentiates agonist-induced contraction of rat mesenteric small artery via Na,K-ATPase-dependent cSrc activation, which increases Ca²⁺ sensitization of vascular smooth muscle cells by MYPT1 phosphorylation. This mechanism may be critical for acute control of vascular tone.

4-Aminopyridine: a pan voltage-gated potassium channel inhibitor that enhances Kv 7.4 currents and inhibits noradrenaline-mediated contraction of rat mesenteric small arteries

BACKGROUND AND PURPOSE

K_v 7.4 and K_v 7.5 channels are regulators of vascular tone. 4-Aminopyridine (4-AP) is considered a broad inhibitor of voltage-gated potassium (K_V ) channels, with little inhibitory effect on K_v 7 family members at mmol concentrations. However, the effect of 4-AP on K_v 7 channels has not been systematically studied. The aim of this study was to investigate the pharmacological activity of 4-AP on K_v 7.4 and K_v 7.5 channels and characterize the effect of 4-AP on rat resistance arteries.

EXPERIMENTAL APPROACH

Voltage clamp experiments were performed on Xenopus laevis oocytes injected with cRNA encoding KCNQ4 or KCNQ5, HEK cells expressing K_v 7.4 channels and on rat, freshly isolated mesenteric artery smooth muscle cells. The effect of 4-AP on tension, membrane potential, intracellular calcium and pH was assessed in rat mesenteric artery segments.

KEY RESULTS

4-AP increased the K_v 7.4-mediated current in oocytes and HEK cells but did not affect K_v 7.5 current. 4-AP also enhanced native mesenteric artery myocyte K⁺ current at sub-mmol concentrations. When applied to NA-preconstricted mesenteric artery segments, 4-AP hyperpolarized the membrane, decreased [Ca²⁺ ]_i and caused concentration-dependent relaxations that were independent of 4-AP-mediated changes in intracellular pH. Application of the K_v 7 channel blocker XE991 and BK_Ca channel blocker iberiotoxin attenuated 4-AP-mediated relaxation. 4-AP also inhibited the NA-mediated signal transduction to elicit a relaxation.

CONCLUSIONS AND IMPLICATIONS

These data show that 4-AP is able to relax NA-preconstricted rat mesenteric arteries by enhancing the activity of K_v 7.4 and BK_Ca channels and attenuating NA-mediated signalling.

PDE1A inhibition elicits cGMP-dependent relaxation of rat mesenteric arteries

BACKGROUND AND PURPOSE

PDE1, a subfamily of cyclic nucleotide PDEs consisting of three isoforms, PDE1A, PDE1B and PDE1C, has been implicated in the regulation of vascular tone. The PDE1 isoform(s) responsible for tone regulation is unknown. This study used isoform-preferring PDE1 inhibitors, Lu AF58027, Lu AF64196, Lu AF66896 and Lu AF67897, to investigate the relative contribution of PDE1 isoforms to regulation of vascular tone.

EXPERIMENTAL APPROACH

In rat mesenteric arteries, expression and localization of Pde1 isoforms were determined by quantitative PCR and in situ hybridization, and physiological impact of PDE1 inhibition was evaluated by isometric tension recordings.

KEY RESULTS

In rat mesenteric arteries, Pde1a mRNA expression was higher than Pde1b and Pde1c. In situ hybridization revealed localization of Pde1a to vascular smooth muscle cells (VSMCs) and only minor appearance of Pde1b and Pde1c. The potency of the PDE1 inhibitors at eliciting relaxation showed excellent correlation with their potency at inhibiting PDE1A. Thus, Lu AF58027 was the most potent at inhibiting PDE1A and was also the most potent at eliciting relaxation in mesenteric arteries. Inhibition of NOS with l-NAME, soluble GC with ODQ or PKG with Rp-8-Br-PET-cGMP all attenuated the inhibitory effect of PDE1 on relaxation, whereas PKA inhibition with H89 had no effect.

CONCLUSIONS AND IMPLICATIONS

Pde1a is the dominant PDE1 isoform present in VSMCs, and relaxation mediated by PDE1A inhibition is predominantly driven by enhanced cGMP signalling. These results imply that isoform-selective PDE1 inhibitors are powerful investigative tools allowing examination of physiological and pathological roles of PDE1 isoforms.

