Machine learning powered tools for automated analysis of muscle sympathetic nerve activity recordings.

Automated analysis and quantification of physiological signals in clinical practice and medical research can reduce manual labor, increase efficiency, and provide more objective, reproducible results. To build a novel platform for the analysis of muscle sympathetic nerve activity (MSNA), we employed state-of-the-art data processing and machine learning applications. Data processing methods for integrated MSNA recordings were developed to evaluate signals regarding the overall quality of the signal, the validity of individual signal peaks regarding the potential to be MSNA bursts and the timing of their occurrence. An overall probability score was derived from this flexible platform to evaluate each individual signal peak automatically. Overall, three deep neural networks were designed and trained to validate individual signal peaks randomly sampled from recordings representing only electrical noise and valid microneurography recordings. A novel data processing method for the whole signal was developed to differentiate between periods of valid MSNA signal recordings and periods in which the signal was not available or lost due to involuntary movement of the recording electrode. A probabilistic model for timing of the signal bursts was implemented as part of the system. Machine Learning algorithms and data processing tools were implemented to replicate the complex decision-making process of manual MSNA analysis. Validation of manual MSNA analysis including intra- and inter-rater validity and a comparison with automated MSNA tools is required. The developed toolbox for automated MSNA analysis can be extended in a flexible way to include algorithms based on other datasets.

Visualization of Spatially-Controlled Vasospasm by Sympathetic Nerve-Mediated ROCK Activation.

Contraction of vascular smooth muscle is regulated primarily by calcium concentration and secondarily by ROCK activity within the cells. In contrast to the wealth of information regarding regulation of calcium concentration, little is known about the spatiotemporal regulation of ROCK activity in live blood vessels. Here, we report ROCK activation in subcutaneous arterioles in a transgenic mouse line that expresses a genetically encoded ROCK biosensor based on the principle of FÓ§rster resonance energy transfer by two-photon excitation in vivo imaging. Rapid vasospasm was induced upon laser ablation of arterioles, concomitant with a transient increase in calcium concentration in arteriolar smooth muscles. Unlike the increase in calcium concentration, vasoconstriction and ROCK activation continued for several minutes after irradiation. Both the ROCK inhibitor, fasudil, and the ganglionic nicotinic acetylcholine receptor blocker, hexamethonium, inhibited laser-induced ROCK activation and reduced the duration of vasospasm at the segments distant from the irradiated point. These observations suggest that vasoconstriction is initially triggered by a rapid surge of cytoplasmic calcium and then maintained by sympathetic nerve-mediated ROCK activation.

Sympathetic nervous system activity and reactivity in women with gestational diabetes mellitus.

INTRODUCTION: Gestational diabetes mellitus (GDM) is associated with vascular dysfunction. Sympathetic nervous system activity (SNA) is an important regulator of vascular function, and is influenced by glucose and insulin. The association between GDM and SNA (re)activity is unknown. We hypothesize that women with GDM would have increased SNA during baseline and during stress. METHODS: Eighteen women with GDM and 18 normoglycemic pregnant women (controls) were recruited. Muscle SNA (MSNA; peroneal microneurography) was assessed at rest, during a cold pressor test (CPT) and during peripheral chemoreflex deactivation (hyperoxia). Spontaneous sympathetic baroreflex gain was quantified versus diastolic pressure at rest and during hyperoxia. RESULTS: Age, gestational age (third trimester) and pre-pregnancy body mass index and baseline MSNA was not different among the groups. Women with GDM had a similar increase in MSNA, but a greater pressor response to CPT compared to controls (% change in MAP 17 ± 7% vs. 9 ± 9%; p = .004). These data are consistent with a greater neurovascular transduction in GDM (% change in total peripheral resistance/% change in burst frequency [BF]: 15.9 ± 30.2 vs. -5.2 ± 16.4, p = .03). Interestingly, women with GDM had a greater reduction in MSNA during hyperoxia (% change in BF -30 ± 19% vs. -6 ± 17%; p = .01). CONCLUSION: Women diagnosed with GDM have similar basal SNA versus normoglycemic pregnant women, but greater neurovascular transduction, meaning a greater influence of the sympathetic nerve activity in these women. We also document evidence of chemoreceptor hyperactivity, which may influence SNA in women with GDM but not in controls.

