Vasoconstrictor Neurons Research Articles

Sympathetic System in Wound Healing: Multistage Control in Normal and Diabetic Skin.

In this review, we discuss sympathetic regulation in normal and diabetic wound healing. Experimental denervation studies have confirmed that sympathetic nerve endings in skin have an important and complex role in wound healing. Vasoconstrictor neurons secrete norepinephrine (NE) and neuropeptide Y (NPY). Both mediators decrease blood flow and interact with inflammatory cells and keratinocytes. NE acts in an ambiguous way depending on receptor type. Beta2-adrenoceptors could be activated near sympathetic endings; they suppress inflammation and re-epithelialization. Alpha1- and alpha2-adrenoceptors induce inflammation and activate keratinocytes. Sudomotor neurons secrete acetylcholine (ACh) and vasoactive intestinal peptide (VIP). Both induce vasodilatation, angiogenesis, inflammation, keratinocytes proliferation and migration. In healthy skin, all effects are important for successful healing. In treatment of diabetic ulcers, mediator balance could be shifted in different ways. Beta2-adrenoceptors blockade and nicotinic ACh receptors activation are the most promising directions in treatment of diabetic ulcers with neuropathy, but they require further research.

Sympathetic Discharge Patterns and Neurovascular Transduction Following Acute Intermittent Hypoxia

BackgroundMuscle sympathetic nerve activity (MSNA) is augmented in patients with sleep apnea. This increase in activity is due to increases in firing probability and mean firing rates of individual vasoconstrictor neurons, in addition to multiple within‐burst firing; however, contributing mechanisms and the impact of such firing patterns on peripheral vascular tone are less clear. We examined the effect of acute intermittent hypoxia (IH) on sympathetic neural firing and recruitment strategies and the relationship between changes in MSNA and neurovascular transduction following acute IH. We hypothesized exposure to acute IH would increase MSNA via an increase in action potential (AP) discharge rates and within‐burst firing. We further hypothesized any change in discharge patterns following acute IH would be positively related to a change in neurovascular transduction.MethodsBlood pressure (brachial arterial catheter) and MSNA (microneurography of the peroneal nerve) were assessed in 17 healthy adults (11M/6F; 31 ± 1 yrs) during quiet normoxic rest prior to and following 30‐min of experimental IH [30‐s hypoxic (0.05 FiO2) exposures alternating with 90‐s room air breathing]. AP patterns were studied from the filtered raw MSNA signal using wavelet‐based methodology. The slope of the relationship between diastolic blood pressure (DBP) and the preceding MSNA burst area at a fixed cardiac cycle lag was used to quantify neurovascular transduction.ResultsIH resulted in 15 events where oxygen saturation was reduced (SpO2: 99 ± 1% vs 92 ± 1%, P<0.01). Compared to baseline, multi‐unit MSNA burst incidence (33 ± 4 to 43 ± 5 bursts/100 heart beats), AP incidence (272 ± 50 to 501 ± 112 AP/100 heart beats) and AP content per integrated burst (7 ± 1 to 10 ± 1 AP/burst; 4 ± 1 to 5 ± 1 clusters/burst) were increased following IH (all P<0.05). Neurovascular transduction was not altered following IH (P=0.34); however, there was a positive relationship between the change (Δ) in neurovascular transduction (mmHg/%/s) and changes in AP discharge rate (AP/100 heart beats; R=0.52, P=0.03) and within‐burst firing (AP/burst; R=0.38, P=0.14) following IH.ConclusionsAcute IH increases sympathetic neuronal discharge by increasing within‐burst firing of low‐threshold axons. A positive relationship exists between increases in the neural discharge rates and neurovascular transduction following acute IH, such that those individuals with the greatest increase in AP firing exhibited the greatest increase in transduction of sympathetic activity into blood pressure. These data may have important implications for mechanistic understanding of sympathetic activation in sleep apnea and its down‐stream effects, including the development of hypertension and cardiovascular disease.Support or Funding InformationFunding: NIH HL130339, Mayo Clinic Center for Biomedical Discovery, Mizzou HES PURE, APS UGSRF.This abstract is from the Experimental Biology 2019 Meeting. There is no full text article associated with this abstract published in The FASEB Journal.

Physiological and pathophysiological firing properties of single postganglionic sympathetic neurons in humans.