Intravital investigation of rat mesenteric small artery tone and blood flow

KEY POINTS

Substantial information on rat mesenteric small artery physiology and pharmacology based on in vitro experiments is available. Little is known about the relevance of this for artery function in vivo. We here present an intravital model where rat mesenteric small artery diameters are studied under isolated and controlled conditions in situ with simultaneous measurement of blood flow. The responses of the isolated arteries vary with the anaesthetic used, and they are quantitatively but not qualitatively different from the responses seen in vitro.

ABSTRACT

Functional characteristics of rat mesenteric small arteries (internal diameter ∼150-200 μm) have been extensively studied in vitro using isometric and isobaric myographs. In vivo, precapillary arterioles (internal diameter < 50 μm) have been studied, but only a few studies have investigated the function of mesenteric small arteries. We here present a novel approach for intravital studies of rat mesenteric small artery segments (∼5 mm long) isolated in a chamber. The agonist-induced changes in arterial diameter and blood flow were studied using video imaging and laser speckle analysis in rats anaesthetized by isoflurane, pentobarbital, ketamine-xylazine, or by a combination of fentanyl, fluanison and midazolam (rodent mixture). The arteries had spontaneous tone. Noradrenaline added to the chamber constricted the artery in the chamber but not the downstream arteries in the intestinal wall. The constriction was smaller when rats were anaesthetized by rodent mixture in comparison with other anaesthetics, where responses were qualitatively similar to those reported in vitro. The contraction was associated with reduction of blood flow, but no flow reduction was seen in the downstream arteries in the intestinal wall. The magnitude of different endothelium-dependent relaxation pathways was dependent on the anaesthesia. Vasomotion was present under all forms of anaesthesia with characteristics similar to in vitro. We have established an intravital method for studying the tone and flow in rat mesenteric arteries. The reactivity of the arteries was qualitatively similar to the responses previously obtained under in vitro conditions, but the choice of anaesthetic affects the magnitude of responses.

Down-regulation of KCa2.3 channels causes erectile dysfunction in mice

Modulation of endothelial calcium-activated K⁺ channels has been proposed as an approach to restore arterial endothelial cell function in disease. We hypothesized that small-conductance calcium-activated K⁺ channels (K_Ca2.3 or SK3) contributes to erectile function. The research was performed in transgenic mice with overexpression (K_Ca2.3^T/T(−Dox)) or down-regulation (K_Ca2.3^T/T(+Dox)) of the K_Ca2.3 channels and wild-type C57BL/6-mice (WT). QPCR revealed that K_Ca2.3 and K_Ca1.1 channels were the most abundant in mouse corpus cavernosum. K_Ca2.3 channels were found by immunoreactivity and electron microscopy in the apical-lateral membrane of endothelial cells in the corpus cavernosum. Norepinephrine contraction was enhanced in the corpus cavernosum of K_Ca2.3^T/T(+Dox)versus K_Ca2.3^T/T(−Dox) mice, while acetylcholine relaxation was only reduced at 0.3 µM and relaxations in response to the nitric oxide donor sodium nitroprusside were unaltered. An opener of K_Ca2 channels, NS309 induced concentration-dependent relaxations of corpus cavernosum. Mean arterial pressure was lower in K_Ca2.3^T/T(−Dox) mice compared with WT and K_Ca2.3^T/T(+Dox) mice. In anesthetized mice, cavernous nerve stimulation augmented in frequency/voltage dependent manner erectile function being lower in K_Ca2.3^T/T(+Dox) mice at low frequencies. Our findings suggest that down-regulation of K_Ca2.3 channels contributes to erectile dysfunction, and that pharmacological activation of K_Ca2.3 channels may have the potential to restore erectile function.

Differential effects of nitric oxide and cyclo-oxygenase inhibition on the diameter of porcine retinal vessels with different caliber during hypoxia ex vivo

BACKGROUND

Hypoxia induced relaxation of larger retinal arterioles has been shown to be mediated by nitric oxide (NO) and cyclo-oxygenase (COX) products both in vivo and in vitro. However, the involvement of smaller retinal vessels in the response is unknown. Therefore, the purpose of the present study was to investigate the effect of blocking the synthesis of NO and COX on hypoxia induced changes in the diameter of smaller porcine retinal vessels at different branching level.

METHODS

Porcine hemiretinas were mounted in a tissue chamber and were constricted with the prostaglandin agonist U46619. Changes in the diameter of arterioles, pre-capillary arterioles and capillaries were studied during hypoxia, in the presence of the COX inhibitor ibuprofen and the NO synthase inhibitor L-NAME.