Hydrogen Protons Modulate Perivascular Axo-axonal Interactions in the Middle Cerebral Artery of Rats.

Previous studies have demonstrated that nicotine can induce relaxation of the middle cerebral artery (MCA). However, whether this relaxation is associated with the activity of sensory calcitonin gene-related peptide (CGRP) nerves and whether this is modulated by hydrogen protons (H), facilitating the release of CGRP from sensory CGRPergic nerve terminals in the MCA, remains unclear. In this study, we examined the role of H in the modulation of neurogenic vasomotor responses in the rat-isolated endothelium-denuded MCA. Wire myography was used to measure vasoreactivity and indicated that nicotine-induced relaxation was sensitive to tetrodotoxin and lidocaine and drastically reduced levels of guanethidine (an adrenergic neuronal blocker), N-nitro-L-arginine (L-NNA), CGRP8-37, vasoactive intestinal polypeptide (VIP)6-28, capsaicin, capsazepine (a transient receptor potential vanilloid-1 inhibitor), and tetraethylammonium. However, this nicotine-induced relaxation was not sensitive to propranolol. Lowering the pH of the buffer solution with HCl caused pH-dependent vasorelaxation and deceased intracellular pH in the MCA rings, which was sensitive to L-NNA, CGRP8-37, VIP6-28, capsazepine, 4-aminopyridine (a voltage-gated potassium channel antagonist), and paxilline (a large conductance Ca-activated K channel antagonist). However, HCl-induced relaxation was not inhibited by glibenclamide (an ATP-sensitive K channel blocker). These results suggested that electrical and chemical activation of cerebral perivascular adrenergic nerves led to the release of H, which then facilitated the release of NO, VIP, and CGRP, resulting in vasorelaxation. Lowering the pH of the buffer solution caused potassium channels of vascular smooth muscle cells and perivascular nerves to open. In conclusion, our results demonstrated that H may act as a modulator on MCA perivascular nerves and/or smooth muscles.

[Ca2+ ] changes in sympathetic varicosities and Schwann cells in rat mesenteric arteries-Relation to noradrenaline release and contraction.

AIM: This study aimed to assess intracellular Ca2+ dynamics in nerve cells and Schwann cells in isolated rat resistance arteries and determine how these dynamics modify noradrenaline release from the nerves and consequent force development. METHODS: Ca2+ in nerves was assessed with confocal imaging, noradrenaline release with amperometry and artery tone with wire myography. Ca2+ in axons was assessed after loading with Oregon Green 488 BAPTA-1 dextran. In other experiments, arteries were incubated with Calcium Green-1-AM which loads both axons and Schwann cells. RESULTS: Schwann cells but not axons responded with a Ca2+ increase to ATP. Electrical field stimulation of nerves caused a frequency-dependent increase in varicose [Ca2+ ] ([Ca2+ ]v ). ω-conotoxin-GVIA (100 nmol/L) reduced the [Ca2+ ]v transient to 2 and 16 Hz by 60% and 27%, respectively; in contrast ω-conotoxin GVIA inhibited more than 80% of the noradrenaline release and force development at 2 and 16 Hz. The KV channel blocker, 4-aminopyridine (10 µmol/L), increased [Ca2+ ]v , noradrenaline release and force development both in the absence and presence of ω-conotoxin-GVIA. Yohimbine (1 µmol/L) increased both [Ca2+ ]v and noradrenaline release but reduced force development. Acetylcholine (10 µmol/L) caused atropine-sensitive inhibition of [Ca2+ ]v , noradrenaline release and force. In the presence of ω-conotoxin-GVIA, acetylcholine caused a further inhibition of all parameters. CONCLUSION: Modification of [Ca2+ ] in arterial sympathetic axons and Schwann cells was assessed separately. KV 3.1 channels may be important regulators of [Ca2+ ]v , noradrenaline release and force development. Presynaptic adrenoceptor and muscarinic receptor activation modify transmitter release through modification of [Ca2+ ]v .