It has long been known from microneurographic recordings in human subjects that the activity of postganglionic sympathetic axons occurs as spontaneous bursts, with muscle sympathetic nerve activity (MSNA) exhibiting strong cardiac rhythmicity via the baroreflex and skin sympathetic nerve activity showing much weaker cardiac modulation. Here we review the firing properties of single sympathetic neurons, obtained using highly selective microelectrodes. Individual vasoconstrictor neurons supplying muscle or skin, or sudomotor neurons supplying sweat glands, always discharge with a low firing probability (~30%) and at very low frequencies (~0.5 Hz). Moreover, they usually fire only once per cardiac interval but can fire greater than four times within a burst. Modeling has shown that this pattern can best be explained by individual neurons being driven by, on average, two preganglionic inputs. Unitary recordings of muscle vasoconstrictor neurons have been made in several pathophysiological states, including heart failure, hypertension, obstructive sleep apnea, bronchiectasis, chronic obstructive pulmonary disease, depression, and panic disorder. The augmented MSNA in each of these diseases features an increase in firing probability and discharge frequency of individual muscle vasoconstrictor neurons above that seen in healthy subjects, yet firing rates rarely exceed 1 Hz. However, unlike patients with heart failure, all patients with respiratory disease or panic disorder, and patients with hyperhidrosis, exhibited an increase in multiple within-burst firing, which emphasizes the different modes by which the sympathetic nervous system grades its output in pathophysiological states of high sympathetic nerve activity.

Autonomic dysreflexia: Somatosympathetic and viscerosympathetic vasoconstrictor responses to innocuous and noxious sensory stimulation below lesion in human spinal cord injury

Autonomic dysreflexia is a dangerous elevation in blood pressure in people with spinal cord injury (SCI), produced by a spinally-mediated reflex activation of sympathetic vasoconstrictor neurones supplying skeletal muscle and the gut. Current dogma states that, apart from visceral inputs - such as those originating from a distended bladder or impacted colon - autonomic dysreflexia is triggered by noxious inputs below the lesion. However, while selective stimulation of small-diameter afferents in muscle or skin evokes a sustained increase in muscle sympathetic nerve activity and blood pressure, and a transient increase in skin sympathetic nerve activity and decrease in skin blood flow in able-bodied subjects, such noxious inputs have no effects on blood pressure and skin blood flow in SCI individuals. Conversely, weak electrical stimulation over the abdominal wall, which in able-bodied subjects is not painful and activates large-diameter cutaneous afferents, causes a marked increase in blood pressure in SCI but not in able-bodied subjects. Moreover, vibration of the penis in spinal-injured men, which is not noxious, caused marked vasoconstriction and increases in blood pressure, similar to those produced by non-noxious distension of the bladder during urodynamics procedures. This suggests that activation of large-diameter somatic afferents, not small-diameter afferents, triggers the increases in vasoconstrictor drive that lead to autonomic dysreflexia, arguing against current dogma on the importance of noxious inputs in triggering autonomic dysreflexia.

Changes in sympathetic neurovascular function following spinal cord injury

The effects of spinal cord injury (SCI) on sympathetic neurovascular transmission have generally been ignored. This review describes changes in sympathetic nerve-mediated activation of arterial vessels to which ongoing sympathetic activity has been reduced or silenced following spinal cord transection in rats. In all vessels studied in rats, SCI markedly enhanced their contractile responses to nerve activity. However, the mechanisms that augment neurovascular transmission differ between the rat tail artery and mesenteric artery. In tail artery, the enhancement of neurovascular transmission cannot be attributed to changes in sensitivity of the vascular muscle to α1- or α2-adrenoceptor agonists. Instead the contribution of L-type Ca2+ channels to activation of the smooth muscle by nerve-released noradrenaline is greatly increased following SCI. By contrast, mesenteric arteries from SCI rats had increased sensitivity to phenylephrine but not to methoxamine. While both phenylephrine and methoxamine are α1-adrenoceptor agonists, only phenylephrine is a substrate for the neuronal noradrenaline transporter. Therefore the selective increase in sensitivity to phenylephrine suggests that the activity of the neuronal noradrenaline transporter is reduced. While present evidence suggests that sympathetic vasoconstrictor neurons do not contribute to the normal regulation of peripheral resistance below a complete SCI in humans, the available evidence does indicate that these experimental findings in animals are likely to apply after SCI in humans and contribute to autonomic dysreflexia.