RESULTS

In the presence of L-NAME hypoxia induced dilatation was significantly smaller in arterioles and capillaries than in precapillary arterioles (p < 0.04), whereas in the presence of ibuprofen the dilatation was significantly smaller in capillaries and pre-capillary arterioles than in arterioles (p < 0.04).

CONCLUSIONS

The mechanisms underlying hypoxia induced dilatation differ among smaller porcine retinal vessels with different caliber ex vivo. This may reflect differences in the responses of retinal vessels to changes in metabolism, and may point to possible targets for pharmacological intervention on the diameter of retinal vessels with different caliber in vivo.

Na-K-ATPase regulates intercellular communication in the vascular wall via cSrc kinase-dependent connexin43 phosphorylation

Communication between vascular smooth muscle cells (VSMCs) is dependent on gap junctions and is regulated by the Na-K-ATPase. The Na-K-ATPase is therefore important for synchronized VSMC oscillatory activity, i.e., vasomotion. The signaling between the Na-K-ATPase and gap junctions is unknown. We tested here the hypothesis that this signaling involves cSrc kinase. Intercellular communication was assessed by membrane capacitance measurements of electrically coupled VSMCs. Vasomotion in isometric myograph, input resistance, and synchronized [Ca2+]i transients were used as readout for intercellular coupling in rat mesenteric small arteries in vitro. Phosphorylation of cSrc kinase and connexin43 (Cx43) were semiquantified by Western blotting. Micromole concentration of ouabain reduced the amplitude of norepinephrine-induced vasomotion and desynchronized Ca2+ transients in VSMC in the arterial wall. Ouabain also increased input resistance in the arterial wall. These effects of ouabain were antagonized by inhibition of tyrosine phosphorylation with genistein, PP2, and by an inhibitor of the Na-K-ATPase-dependent cSrc activation, pNaKtide. Moreover, inhibition of cSrc phosphorylation increased vasomotion amplitude and decreased the resistance between cells in the vascular wall. Ouabain inhibited the electrical coupling between A7r5 cells, but pNaKtide restored the electrical coupling. Ouabain increased cSrc autophosphorylation of tyrosine 418 (Y418) required for full catalytic activity whereas pNaKtide antagonized it. This cSrc activation was associated with Cx43 phosphorylation of tyrosine 265 (Y265). Our findings demonstrate that Na-K-ATPase regulates intercellular communication in the vascular wall via cSrc-dependent Cx43 tyrosine phosphorylation.

GLP-1 inhibits VEGFA-mediated signaling in isolated human endothelial cells and VEGFA-induced dilation of rat mesenteric arteries

We investigated the acute effects of glucagon-like peptide-1 (GLP-1), GLP-1(1–36), and GLP-1(7–36) on vascular endothelial growth factor-A (VEGFA)-induced endothelium-dependent signaling and vasodilation. Our hypothesis was that GLP-1 released from intestinal l-cells modulates processes related to PLCγ activation, Src, and endothelial NOS (eNOS) signaling, thereby controlling endothelial vessel tone. By using RT-PCR analysis, we found mRNA for the GLP-1 receptor (GLP-1R) in human dermal microvascular endothelial cells (HDMEC), human retinal microvascular endothelial cells, and rat arteries. In isolated rat mesenteric resistance arteries precontracted with the thromboxane analog U46619 to 80–90% of maximum contraction, VEGFA (25 ng/ml) caused a small and gradual relaxation (28.9 ± 3.9%). Pretreatment of arteries with either GLP-1(1–36) (500 nM) or GLP-1(7–36) (1 nM) abolished the VEGFA-induced relaxation. VEGFA-induced relaxations were also inhibited in endothelial-denuded arteries and in arteries pretreated with the nitric oxide synthase (NOS) inhibitor, Nω-nitro-l-arginine methyl ester (100 μM). In vivo studies on male Wistar rats also revealed that GLP-1(7–36) inhibited VEGFA-induced vasodilation of the same arteries. In isolated endothelial cells, GLP-1(1–36) and GLP-1(7–36) caused a reduction in VEGFA-induced phosphorylation of PLCγ. Ca2+ imaging of endothelial cells and rat mesenteric resistance arteries using fura-2, revealed that both GLP-1 analogs caused a reduction in VEGFA-induced Ca2+ signaling. GLP-1(1–36) also reduced VEGFA-induced eNOS phosphorylation in HDMEC. In conclusion, GLP-1 reduced relaxation induced by VEGFA in resistance arteries by inhibiting VEGFR2-mediated Ca2+ signaling and endothelial NO synthesis. GLP-1, on its own, also induced phosphorylation of Src and ERK1/2 that can lead to proliferation and is implicated in vessel permeability.