Elevated Muscle Sympathetic Nerve Activity Contributes to Central Artery Stiffness in Young and Middle-Age/Older Adults.

Muscle sympathetic nerve activity (MSNA) influences the mechanical properties (ie, vascular smooth muscle tone and stiffness) of peripheral arteries, but it remains controversial whether MSNA contributes to stiffness of central arteries, such as the aorta and carotids. We examined whether elevated MSNA (age-related) would be independently associated with greater stiffness of central (carotid-femoral pulse wave velocity [PWV]) and peripheral (carotid-brachial PWV) arteries, in addition to lower carotid compliance coefficient, in healthy men and women (n=88, age: 19-73 years, 52% men). We also examined whether acute elevations in MSNA without increases in mean arterial pressure using graded levels of lower body negative pressure would augment central and peripheral artery stiffness in young (n=15, 60% men) and middle-age/older (MA/O, n=14, 43% men) adults. Resting MSNA burst frequency (bursts·min-1) was significantly correlated with carotid-femoral PWV ( R=0.44, P<0.001), carotid-brachial PWV ( R=0.32, P=0.004), and carotid compliance coefficient ( R=0.28, P=0.01) after controlling for sex, mean arterial pressure, heart rate, and waist-to-hip ratio (central obesity), but these correlations were abolished after further controlling for age (all P>0.05). In young and MA/O adults, MSNA was elevated during lower body negative pressure ( P<0.001) and produced significant increases in carotid-femoral PWV (young: Δ+1.3±0.3 versus MA/O: Δ+1.0±0.3 m·s-1, P=0.53) and carotid-brachial PWV (young: Δ+0.7±0.3 versus MA/O: Δ+0.7±0.5 m·s-1, P=0.92), whereas carotid compliance coefficient during lower body negative pressure was significantly reduced in young but not MA/O (young: Δ-0.04±0.01 versus MA/O: Δ0.001±0.008 mm2·mm Hg-1, P<0.01). Collectively, these data demonstrate the influence of MSNA on central artery stiffness and its potential contribution to age-related increases in stiffness of both peripheral and central arteries.

Pro-contractile role of chloride in arterial smooth muscle: Postnatal decline potentially governed by sympathetic nerves.

NEW FINDINGS: What is the topic of this review? This symposium report discusses the previously unrecognized pro-contractile role of chloride ions in rat arteries at early stages of postnatal development. What advances does it highlight? It highlights the postnatal decline in the contribution of chloride ions to regulation of arterial contractile responses and potential trophic role of sympathetic nerves in these developmental alterations. ABSTRACT: Chloride ions are important for smooth muscle contraction in adult vasculature. Arterial smooth muscle undergoes structural and functional remodelling during early postnatal development, including changes in K+ currents, Ca2+ handling and sensitivity. However, developmental change in the contribution of Cl- to regulation of arterial contraction has not yet been explored. Here, we provide the first evidence that the role of Cl- in α1 -adrenergic arterial contraction prominently decreases during early postnatal ontogenesis. The trophic influence of sympathetic nerves is a potential mechanism for postnatal decline of the contribution of Cl- to the vascular contraction.

Reducing Dietary Sodium to 1000 mg per Day Reduces Neurovascular Transduction Without Stimulating Sympathetic Outflow.