Sympathetic activation and endothelial dysfunction in polycystic ovary syndrome are not explained by either obesity or insulin resistance.

Polycystic ovary syndrome (PCOS) is a common endocrine condition underpinned by insulin resistance and associated with increased risk of obesity, type 2 diabetes and adverse cardiovascular risk profile. Previous data suggest autonomic imbalance [elevated sympathetic nervous system (SNS) activity and decreased heart rate variability (HRV)] as well as endothelial dysfunction in PCOS. However, it is not clear whether these abnormalities are driven by obesity and metabolic disturbance or whether they are independently related to PCOS. We examined multiunit and single-unit muscle SNS activity (by microneurography), HRV (time and frequency domain analysis) and endothelial function [ischaemic reactive hyperaemia index (RHI) using the EndoPAT device] in 19 overweight/obese women with PCOS (BMI: 31·3 ± 1·5 kg/m(2), age: 31·3 ± 1·6 years) and compared them with 21 control overweight/obese women (BMI: 33·0 ± 1·4 kg/m(2), age: 28·2 ± 1·6 years) presenting a similar metabolic profile (fasting total, HDL and LDL cholesterol, glucose, triglycerides, insulin sensitivity and blood pressure). Women with PCOS had elevated multiunit muscle SNS activity (41 ± 2 vs 33 ± 3 bursts per 100 heartbeats, P < 0·05). Single-unit analysis showed that vasoconstrictor neurons were characterized by elevated firing rate and probability and incidence of multiple spikes (P < 0·01 for all parameters). Women with PCOS also had impaired endothelial function (RHI: 1·77 ± 0·14 vs 2·18 ± 0·14, P < 0·05). HRV did not differ between the groups. Women with PCOS have increased sympathetic drive and impaired endothelial function independent of obesity and metabolic disturbances. Sympathetic activation and endothelial dysfunction may confer greater cardiovascular risk in women with PCOS.

Virtual leak channels modulate firing dynamics and synaptic integration in rat sympathetic neurons: implications for ganglionic transmission in vivo.

The synaptic organization of paravertebral sympathetic ganglia enables them to relay activity from the spinal cord to the periphery and thereby control autonomic functions, including blood pressure and body temperature. The present experiments were done to reconcile conflicting observations in tissue culture, intact isolated ganglia and living animals. By recording intracellularly from dissociated neurons and intact ganglia, we found that when electrode damage makes cells leaky it could profoundly distort cellular excitability and the integration of synaptic potentials. The experiments relied on the dynamic clamp method, which allows the creation of virtual ion channels by injecting current into a cell based upon a mathematical model and using rapid feedback between the model and cell. The results support the hypothesis that sympathetic ganglia can produce a 2.4-fold amplification of presynaptic activity. This could aid understanding of the neural hyperactivity that is believed to drive high blood pressure in some patients. The excitability of rat sympathetic neurons and integration of nicotinic EPSPs were compared in primary cell culture and in the acutely isolated intact superior cervical ganglion using whole cell patch electrode recordings. When repetitive firing was classified by Hodgkin's criteria in cultured cells, 18% displayed tonic class 1 excitability, 36% displayed adapting class 2 excitability and 46% displayed phasic class 3 excitability. In the intact ganglion, 71% of cells were class 1 and 29% were class 2. This diverges from microelectrode reports that nearly 100% of superior cervical ganglion neurons show phasic class 3 firing. The hypothesis that the disparity between patch and microelectrode data arises from a shunt conductance was tested using the dynamic clamp in cell culture. Non-depolarizing shunts of 3-10nS converted cells from classes 1 and 2 to class 3 dynamics with current-voltage relations that replicated microelectrode data. Primary and secondary EPSPs recorded from the intact superior cervical ganglion were modelled as virtual synapses in cell culture using the dynamic clamp. Stimulating sympathetic neurons with virtual synaptic activity, designed to replicate in vivo recordings of EPSPs in muscle vasoconstrictor neurons, produced a 2.4-fold amplification of presynaptic activity. This gain in postsynaptic output did not differ between neurons displaying the three classes of excitability. Mimicry of microelectrode damage by virtual leak channels reduced and eventually obliterated synaptic gain by inhibiting summation of subthreshold EPSPs. These results provide a framework for interpreting sympathetic activity recorded from intact animals and support the hypothesis that paravertebral ganglia function as activity-dependent amplifiers of spinal output from preganglionic circuitry.