Negative News: Cl− and HCO3− in the Vascular Wall

Cl− and HCO3− are the most prevalent membrane-permeable anions in the intra- and extracellular spaces of the vascular wall. Outwardly directed electrochemical gradients for Cl− and HCO3− permit anion channel opening to depolarize vascular smooth muscle and endothelial cells. Transporters and channels for Cl− and HCO3− also modify vascular contractility and structure independently of membrane potential. Transport of HCO3− regulates intracellular pH and thereby modifies the activity of enzymes, ion channels, and receptors. There is also evidence that Cl− and HCO3− transport proteins affect gene expression and protein trafficking. Considering the extensive implications of Cl− and HCO3− in the vascular wall, it is critical to understand how these ions are transported under physiological conditions and how disturbances in their transport can contribute to disease development. Recently, sensing mechanisms for Cl− and HCO3− have been identified in the vascular wall where they modify ion transport and vasomotor function, for instance, during metabolic disturbances. This review discusses current evidence that transport (e.g., via NKCC1, NBCn1, Ca2+-activated Cl− channels, volume-regulated anion channels, and CFTR) and sensing (e.g., via WNK and RPTPγ) of Cl− and HCO3− influence cardiovascular health and disease.

The vasodilating effects of insulin and lactate are increased in precapillary arterioles in the porcine retina ex vivo

PURPOSE

Disturbances in the diameter regulation of the larger retinal vessels are involved in the pathophysiology of a number of vision threatening retinal diseases, but the role of the smaller vessels has received less attention. Therefore, an experimental model was developed for studying diameter changes in retinal vessels at all branching levels secondary to pharmacological interventions ex vivo.

METHODS

Porcine hemiretinas were mounted in a tissue chamber that allowed the control of temperature, pH and oxygen saturation. The chamber was positioned in a fluorescence microscope, and changes in the diameter of larger arterioles, precapillary arterioles and capillaries were studied after intravascular and extravascular application of the thromboxane analogue U46619, lactate, the glutamate agonist N-Methyl-D-aspartic acid (NMDA) and insulin.

RESULTS

U46619 induced significant contraction of all vessel calibres after extravascular application, but had no significant effect on vessel diameters after intravascular application. In contracted vessels, lactate induced significant dilatation in all vessel calibres, and the effect was more pronounced after intravascular than after extravascular application where only precapillary arterioles dilated significantly. N-Methyl-D-aspartic acid (NMDA) induced similar dilatation in the three vessel types after intra- and extravascular application, whereas the dilating effect of insulin was significantly more pronounced in the precapillary arterioles than in the other vessel types.

CONCLUSION

The diameter regulation in precapillary retinal arterioles differs from that of other retinal vascular branching levels and may play a special role in the pathophysiology of retinal vascular disease.

Extracellular HCO3- is sensed by mouse cerebral arteries: Regulation of tone by receptor protein tyrosine phosphatase γ

We investigate sensing and signaling mechanisms for H(+), [Formula: see text] and CO2 in basilar arteries using out-of-equilibrium solutions. Selectively varying pHo, [[Formula: see text]]o, or pCO2, we find: (a) lowering pHo attenuates vasoconstriction and vascular smooth muscle cell (VSMC) Ca(2+)-responses whereas raising pHo augments vasoconstriction independently of VSMC [Ca(2+)]i, (b) lowering [[Formula: see text]]o increases arterial agonist-sensitivity of tone development without affecting VSMC [Ca(2+)]i but c) no evidence that CO2 has direct net vasomotor effects. Receptor protein tyrosine phosphatase (RPTP)γ is transcribed in endothelial cells, and direct vasomotor effects of [Formula: see text] are absent in arteries from RPTPγ-knockout mice. At pHo 7.4, selective changes in [[Formula: see text]]o or pCO2 have little effect on pHi At pHo 7.1, decreased [[Formula: see text]]o or increased pCO2 causes intracellular acidification, which attenuates vasoconstriction. Under equilibrated conditions, anti-contractile effects of CO2/[Formula: see text] are endothelium-dependent and absent in arteries from RPTPγ-knockout mice. With CO2/[Formula: see text] present, contractile responses to agonist-stimulation are potentiated in arteries from RPTPγ-knockout compared to wild-type mice, and this difference is larger for respiratory than metabolic acidosis. In conclusion, decreased pHo and pHi inhibit vasoconstriction, whereas decreased [[Formula: see text]]o promotes vasoconstriction through RPTPγ-dependent changes in VSMC Ca(2+)-sensitivity. [Formula: see text] serves dual roles, providing substrate for pHi-regulating membrane transporters and modulating arterial responses to acid-base disturbances.