The American Heart Association recommends no more than 1500 mg of sodium/day as ideal. Some cohort studies suggest low-sodium intake is associated with increased cardiovascular mortality. Extremely low-sodium diets (≤500 mg/d) elicit activation of the renin-angiotensin-aldosterone system and stimulate sympathetic outflow. The effects of an American Heart Association-recommended diet on sympathetic regulation of the vasculature are unclear. Therefore, we assessed whether a 1000 mg/d diet alters sympathetic outflow and sympathetic vascular transduction compared with the more commonly recommended 2300 mg/d. We hypothesized that sodium reduction from 2300 to 1000 mg/d would not affect resting sympathetic outflow but would reduce sympathetic transduction in healthy young adults. Seventeen participants (age: 26±2 years, 9F/8M) completed 10-day 2300 and 1000 mg/d sodium diets in this randomized controlled feeding study (crossover). We measured resting renin activity, angiotensin II, aldosterone, blood pressure, muscle sympathetic nerve activity, and norepinephrine. We quantified beat-by-beat changes in mean arterial pressure and leg vascular conductance (femoral artery ultrasound) following spontaneous sympathetic bursts to assess sympathetic vascular transduction. Reducing sodium to 1000 mg/d increased renin activity, angiotensin II, and aldosterone ( P<0.01 for all) but did not alter mean arterial pressure (78±2 versus 77±2 mm Hg, P=0.56), muscle sympathetic nerve activity (13.9±1.3 versus 13.9±0.8 bursts/min, P=0.98), or plasma/urine norepinephrine. Sympathetic vascular transduction decreased ( P<0.01). These data suggest that reducing sodium from 2300 to 1000 mg/d stimulates the renin-angiotensin-aldosterone system, does not increase resting basal sympathetic outflow, and reduces sympathetic vascular transduction in normotensive adults.

Muscle sympathetic nerve activity and hemodynamic responses to venous distension: does sex play a role?

Peripheral venous distension mechanically stimulates type III/IV sensory fibers in veins and evokes pressor and sympathoexcitatory reflex responses in humans. As young women have reduced venous compliance and impaired sympathetic transduction, we tested the hypothesis that pressor and sympathoexcitatory responses to venous distension may be attenuated in women compared with men. Mean arterial pressure (photoplethysmography), heart rate (HR), stroke volume (SV; Modelflow), cardiac output (CO = HR × SV), muscle sympathetic nerve activity (MSNA), femoral artery blood flow, and femoral artery conductance (Doppler ultrasound) were quantified in eight men (27 ± 4 yr) and nine women (28 ± 4 yr) before [control (CON)], during (INF), and immediately after (post-INF) a local infusion of saline [5% of the total forearm volume (30 ml/min); the infusion time was 2 ± 1 and 1 ± 1 min ( P = 0.0001) for men and women, respectively] through a retrograde catheter inserted into an antecubital vein, to which venous drainage and arterial supply had been occluded. Mean arterial pressure increased during and after infusion in both groups (vs. the CON group, P < 0.05), but women showed a smaller pressor response in the post-INF period (Δ+7.2 ± 2.0 vs. Δ+18.3 ± 3.9 mmHg in men, P = 0.019). MSNA increased and femoral artery conductance decreased similarly in both groups (vs. the CON group, P < 0.05) at post-INF. Although HR changes were similar, increases in SV (Δ+20.4 ± 8.6 vs. Δ+2.6 ± 2.7 ml, P = 0.05) and CO (Δ+0.84 ± 0.17 vs. Δ+0.34 ± 0.10 l/min, P = 0.024) were greater in men compared with women. Therefore, venous distension evokes a smaller pressor response in young women due to attenuated cardiac adjustments rather than reduced venous compliance or sympathetic transduction. NEW & NOTEWORTHY We found that the pressor response to venous distension was attenuated in young women compared with age-matched men. This was due to attenuated cardiac adjustments rather than reduced venous compliance, sympathetic activation, or impaired transduction and vascular control. Collectively, these findings suggest that an attenuated venous distension reflex could be involved in orthostatic intolerance in young women.

Response Gene to Complement 32 Maintains Blood Pressure Homeostasis by Regulating α-Adrenergic Receptor Expression.