Localized hypertrichosis after index finger revascularization and complex regional pain syndrome.

Localized hypertrichosis after index finger revascularization and complex regional pain syndrome.

Reflex inhibition of cutaneous and muscle vasoconstrictor neurons during stimulation of cutaneous and muscle nociceptors

Cutaneous (CVC) and muscle (MVC) vasoconstrictor neurons exhibit typical reflex patterns to physiological stimulation of somatic and visceral afferent neurons. Here we tested the hypothesis that CVC neurons are inhibited by stimulation of cutaneous nociceptors but not of muscle nociceptors and that MVC neurons are inhibited by stimulation of muscle nociceptors but not of cutaneous nociceptors. Activity in the vasoconstrictor neurons was recorded from postganglionic axons isolated from the sural nerve or the lateral gastrocnemius-soleus nerve in anesthetized rats. The nociceptive afferents were excited by mechanical stimulation of the toes of the ipsilateral hindpaw (skin), by hypertonic saline injected into the ipsi- or contralateral gastrocnemius-soleus muscle, or by heat or noxious cold stimuli applied to the axons in the common peroneal nerve or tibial nerve. The results show that CVC neurons are inhibited by noxious stimulation of skin but not by noxious stimulation of skeletal muscle and that MVC neurons are inhibited by noxious stimulation of skeletal muscle but not by noxious stimulation of skin. These inhibitory reflexes are mostly lateralized and are most likely organized in the spinal cord. Stimulation of nociceptive cold-sensitive afferents does not elicit inhibitory or excitatory reflexes in CVC or MVC neurons. The reflex inhibition of activity in CVC or MVC neurons generated by stimulation of nociceptive cutaneous or muscle afferents during tissue injury leads to local increase of blood flow, resulting in an increase of transport of immunocompetent cells, proteins, and oxygen to the site of injury and enhancing the processes of healing.

Hypertension Editors’ Picks

<i>Hypertension</i> Editors’ Picks

Cardiorespiratory coupling of sympathetic outflow in humans: a comparison of respiratory and cardiac modulation of sympathetic nerve activity to skin and muscle

What is the central question of this study?Muscle sympathetic nerve activity (MSNA) is well known to be modulated by the arterial baroreceptors and respiration, but what are the magnitudes of cardiac and respiratory modulation of skin sympathetic nerve activity (SSNA), which primarily subserves thermoregulation?What is the main finding and what is its importance?Using direct microelectrode recordings of MSNA and SSNA in awake humans, we show that the magnitude of respiratory modulation of SSNA is identical to that of MSNA, the primary difference between the two sources of sympathetic outflow being the greater cardiac modulation of MSNA. This emphasises the role of the baroreceptors in entraining sympathetic outflow to muscle. It is well known that microelectrode recordings of skin sympathetic nerve activity (SSNA) in awake human subjects reveal spontaneous bursts of activity with no overt modulation by changes in blood pressure or respiration, in contrast to the clear cardiac and respiratory modulation of muscle sympathetic nerve activity (MSNA). However, cross-correlation analysis has revealed that, like individual muscle vasoconstrictor neurones, the firing of individual cutaneous vasoconstrictor neurones is temporally coupled to both the cardiac and respiratory rhythms during cold-induced cutaneous vasoconstriction, and the same is true of single sudomotor neurones during heat-induced sweating. Here, we used cross-correlation analysis to determine whether SSNA exhibits cardiac and respiratory modulation in thermoneutral conditions and to compare respiratory and cardiac modulation of SSNA with that of MSNA. Oligounitary recordings of spontaneous SSNA (n = 20) and MSNA (n = 18) were obtained during quiet, unrestrained breathing. Respiration was recorded by a strain-gauge transducer around the chest and ECG recorded by surface electrodes. Respiratory and cardiac modulation of SSNA and MSNA were quantified by fitting polynomial equations to the cross-correlation histograms constructed between the sympathetic spikes and respiration or ECG. The amplitude of the respiratory modulation (52.5 ± 3.4%) of SSNA was not significantly different from the amplitude of the cardiac modulation (46.6 ± 3.2%). Both were comparable to the respiratory modulation of MSNA (47.7 ± 4.2%), while cardiac modulation of MSNA was significantly higher (89.8 ± 1.5%). We conclude that SSNA and MSNA share similar levels of respiratory modulation, the primary difference between the two sources of sympathetic outflow being the marked cardiac modulation of MSNA provided by the baroreceptors.