Na+, HCO3--cotransporter NBCn1 increases pHi gradients, filopodia, and migration of smooth muscle cells and promotes arterial remodelling

AIMS

Arterial remodelling can cause luminal narrowing and obstruct blood flow. We tested the hypothesis that cellular acid-base transport facilitates proliferation and migration of vascular smooth muscle cells (VSMCs) and enhances remodelling of conduit arteries.

METHODS AND RESULTS

[Formula: see text]-cotransport via NBCn1 (Slc4a7) mediates net acid extrusion and controls steady-state intracellular pH (pHi) in VSMCs of mouse carotid arteries and primary aortic explants. Carotid arteries undergo hypertrophic inward remodelling in response to partial or complete ligation in vivo, but the increase in media area and thickness and reduction in lumen diameter are attenuated in arteries from NBCn1 knock-out compared with wild-type mice. With [Formula: see text] present, gradients for pHi (∼0.2 units magnitude) exist along the axis of VSMC migration in primary explants from wild-type but not NBCn1 knock-out mice. Knock-out or pharmacological inhibition of NBCn1 also reduces filopodia and lowers initial rates of VSMC migration after scratch-wound infliction. Interventions to reduce H(+)-buffer mobility (omission of [Formula: see text] or inhibition of carbonic anhydrases) re-establish axial pHi gradients, filopodia, and migration rates in explants from NBCn1 knock-out mice. The omission of [Formula: see text] also lowers global pHi and inhibits proliferation in primary explants.

CONCLUSION

Under physiological conditions (i.e. with [Formula: see text] present), NBCn1-mediated [Formula: see text] uptake raises VSMC pHi and promotes filopodia, VSMC migration, and hypertrophic inward remodelling. We propose that axial pHi gradients enhance VSMC migration whereas global acidification inhibits VSMC proliferation and media hypertrophy after carotid artery ligation. These findings support a key role of acid-base transport, particularly via NBCn1, for development of occlusive artery disease.

Chronic selective serotonin reuptake inhibition modulates endothelial dysfunction and oxidative state in rat chronic mild stress model of depression

Major depression is known to be associated with cardiovascular abnormalities, and oxidative stress has been suggested to play a role. We tested the hypothesis that antidepressant treatment reduces oxidative stress and endothelial dysfunctions in the chronic mild stress (CMS) model of depression in rats. Rats with >30% reduction in sucrose intake after 4 wk of CMS were defined in the study as CMS-susceptible and compared with unstressed controls. Sixteen CMS-susceptible and eight unstressed rats were treated during weeks 5 to 8 of the CMS protocol with escitalopram. Escitalopram-treated rats with >20% recovery in the sucrose consumption during the last 2 wk of treatment were defined as escitalopram responders. Rats that did not reach these criteria were defined as escitalopram nonresponders. In the open field test, escitalopram responders demonstrated anxiolytic effect of treatment. In mesenteric small arteries, escitalopram affected neither NO nor cyclooxygenase-1 (COX-1)-mediated vasodilation. Escitalopram potentiated endothelium-dependent hyperpolarization-like response, which was suppressed in the vehicle-treated CMS-susceptible rats and reduced COX-2-dependent relaxation, which was elevated in the vehicle-treated CMS-susceptible rats. Escitalopram did not affect blood pressure and heart rate, which were elevated in the vehicle-treated CMS-susceptible rats. Oxidative stress markers were changed in association with CMS in liver, heart, and brain. Escitalopram normalized oxidative stress markers in the majority of tissues. This study demonstrates that the antidepressant effect of escitalopram is associated with partial improvement of endothelial function in small arteries affecting COX-2 and endothelium-dependent hyperpolarization-like pathways.