RATIONALE: Hypertension prevalence is much higher among children and adolescents with low birth weight and greater postnatal weight gain than in individuals with normal birth weight. However, the cause and molecular mechanisms underlying this complication remain largely unknown. Our previous studies have shown that RGC-32 (response gene to complement 32)-deficient (RGC-32-/-) mice are born significantly smaller but grow faster than their WT (wild type) controls, which allows adult RGC-32-/- mice to attain body weights similar to those of control mice. OBJECTIVE: The objective of this study is to determine whether RGC-32-/- mice develop hypertension, and if so, to elucidate the underlying mechanisms. METHODS AND RESULTS: By using a radiotelemetry system, we found that RGC-32-/- mice exhibit higher mean arterial pressure than WT mice (101±4 versus 119±5 mm Hg), which enabled us to use RGC-32-/- mice to study the mechanisms underlying low birth weight-related hypertension. The increased blood pressure in RGC-32-/- mice was associated with increased vascular tone and decreased distensibility of small resistance arteries. The increased vascular tone was because of an increase in the relative contribution of sympathetic versus parasympathetic activity and was linked to increased expression of AT1R (angiotensin II type I receptor) and α1-AdR (α1-adrenergic receptor) in arterial smooth muscles. Mechanistically, RGC-32 regulated AT1R gene transcription by interacting with Sp1 (specificity protein 1) transcription factor and further blocking its binding to the AT1R promoter, leading to suppression of AT1R expression. The attenuation of AT1R leads to reduction in α1-AdR expression, which was critical for the balance of sympathetic versus parasympathetic control of vascular tone. Of importance, downregulation of RGC-32 in arterial smooth muscles was also associated with low birth weight and hypertension in humans. CONCLUSIONS: Our results indicate that RGC-32 is a novel protein factor vital for maintaining blood pressure homeostasis, especially in individuals with low birth weight.

Interaction Between Pannexin 1 and Caveolin-1 in Smooth Muscle Can Regulate Blood Pressure.

Objective- Sympathetic nerve innervation of vascular smooth muscle cells (VSMCs) is a major regulator of arteriolar vasoconstriction, vascular resistance, and blood pressure. Importantly, α-adrenergic receptor stimulation, which uniquely couples with Panx1 (pannexin 1) channel-mediated ATP release in resistance arteries, also requires localization to membrane caveolae. Here, we test whether localization of Panx1 to Cav1 (caveolin-1) promotes channel function (stimulus-dependent ATP release and adrenergic vasoconstriction) and is important for blood pressure homeostasis. Approach and Results- We use in vitro VSMC culture models, ex vivo resistance arteries, and a novel inducible VSMC-specific Cav1 knockout mouse to probe interactions between Panx1 and Cav1. We report that Panx1 and Cav1 colocalized on the VSMC plasma membrane of resistance arteries near sympathetic nerves in an adrenergic stimulus-dependent manner. Genetic deletion of Cav1 significantly blunts adrenergic-stimulated ATP release and vasoconstriction, with no direct influence on endothelium-dependent vasodilation or cardiac function. A significant reduction in mean arterial pressure (total=4 mm Hg; night=7 mm Hg) occurred in mice deficient for VSMC Cav1. These animals were resistant to further blood pressure lowering using a Panx1 peptide inhibitor Px1IL2P, which targets an intracellular loop region necessary for channel function. Conclusions- Translocalization of Panx1 to Cav1-enriched caveolae in VSMCs augments the release of purinergic stimuli necessary for proper adrenergic-mediated vasoconstriction and blood pressure homeostasis.

Electrical dynamics of isolated cerebral and skeletal muscle endothelial tubes: Differential roles of G-protein-coupled receptors and K+ channels.