Clinical Implications

HomeHypertensionVol. 61, No. 2Clinical Implications Free AccessResearch ArticlePDF/EPUBAboutView PDFView EPUBSections ToolsAdd to favoritesDownload citationsTrack citationsPermissions ShareShare onFacebookTwitterLinked InMendeleyReddit Jump toFree AccessResearch ArticlePDF/EPUBClinical Implications Originally published1 Feb 2013https://doi.org/10.1161/HYP.0b013e318284b8b8Hypertension. 2013;61:267Autoantibodies in Primary Aldosteronism (page 526)Primary aldosteronism (PA) is the most frequent endocrine cause of hypertension. The differential diagnosis between its 2 main subtypes, for example—aldosterone-producing adenoma and idiopathic hyperaldosteronism, is the key for selecting surgical or medical treatment, but challenging.In this study, we detected autoantibodies against the AT1 receptor (AT1AA) at a raised titer in serum of PA patients, and particularly in those with an aldosterone-producing adenoma as compared with those with idiopathic hyperaldosteronism or essential hypertension with similar blood pressure (BP) elevation. Of clinical relevance, the AT1AA titer provided a discrimination of aldosterone-producing adenoma from idiopathic hyperaldosteronism, thus suggesting that the AT1AA titer can help in selecting the PA patients to be submitted to adrenal vein sampling. The latter test, which is expensive, invasive, and not widely available, is recommended by the Endocrine Society Guidelines in all patients who are candidates for adrenalectomy.Finally, the mechanisms underlying PA have eluded identification thus far notwithstanding a long quest. As these AT1AA are suggested to play an agonistic effect, including stimulation of aldosterone secretion, their presence in aldosterone-producing adenoma patients could explain the persistent over-secretion of aldosterone in spite of the high BP, the suppression of the renin-angiotensin system and the hypokalemia, all factors that would be expected to blunt aldosterone secretion. Hence our data might shed a new light on the pathophysiology of PA.Download figureDownload PowerPointRenal Denervation and Sympathetic Activity (page title>Autoantibodies in Primary Aldosteronism (page 457)Catheter-based renal denervation has emerged as a novel treatment option for patients with resistant hypertension. The limited number of studies available thus far suggest that the procedure is safe and results in a significant and sustained BP reduction in the majority of treated patients with resistant hypertension. Although ablation of efferent renal nerves, as evident form a mean reduction in renal noradrenaline spillover of 47%, is likely to contribute to the BP lowering effects via alteration of renin release, renal blood flow, and tubular sodium retention, inhibition of central sympathetic outflow has also been documented by applying multiunit muscle sympathetic nerve activity recordings.Here, we examined whether renal denervation may differentially influence the sympathetic discharge pattern of single vasoconstrictor neurons in patients with resistant hypertension and found that all properties of single-unit muscle sympathetic nerve activity, including firing rates of individual vasoconstrictor fibers, firing probability, and multiple firing incidence of single units within a cardiac cycle, were substantially reduced. These findings indicate that renal denervation may restrain abnormal patterns of sympathetic nerve firing commonly evident in patients with resistant hypertension and exhibit renal signaling via afferent nerves projecting to certain brain stem nuclei as a likely contributor to the BP rise in resistant hypertension.Download figureDownload PowerPointCombination Therapy, CV Events, and BP Control (page Sympathetic Activity (page title>Autoantibodies in Primary Aldosteronism (page 309)Combination therapy is required to achieve current BP targets in ≈75% of hypertensive patients. Previous studies indicate that initial use of a combination accelerates the time course over which BP control is attained. However, the optimal time frame for achieving BP control in clinical practice has never been defined, and criteria for using initial combination therapy remain uncertain. In a retrospective, matched cohort study, the impact of initial versus delayed combination therapy on the risk of developing a cardiovascular (CV) event was assessed in a community practice setting. The mean baseline BP was ≈150/85 mm Hg, and two thirds had stage I hypertension. Initial combination treatment led to more rapid and more effective BP control and was associated with a 34% risk reduction for CV events or death relative to patients initiated on monotherapy and subsequently uptitrated to combination treatment by their practicing physicians. Fewer patients who experienced a CV event achieved target BP, and on-treatment BP parameters accounted for most of the difference in CV event rates.Initial combination therapy was also associated with a significant reduction in healthcare resource utilization. These results suggest that combination therapy should be initiated routinely in most patients likely to require multiple agents to reach goal BP, and that initial combination therapy is a superior treatment strategy for a large segment of the hypertensive population including many patients with mild hypertension.Download figureDownload PowerPoint Previous Back to top Next FiguresReferencesRelatedDetails February 2013Vol 61, Issue 2 Advertisement Article InformationMetrics © 2013 American Heart Association, Inc.https://doi.org/10.1161/HYP.0b013e318284b8b8 Originally publishedFebruary 1, 2013 PDF download Advertisement