The giraffe kidney tolerates high arterial blood pressure by high renal interstitial pressure and low glomerular filtration rate

BACKGROUND

The tallest animal on earth, the giraffe (Giraffa camelopardalis) is endowed with a mean arterial blood pressure (MAP) twice that of other mammals. The kidneys reside at heart level and show no sign of hypertension-related damage. We hypothesized that a species-specific evolutionary adaption in the giraffe kidney allows normal for size renal haemodynamics and glomerular filtration rate (GFR) despite a MAP double that of other mammals.

METHODS

Fourteen anaesthetized giraffes were instrumented with vascular and bladder catheters to measure glomerular filtration rate (GFR) and effective renal plasma flow (ERPF). Renal interstitial hydrostatic pressure (RIHP) was assessed by inserting a needle into the medullary parenchyma. Doppler ultrasound measurements provided renal artery resistive index (RI). Hormone concentrations as well as biomechanical, structural and histological characteristics of vascular and renal tissues were determined.

RESULTS

GFR averaged 342 ± 99 mL min(-1) and ERPF 1252 ± 305 mL min(-1) . RIHP varied between 45 and 140 mmHg. Renal pelvic pressure was 39 ± 2 mmHg and renal venous pressure 32 ± 4 mmHg. A valve-like structure at the junction of the renal and vena cava generated a pressure drop of 12 ± 2 mmHg. RI was 0.27. The renal capsule was durable with a calculated burst pressure of 600 mmHg. Plasma renin and AngII were 2.6 ± 0.5 mIU L(-1) and 9.1 ± 1.5 pg mL(-1) respectively.

CONCLUSION

In giraffes, GFR, ERPF and RI appear much lower than expected based on body mass. A strong renal capsule supports a RIHP, which is >10-fold that of other mammals effectively reducing the net filtration pressure and protecting against the high MAP.

Cation-coupled bicarbonate transporters

Cation-coupled HCO3(-) transport was initially identified in the mid-1970s when pioneering studies showed that acid extrusion from cells is stimulated by CO2/HCO3(-) and associated with Na(+) and Cl(-) movement. The first Na(+)-coupled bicarbonate transporter (NCBT) was expression-cloned in the late 1990s. There are currently five mammalian NCBTs in the SLC4-family: the electrogenic Na,HCO3-cotransporters NBCe1 and NBCe2 (SLC4A4 and SLC4A5 gene products); the electroneutral Na,HCO3-cotransporter NBCn1 (SLC4A7 gene product); the Na(+)-driven Cl,HCO3-exchanger NDCBE (SLC4A8 gene product); and NBCn2/NCBE (SLC4A10 gene product), which has been characterized as an electroneutral Na,HCO3-cotransporter or a Na(+)-driven Cl,HCO3-exchanger. Despite the similarity in amino acid sequence and predicted structure among the NCBTs of the SLC4-family, they exhibit distinct differences in ion dependency, transport function, pharmacological properties, and interactions with other proteins. In epithelia, NCBTs are involved in transcellular movement of acid-base equivalents and intracellular pH control. In nonepithelial tissues, NCBTs contribute to intracellular pH regulation; and hence, they are crucial for diverse tissue functions including neuronal discharge, sensory neuron development, performance of the heart, and vascular tone regulation. The function and expression levels of the NCBTs are generally sensitive to intracellular and systemic pH. Animal models have revealed pathophysiological roles of the transporters in disease states including metabolic acidosis, hypertension, visual defects, and epileptic seizures. Studies are being conducted to understand the physiological consequences of genetic polymorphisms in the SLC4-members, which are associated with cancer, hypertension, and drug addiction. Here, we describe the current knowledge regarding the function, structure, and regulation of the mammalian cation-coupled HCO3(-) transporters of the SLC4-family.

Voltage-gated sodium channels contribute to action potentials and spontaneous contractility in isolated human lymphatic vessels