Electrical dynamics of freshly isolated cerebral endothelium have not been determined independently of perivascular nerves and smooth muscle. We tested the hypothesis that endothelium of cerebral and skeletal muscle arteries differentially utilizes purinergic and muscarinic signaling pathways to activate endothelium-derived hyperpolarization. Changes in membrane potential (Vm) were recorded in intact endothelial tubes freshly isolated from posterior cerebral and superior epigastric arteries of male and female C57BL/6 mice (age: 3-8 months). Vm was measured in response to activation of purinergic (P2Y) and muscarinic (M3) receptors in addition to small- and intermediate-conductance Ca2+-activated K+ (SKCa/IKCa) and inward rectifying K+ (KIR) channels using ATP (100 µmol·L-1), acetylcholine (ACh; 10 µmol·L-1), NS309 (0.01-10 µmol·L-1), and 15 mmol·L-1 KCl, respectively. Intercellular coupling was demonstrated via transfer of propidium iodide dye and electrical current (±0.5-3 nA) through gap junctions. With similarities observed across gender, peak hyperpolarization to ATP and ACh in skeletal muscle endothelial tubes was ~twofold and ~sevenfold higher, respectively, vs cerebral endothelial tubes, whereas responses to NS309 were similar (from resting Vm ~-30 mV to maximum ~-80 mV). Hyperpolarization (~8 mV) occurred during 15 mmol·L-1 KCl treatment in cerebral but not skeletal muscle endothelial tubes. Despite weaker hyperpolarization during endothelial GPCR stimulation in cerebral vs skeletal muscle endothelium, the capability for robust SKCa/IKCa activity is preserved across brain and skeletal muscle. As vascular reactivity decreases with aging and cardiovascular disease, endothelial K+ channel activity may be calibrated to restore blood flow to respective organs regardless of gender.

Optogenetic investigation of the variable neurovascular coupling along the interhemispheric circuits.

The interhemispheric circuit connecting the left and the right mammalian brain plays a key role in integration of signals from the left and the right side of the body. The information transfer is carried out by modulation of simultaneous excitation and inhibition. Hemodynamic studies of this circuit are inconsistent since little is known about neurovascular coupling of mixed excitatory and inhibitory signals. We investigated the variability in hemodynamic responses driven by the interhemispheric circuit during optogenetic and somatosensory activation. We observed differences in the neurovascular response based on the stimulation site - cell bodies versus distal projections. In half of the experiments, optogenetic stimulation of the cell bodies evoked a predominant post-synaptic inhibition in the other hemisphere, accompanied by metabolic oxygen consumption without coupled functional hyperemia. When the same transcallosal stimulation resulted in predominant post-synaptic excitation, the hemodynamic response was biphasic, consisting of metabolic dip followed by functional hyperemia. Optogenetic suppression of the postsynaptic excitation abolished the coupled functional hyperemia. In contrast, light stimulation at distal projections evoked consistently a metabolic response. Our findings suggest that functional hyperemia requires signals originating from the cell body and the hemodynamic response variability appears to reflect the balance between the post-synaptic excitation and inhibition.

Exaggerated Vasoconstriction to Spontaneous Bursts of Muscle Sympathetic Nerve Activity in Healthy Young Black Men.

Blacks have the highest prevalence of hypertension, putting them at greater risk of cardiovascular disease and death. Previous studies have reported that, relative to whites, healthy black men have augmented pressor responses to sympathoexcitatory stressors. Although important, these studies do not inform about the resting state and the influence of spontaneous changes in resting muscle sympathetic nerve activity (MSNA). Likewise, little is known about the transduction of MSNA into a vascular response at rest on a beat-to-beat basis. Accordingly, we tested the hypothesis that relative to whites, blacks would exhibit greater vasoconstriction and pressor responses following spontaneous bursts of MSNA. Mean arterial pressure, common femoral artery blood flow, and MSNA were continuously recorded during 20 minutes of supine rest in 35 young healthy men (17 blacks and 18 whites). Signal averaging was used to characterize changes in leg vascular conductance, total vascular conductance, and mean arterial pressure following spontaneous MSNA bursts. Blacks demonstrated significantly greater decreases in leg vascular conductance (blacks: -15.0±1.0%; whites: -11.5±1.2%; P=0.042) and total vascular conductance (blacks: -8.6±0.9%; whites: -5.1±0.4%; P=0.001) following MSNA bursts, which resulted in greater mean arterial pressure increases (blacks: +5.2±0.6 mm Hg; whites: +3.9±0.3 mm Hg; P=0.04). These exaggerated responses in blacks compared with whites were present whether MSNA bursts occurred in isolation (singles) or in combination (multiples) and were graded with increases in burst height. Collectively, these findings suggest that healthy young black men exhibit augmented sympathetic vascular transduction at rest and provide novel insight into potential mechanism(s) by which this population may develop hypertension later in life.