Cardiac‐locked bursts of muscle sympathetic nerve activity are absent in familial dysautonomia

Familial dysautonomia (Riley-Day syndrome) is an hereditary sensory and autonomic neuropathy (HSAN type III), expressed at birth, that is associated with reduced pain and temperature sensibilities and absent baroreflexes, causing orthostatic hypotension as well as labile blood pressure that increases markedly during emotional excitement. Given the apparent absence of functional baroreceptor afferents, we tested the hypothesis that the normal cardiac-locked bursts of muscle sympathetic nerve activity (MSNA) are absent in patients with familial dysautonomia. Tungsten microelectrodes were inserted percutaneously into muscle or cutaneous fascicles of the common peroneal nerve in 12 patients with familial dysautonomia. Spontaneous bursts of MSNA were absent in all patients, but in five patients we found evidence of tonically firing sympathetic neurones, with no cardiac rhythmicity, that increased their spontaneous discharge during emotional arousal but not during a manoeuvre that unloads the baroreceptors. Conversely, skin sympathetic nerve activity (SSNA), recorded in four patients, appeared normal. We conclude that the loss of phasic bursts of MSNA and the loss of baroreflex modulation of muscle vasoconstrictor drive contributes to the poor control of blood pressure in familial dysautonomia, and that the increase in tonic firing of muscle vasoconstrictor neurones contributes to the increase in blood pressure during emotional excitement.

Abstract 19395: Increased Multiple Firing of Single Muscle Vasoconstrictor Neurons during Lower Body Positive Pressure in Human Heart Failure

Introduction: Functionally specific efferent renal sympathetic nerves fire selectively in response to distinct hemodynamic afferent inputs. Using norepinephrine spillover, our group detected a heart failure (HF)-specific cardio-cardiac sympathoexcitatory reflex stimulated by high atrial pressure. Efferent discharge directed at other vascular beds similarly activated by atrial stretch might not be detected by multi-unit sympathetic recordings. Hypothesis: In contrast to healthy controls (CON), in HF some muscle sympathetic fibers respond paradoxically (i.e. increase firing) to increases in filling pressure. Methods: Single-unit muscle sympathetic nerve activity (sMSNA; microneurography) and peripheral venous pressure (PVP; forearm catheter) were recorded from 11 CON and 10 treated HF (LVEF 27±6%) patients at baseline and during non-hypotensive lower body negative pressure (LBNP; -10mmHg) and non-hypertensive lower body positive pressure (LBPP; +10mmHg). sMSNA firing frequency and incidence, and the percentage of multiple within-burst firing was calculated for each condition. Results: PVP was reduced with LBNP (p&lt;0.05) and increased with LBPP (p&lt;0.05) in both groups, with PVP greater in HF across each condition (p&lt;0.05). Blood pressure and heart rate were unchanged by LBNP or LBPP in both groups (p&gt;0.05). We identified 26 and 22 single-units in CON and HF patients, respectively. sMSNA firing frequency was similar (p&gt;0.05) in CON and HF at baseline (30±18 vs. 35±15 spikes/min) and during LBNP (45±24 vs. 42±23 spikes/min) but higher in HF during LBPP (28±17 vs. 40±22 spikes/min, p=0.05). sMSNA firing incidence was similar (p&gt;0.05) in both groups across each condition. The percentage of within-burst multiple spike firing was similar (p&gt;0.05) in CON and HF during baseline (24±9% vs. 24±7%) and LBNP (25±11% vs. 28±14%) but higher in HF during LBPP (17±8% vs. 34±16%, p&lt;0.001). Conclusions: In HF, increases in PVP with non-hypertensive LBPP cause a paradoxical increase in muscle sympathetic single-unit firing frequency and multiple within-burst firing, compared to CON. This provides evidence of a sympathoexcitatory cardiac-skeletal muscle vasoconstrictor reflex in human HF that may be attenuated by normalizing filling pressures.