Voltage-gated sodium channels (VGSC) play a key role for initiating action potentials (AP) in excitable cells. VGSC in human lymphatic vessels have not been investigated. In the present study, we report the electrical activity and APs of small human lymphatic collecting vessels, as well as mRNA expression and function of VGSC in small and large human lymphatic vessels. The VGSC blocker TTX inhibited spontaneous contractions in six of 10 spontaneously active vessels, whereas ranolazine, which has a narrower VGSC blocking profile, had no influence on spontaneous activity. TTX did not affect noradrenaline-induced contractions. The VGSC opener veratridine induced contractions in a concentration-dependent manner (0.1-30 μm) eliciting a stable tonic contraction and membrane depolarization to -18 ± 0.6 mV. Veratridine-induced depolarizations and contractions were reversed ∼80% by TTX, and were dependent on Ca(2+) influx via L-type calcium channels and the sodium-calcium exchanger in reverse mode. Molecular analysis determined NaV 1.3 to be the predominantly expressed VGSC isoform. Electrophysiology of mesenteric lymphatics determined the resting membrane potential to be -45 ± 1.7 mV. Spontaneous APs were preceded by a slow depolarization of 5.3 ± 0.6 mV after which a spike was elicited that almost completely repolarized before immediately depolarizing again to plateau. Vessels transiently hyperpolarized prior to returning to the resting membrane potential. TTX application blocked APs. We have shown that VGSC are necessary for initiating and maintaining APs and spontaneous contractions in human lymphatic vessels and our data suggest the main contribution from comes NaV 1.3. We have also shown that activation of these channels augments the contractile activity of the vessels.

The bestrophin- and TMEM16A-associated Ca(2+)- activated Cl(–) channels in vascular smooth muscles

The presence of Ca²⁺-activated Cl^– currents (I_Cl(Ca)) in vascular smooth muscle cells (VSMCs) is well established. I_Cl(Ca) are supposedly important for arterial contraction by linking changes in [Ca²⁺]_i and membrane depolarization. Bestrophins and some members of the TMEM16 protein family were recently associated with I_Cl(Ca). Two distinct I_Cl(Ca) are characterized in VSMCs; the cGMP-dependent I_Cl(Ca) dependent upon bestrophin expression and the ‘classical’ Ca²⁺-activated Cl^– current, which is bestrophin-independent. Interestingly, TMEM16A is essential for both the cGMP-dependent and the classical I_Cl(Ca). Furthermore, TMEM16A has a role in arterial contraction while bestrophins do not. TMEM16A’s role in the contractile response cannot be explained however only by a simple suppression of the depolarization by Cl^– channels. It is suggested that TMEM16A expression modulates voltage-gated Ca²⁺ influx in a voltage-independent manner and recent studies also demonstrate a complex role of TMEM16A in modulating other membrane proteins.

Dorzolamide-induced relaxation of porcine retinal arterioles in vitro depends on nitric oxide but not on acidosis in vascular smooth muscle cells

The carbonic anhydrase inhibitor dorzolamide can induce relaxation of retinal arterioles with a consequent increase in blood flow and oxygenation of the retina. It has been shown that the mechanisms underlying this relaxation are independent of extracellular acidosis and CO2. The purpose of the present study was to investigate the possible involvement of nitric oxide (NO) and intracellular acidosis in dorzolamide-induced relaxation of retinal arterioles. Porcine retinal arterioles were mounted in a wire myograph and dorzolamide induced relaxation was studied after 1) the addition of the NO synthase inhibitor l-NAME (3 × 10(-4) M) or the guanylyl cyclase inhibitor ODQ (3 × 10(-6) M), and 2) after loading the smooth muscle cells with the pH sensitive fluorophore SNARF-1-AM and studying changes in vascular tone and intracellular fluorescence after the induction of hypoxia, addition of lactate (10(-2) M), and extracellular acidification (pH = 7.0) alone and in the presence of dorzolamide (10(-3) M). Dorzolamide significantly relaxed retinal arterioles (p < 0.03), and the effect was significantly higher in the presence of perivascular tissue than in isolated vessels at the highest concentration (p < 0.01). In the presence of perivascular tissue dorzolamide-induced relaxation could be reduced by NO inhibition (p < 0.02). Dorzolamide increased intracellular acidification (p < 0.02) during extracellular acidosis, but there was no relation between relaxation and intracellular acidosis. In conclusion, dorzolamide-induced vasorelaxation depends on NO and the perivascular retinal tissue, but is independent of acidification in the extracellular and the intracellular space of retinal vascular smooth muscle cells. Other factors than NO and acidification are involved in dorzolamide-induced relaxation of retinal arterioles.