Probenecid Inhibits α-Adrenergic Receptor-Mediated Vasoconstriction in the Human Leg Vasculature.

Coordination of vascular smooth muscle cell tone in resistance arteries plays an essential role in the regulation of peripheral resistance and overall blood pressure. Recent observations in animals have provided evidence for a coupling between adrenoceptors and Panx1 (pannexin-1) channels in the regulation of sympathetic nervous control of peripheral vascular resistance and blood pressure; however, evidence for a functional coupling in humans is lacking. We determined Panx1 expression and effects of treatment with the pharmacological Panx1 channel inhibitor probenecid on the vasoconstrictor response to α1- and α2-adrenergic receptor stimulation in the human forearm and leg vasculature of young healthy male subjects (23±3 years). By use of immunolabeling and confocal microscopy, Panx1 channels were found to be expressed in vascular smooth muscle cells of arterioles in human leg skeletal muscle. Probenecid treatment increased (P<0.05) leg vascular conductance at baseline by ≈15% and attenuated (P<0.05) the leg vasoconstrictor response to arterial infusion of tyramine (α1- and α2-adrenergic receptor stimulation) by ≈15%, whereas the response to the α1-agonist phenylephrine was unchanged. Inhibition of α1-adrenoceptors prevented the probenecid-induced increase in baseline leg vascular conductance, but did not alter the effect of probenecid on the vascular response to tyramine. No differences with probenecid treatment were detected in the forearm. These observations provide the first line of evidence in humans for a functional role of Panx1 channels in setting resting tone via α1-adrenoceptors and in the constrictive effect of noradrenaline via α2-adrenoceptors, thereby contributing to the regulation of peripheral vascular resistance and blood pressure in humans.

Sex-specific factors regulating pressure and flow.

NEW FINDINGS: What is the topic of this review? There are sex- and sex-hormone-specific differences in autonomic control of blood pressure, central haemodynamics and cerebral blood flow. What advances does it highlight? Sex differences in autonomic control of blood pressure may underlie other sex-specific characteristics associated with cerebral blood flow, which can, in turn, affect tissue function. Over the last decade, there have been many published reports on sex differences in blood pressure regulation between young men and young women. The autonomic nervous system is a primary contributor to both acute and long-term blood pressure regulation. Sex differences in blood pressure regulation are likely to have effects that extend beyond mean arterial pressure and that can affect blood flow and tissue function. This short review includes recent literature from our laboratory focusing on autonomic control of the circulation, specifically age- and sex-hormone-related differences in central haemodynamics and cerebral blood flow, and discusses potential clinical implications.

Effect of Nerve Growth Factor on Innervation of Perivascular Nerves in Neovasculatures of Mouse Cornea.

Angiogenesis, which is the generation of new vascular networks from existing blood vessels, occurs under normal and pathophysiological conditions. Perivascular nerves, which innervate mature vasculatures, maintain vascular tone and regulate tissue blood flow. However, little is known whether perivascular nerves innervate newborn blood vessels. Therefore, the aim of this study was to investigate the distribution and characterization of perivascular nerves in neovasculatures, which were generated by the mouse corneal micropocket method. Under anesthesia, a pellet containing basic fibroblast growth factor (bFGF) (100 ng/pellet) was implanted into a mouse cornea in one side of the eyeball. Nerve growth factor (NGF) was locally (2 or 20 ng) applied with the pellet, or subcutaneously (40 ng/h for 7 d) administered with an osmotic mini-pump. After the implantation, vascular endothelial cells, smooth muscle cells, and perivascular nerves in the cornea were immunohistochemically studied. Neovessels generated from existing limbal vessels were observed in pellet-implanted cornea. Immunostaining of neovasculatures showed the presence of CD31-like immunoreactive (LI) endothelial cells and α-smooth muscle actin-LI vascular smooth muscles. Perivascular nerves immunostained by protein gene product (PGP) 9.5, an axonal marker, were found in the existing limbal vessels, but they were not observed in neovasculatures. Local and subcutaneous treatment of NGF inhibits bFGF-derived angiogenesis and resulted in loop-shaped vessels that had many anastomoses, and produced innervation of PGP 9.5-LI perivascular nerves around bFGF-derived neovessels. These findings suggest that neovasculatures have no innervation of perivascular nerves, and that NGF facilitates innervations of perivascular nerves to regulate the blood flow in neovessels.