Substantial Reduction in Single Sympathetic Nerve Firing After Renal Denervation in Patients With Resistant Hypertension

Renal denervation (RDN) has been shown to reduce blood pressure (BP) and muscle sympathetic nerve activity (MSNA) in patients with resistant hypertension. The mechanisms underlying sympathetic neural inhibition are unknown. We examined whether RDN differentially influences the sympathetic discharge pattern of vasoconstrictor neurons in patients with resistant hypertension. Standardized office BP, single-unit MSNA, and multi-unit MSNA were obtained at baseline and at 3-month follow-up in 35 patients with resistant hypertension. Twenty-five patients underwent RDN, and 10 patients underwent repeated measurements without RDN (non-RDN). Baseline BP averaged 164/93 mm Hg (RDN) and 164/87 mm Hg (non-RDN) despite use of an average of 4.8 ± 0.4 and 4.4 ± 0.5 antihypertensive drugs, respectively. Mean office BP decreased significantly by -13/-6 mm Hg for systolic BP (P<0.001) and diastolic BP (P<0.05) with RDN but not in non-RDN at 3-month follow-up. RDN moderately decreased multi-unit MSNA (79 ± 3 versus 73 ± 4 bursts/100 heartbeats; P<0.05), whereas all properties of single-unit MSNA including firing rates of individual vasoconstrictor fibers (43 ± 5 versus 27 ± 3 spikes/100 heartbeats; P<0.01), firing probability (30 ± 2 versus 22 ± 2% per heartbeat; P<0.02), and multiple firing incidence of single units within a cardiac cycle (8 ± 1 versus 4 ± 1% per heartbeat; P<0.05) were substantially reduced at follow-up. BP, single-unit MSNA, and multi-unit MSNA remained unaltered in the non-RDN cohort at follow-up. RDN results in the substantial and rapid reduction in firing properties of single sympathetic vasoconstrictor fibers, this being more pronounced than multi-unit MSNA inhibition. Whether the earlier changes in single-unit firing patterns may predict long-term BP response to RDN warrants further exploration.

788 PROFOUND REDUCTION IN SYMPATEHTIC NERVE FIRING PATTERN AFTER RENAL DENERVATION IN RESISTANT HYPERTENSION

Objective: Renal denervation (RDN) has been shown to reduce blood pressure (BP) and sympathetic activation in resistant hypertension (RH). The underlying mechanisms involved in sympathetic neural inhibition are unknown. We examined whether RDN may influence differentially sympathetic discharge of vasoconstrictor neurons in patients with RH. Design and method: Standardized office BP, multi-unit and single-unit muscle sympathetic nerve activity (MSNA) measured by microneurography were obtained in 23 patients with RH (57 ± 12yrs) before and 3 months after RDN. Results: Baseline BP averaged 166/74 ± 18/20mmHg despite the use of 4.8 ± 2.0 antihypertensive drugs. RDN was performed with an average of 9.4 ± 1.8 total ablations. SBP decreased significantly by -11 ± 18 mmHg (p < 0.01) and DBP by -5 ± 10 mmHg (p < 0.05). While multi-unit MSNA was only moderately reduced (78 ± 17 vs 71 ± 17bursts/100 heart beats (hb); p < 0.05), all properties of single-unit MSNA including firing rates of individual vasoconstrictor fibers (44 ± 25 vs 27 ± 17 spikes/100hb; p < 0.01), firing probability (31 ± 12 vs 22 ± 12% of hb; p < 0.02), and the multiple firing incidence of single units (9 ± 8 vs 4 ± 4% of hb; p < 0.05) were substantially reduced at 3 month follow up. The DBP reduction was significantly related to multi-unit MSNA decrease (r = 0.54; p < 0.01), but not to single-unit firing pattern. Conclusions: In patients with RH, RDN results in a marked and rapid reduction in the single-unit sympathetic firing discharge pattern of active vasoconstrictor fibres that appears to be more pronounced than multi-unit MSNA reduction. Whether early assessment of the changes in single-unit firing characteristics may be useful to predict the BP response to RDN warrants further investigation.