Human lymphatic vessel contractile activity is inhibited in vitro but not in vivo by the calcium channel blocker nifedipine

Calcium channel blockers (CCB) are widely prescribed anti-hypertensive agents. The commonest side-effect, peripheral oedema, is attributed to a larger arterial than venous dilatation causing increased fluid filtration. Whether CCB treatment is detrimental to human lymphatic vessel function and thereby exacerbates oedema formation is unknown. We observed that spontaneous lymphatic contractions in isolated human vessels (thoracic duct and mesenteric lymphatics) maintained under isometric conditions were inhibited by therapeutic concentrations (nanomolar) of the CCB nifedipine while higher than therapeutic concentrations of verapamil (micromolar) were necessary to inhibit activity. Nifedipine also inhibited spontaneous action potentials measured by sharp microelectrodes. Furthermore, noradrenaline did not elicit normal increases in lymphatic vessel tone when maximal constriction was reduced to 29.4 ± 4.9% of control in the presence of 20 nmol l(-1) nifedipine. Transcripts for the L-type calcium channel gene CACNA1C were consistently detected from human thoracic duct samples examined and the CaV1.2 protein was localized by immunoreactivity to lymphatic smooth muscle cells. While human lymphatics ex vivo were highly sensitive to nifedipine, this was not apparent in vivo when nifedipine was compared to placebo in a randomized, double-blinded clinical trial: conversely, lymphatic vessel contraction frequency was increased and refill time was faster despite all subjects achieving target nifedipine plasma concentrations. We conclude that human lymphatic vessels are highly sensitive to nifedipine in vitro but that care must be taken when extrapolating in vitro observations of lymphatic vessel function to the clinical situation, as similar changes in lymphatic function were not evident in our clinical trial comparing nifedipine treatment to placebo.

The contribution of K(+) channels to human thoracic duct contractility

In smooth muscle cells, K(+) permeability is high, and this highly influences the resting membrane potential. Lymph propulsion is dependent on phasic contractions generated by smooth muscle cells of lymphatic vessels, and it is likely that K(+) channels play a critical role in regulating contractility in this tissue. The aim of this study was to investigate the contribution of distinct K(+) channels to human lymphatic vessel contractility. Thoracic ducts were harvested from 43 patients and mounted in a wire myograph for isometric force measurements or membrane potential recordings with an intracellular microelectrode. Using K(+) channel blockers and activators, we demonstrate a functional contribution to human lymphatic vessel contractility from all the major classes of K(+) channels [ATP-sensitive K(+) (KATP), Ca(2+)-activated K(+), inward rectifier K(+), and voltage-dependent K(+) channels], and this was confirmed at the mRNA level. Contraction amplitude, frequency, and baseline tension were altered depending on which channel was blocked or activated. Microelectrode impalements of lymphatic vessels determined an average resting membrane potential of -43.1 ± 3.7 mV. We observed that membrane potential changes of <5 mV could have large functional effects with contraction frequencies increasing threefold. In general, KATP channels appeared to be constitutively open since incubation with glibenclamide increased contraction frequency in spontaneously active vessels and depolarized and initiated contractions in previously quiescent vessels. The largest change in membrane voltage was observed with the KATP opener pinacidil, which caused 24 ± 3 mV hyperpolarization. We conclude that K(+) channels are important modulators of human lymphatic contractility.

Constriction of porcine retinal arterioles induced by endothelin-1 and the thromboxane analogue U46619 in vitro decreases with increasing vascular branching level

PURPOSE

The retinal blood flow depends on the diameter of retinal arterioles, but diameter changes in these vessels have hitherto only been assessed in vessels larger than approximately 100 μm. Therefore, a new method was developed for studying diameter changes along the vascular tree of arterioles in whole perfused segments of porcine retinas, and the effect of known vasoconstrictors on the diameter of retinal arterioles at different branching levels were studied.

METHODS

Thirty-four whole-mounted porcine retinas were placed in a specially designed tissue chamber. On the basis of video recordings through an inverted microscope, the diameter of retinal arterioles was measured at five different branching levels before and after addition of a high potassium concentration, or increasing concentrations of endothelin-1, the prostaglandin analogue U46619, noradrenaline or none (time controls).

RESULTS

The baseline diameter ranged from 136 μm (95% CI 132-140 μm) for 1st order arterioles to 33 μm (95% CI 21-44 μm) for 5th order arterioles. In 1st order arterioles, endothelin produced 56.6% (95% CI 47.6-64.0) and U46619 14.6% (95% CI 5.7-22.6) relative constriction compared with baseline, which for both compounds decreased significantly with increasing branching level (p<0.0001 and p<0.0001, respectively). The change in diameter during addition of noradrenaline did not differ significantly from the time controls (p=0.07).

CONCLUSIONS

The effect of retinal vasoconstrictors differs among larger and smaller arterioles. The study highlights the need for investigating diameter regulation in smaller retinal arterioles as a basis for understanding normal and pathological changes in retinal blood flow.