Effect of age on cutaneous vasomotor responses during local skin heating.

This study examined the effect of ageing on the low-frequency oscillations (vasomotion) of skin blood flow in response to local heating (LH). Skin blood flow was assessed by laser-Doppler flowmetry on the forearm at rest (33°C) and in response to LH of the skin to both 42°C and 44°C in 14 young (24±1years) and 14 older (64±1years) participants. Vasomotion was analyzed using a wavelet transform to investigate power of the frequency intervals associated with endothelial, neural, myogenic, respiratory, and cardiac activities of the laser-Doppler signal. Laser-Doppler flux increased in both groups with LH (both d>1.8, p<0.001). Endothelial activity increased in both groups following LH to 42°C (young d=1.4, p<0.001; older d=1.2, p=0.005) and 44°C (young d=1.4, p=0.001; older d=1.5, p=0.005). Endothelial activity was higher in the young compared to older group during LH to 42°C (d=1.4, p=0.017) and 44°C (d=1.5, p=0.004). In response to LH to 42°C and 44°C, neural activity in both groups was decreased (both groups and conditions: d>1.2, p<0.001). Myogenic activity increased in the younger group following LH to 44°C (d=1, p=0.042), while in the older group, myogenic activity increased following LH to 42°C (d=1.2, p=0.041) and 44°C (d=1.1, p=0.041). Respiratory and cardiac activities increased in both groups during LH to 42°C and 44°C (All: d>0.9, p<0.017). There were no differences in wavelet amplitude between younger and older in the neural (d=0.1, p>0.7), myogenic (d=0.3, p>0.7), respiratory (d=0.4, p>0.6), and cardiac (d=0.1, p>0.7) frequency intervals. These data indicate that LH increases cutaneous endothelial and myogenic activity, while decreasing neural activity. Furthermore, ageing reduces the increase in cutaneous endothelial activity in response to LH.

Differential Contribution of Nerve-Derived Noradrenaline to High K+-Induced Contraction Depending on Type of Artery.

High K+-induced contraction of arterial smooth muscle is thought to be mediated by membrane depolarization and subsequent activation of voltage-dependent Ca2+ channels (VDCCs). In line with this, this study found that contraction induced by 80 mM K+ was almost abolished by nifedipine (1 µM), a VDCC inhibitor, in isolated rat aorta, and was markedly suppressed in the iliac artery. However, nifedipine (1 µM) only partially suppressed high K+-induced contraction in the tail artery. The contractions remaining in the arteries were further reduced by non-selective cation channel (NSCC) inhibitors, including 2-aminoethoxydiphenyl borate (2-APB) (100 µM), SK&F96365 (10 µM), and 3,4-dihydro-6,7-dimethoxy-α-phenyl-N,N-bis[2-(2,3,4-trimethoxyphenyl)ethyl]-1-isoquinolineacetamide hydrochloride (LOE908) (10 µM). In particular, sustained tonic contraction was nearly abolished. Prazosin (0.3 µM), an α1-adrenoceptor antagonist, partially inhibited high K+-induced contraction in the tail and iliac arteries, but had no effect in the aorta. Consistently, tyramine potently induced contraction in the tail and iliac arteries, but not in the aorta. Furthermore, the inhibition by prazosin and NSCC inhibitors of the high K+-induced contraction in the presence of nifedipine was comparable. These results suggest that depending on the type of artery, high K+-induced contraction is mediated by Ca2+ influx not only through VDCCs but also through NSCCs, the activation of which is due to the activation of α1-adrenoceptors by the released noradrenaline from sympathetic nerve terminals resulting from high K+ stimulation.