489 Activation by Human Heart Failure of a Sympathoexcitatory Cardiopulmonary Reflex

Functionally specific efferent renal sympathetic nerve fibers fire in response to discrete afferent stimuli. We hypothesize that individual efferent sympathetic fibers directed at other vascular beds also may be activated reflexively by specific afferent inputs. Previous work from our group, involving quantification of cardiac norepinephrine spillover, detected activation in heart failure (HF) by high atrial pressure of a normally quiescent cardio-cardiac sympatho-excitatory reflex. Whether individual vasoconstrictor neurons directed at skeletal muscle vessels respond concordantly or differentially to changes in atrial pressure is unknown.

Sympathetic nervous system activity in health and disease—advances with microneurographic recordings

Sympathetic nervous system activity in health and disease—advances with microneurographic recordings

Somatosympathetic Vasoconstrictor Reflexes in Human Spinal Cord Injury: Responses to Innocuous and Noxious Sensory Stimulation below Lesion

It is known that the sudden increases in blood pressure associated with autonomic dysreflexia in people with spinal cord injury (SCI) are due to a spinally mediated reflex activation of sympathetic vasoconstrictor neurons supplying skeletal muscle and the gut. Apart from visceral inputs, such as those originating from a distended bladder, there is a prevailing opinion that autonomic dysreflexia can be triggered by noxious stimulation below the lesion. However, do noxious inputs really cause an increase in blood pressure in SCI? Using microelectrodes inserted into a peripheral nerve to record sympathetic nerve activity we had previously shown that selective stimulation of small-diameter afferents in muscle or skin, induced by bolus injection of hypertonic saline into the tibialis anterior muscle or the overlying skin, evokes a sustained increase in muscle sympathetic nerve activity and blood pressure and a transient increase in skin sympathetic nerve activity and decrease in skin blood flow in able-bodied subjects. We postulated that these sympathetic responses would be exaggerated in SCI, with a purely noxious stimulus causing long-lasting increases in blood pressure and long-lasting decreases in skin blood flow. Surprisingly, though, we found that intramuscular or subcutaneous injection of hypertonic saline into the leg caused negligible changes in these parameters. Conversely, weak electrical stimulation over the abdominal wall, which in able-bodied subjects is not painful and activates large-diameter cutaneous afferents, caused a marked increase in blood pressure in SCI but not in able-bodied subjects. This suggests that it is activation of large-diameter somatic afferents, not small-diameter afferents, that triggers increases in sympathetic outflow in SCI. Whether the responses to activation of large-diameter afferents reflect plastic changes in the spinal cord in SCI is unknown.

Firing patterns of muscle vasoconstrictor neurons in respiratory disease

Because the cardiovascular system and respiration are so intimately coupled, disturbances in respiratory control often lead to disturbances in cardiovascular control. Obstructive Sleep Apnea (OSA), Chronic Obstructive Pulmonary Disease (COPD), and Bronchiectasis (BE) are all associated with a greatly elevated muscle vasoconstrictor drive (muscle sympathetic nerve activity, MSNA). Indeed, the increase in MSNA is comparable to that seen in congestive heart failure (CHF), in which the increase in MSNA compensates for the reduced cardiac output and thereby assists in maintaining blood pressure. However, in OSA – but not COPD or BE – the increase in MSNA can lead to hypertension. Here, the features of the sympathoexcitation in OSA, COPD, and BE are reviewed in terms of the firing properties of post-ganglionic muscle vasoconstrictor neurons. Compared to healthy subjects with low levels of resting MSNA, single-unit recordings revealed that the augmented MSNA seen in OSA, BE, COPD, and CHF were each associated with an increase in firing probability and mean firing rates of individual neurons. However, unlike patients with heart failure, all patients with respiratory disease exhibited an increase in multiple within-burst firing which, it is argued, reflects an increase in central sympathetic drive. Similar patterns to those seen in OSA, COPD, and BE were seen in healthy subjects during an acute increase in muscle vasoconstrictor drive. These observations emphasize the differences by which the sympathetic nervous system grades its output in health and disease, with an increase in firing probability of active neurons and recruitment of additional neurons being the dominant mechanisms.