Activation of sulphonylurea-sensitive channels and the NO-cGMP pathway decreases the heart rate response to sympathetic nerve stimulation

OBJECTIVES: Activation of ATP sensitive K+ channels (K(ATP)) and the NO-cGMP pathway have both been implicated in reducing norepinephrine (NE) release from cardiac sympathetic nerves during stimulation. Our aim was to test whether these pathways could interact and modulate cardiac excitability during sympathetic nerve stimulation (SNS). METHODS: The effect of inhibitors and activators of K(ATP) channels and the NO-cGMP pathway on the heart rate (HR) response to cardiac SNS in the isolated guinea pig (Cavia porcellus) double atrial/right stellate ganglion preparation was studied (n=48). RESULTS: The K(ATP) channel activator, diazoxide (100 microM, n=6) or hypoxia (0% O2/5% CO2, n=6) significantly attenuated the HR response to 3 Hz SNS by -10+/-4% and -27+/-6% respectively; an effect that was reversed by the K(ATP) channel inhibitor, glibenclamide (30 microM). Glibenclamide (n=6) on its own enhanced the HR response to SNS by 20+/-8%. Bath applied NE (0.1-0.7 microM, n=6) did not affect the HR response to diazoxide, although an increased response to glibenclamide was observed at 0.3 and 0.5 microM NE. In the presence of 8-Br-cGMP (0.5 mM, n=7), diazoxide further decreased the HR response SNS (19+/-3%). The NO synthase inhibitor, N-omega-nitro-L-arginine (100 microM) significantly increased the HR response (13+/-3%) to SNS in the presence of diazoxide (100 microM, n=6). This effect was reversed with excess (1 mM) L-arginine. Conversely, the NO donor, sodium nitroprusside (SNP, 20-100 microM) significantly attenuated the HR response to SNS. The addition of glibenclamide (30 microM, n=10) could still enhance the HR response (42+/-15%) to SNS. Similar results were seen with the cyclic GMP analogue, 8-Br-cGMP (0.5 mM, n=12). CONCLUSIONS: Our results indicate that NO and sulphonlyurea-sensitive channels act in a complementary fashion, but appear to be independent of each other in the regulation of HR during cardiac SNS activation.     

Circadian variations of stellate ganglion nerve activity in ambulatory dogs.

BACKGROUND: The presence of circadian variations in sympathetic outflow from the stellate ganglia is unclear. OBJECTIVES: The purpose of this study was to continuously record stellate ganglion nerve activity (SGNA) in ambulatory dogs. METHODS: We performed continuous 24-hour left (N = 3) or bilateral (N = 3) SGNA recordings in normal ambulatory dogs using implanted Data Sciences International transmitters. We also performed simultaneous ECG recording (n = 5) or simultaneous ECG and blood pressure recordings (n = 1). RESULTS: The total duration of continuous ambulatory recording averaged 41.5 +/- 16.6 days. Five dogs had persistent stable recording, and one dog developed hardware malfunction in week 3. SGNA was followed immediately (<1 second) by heart rate and blood pressure elevation and a reduced standard deviation of consecutive activation cycle length (SDNN) from 236 +/- 93 ms to 121 +/- 51 ms (P = 0.007). Heart rate correlated significantly with SGNA. When there was a sudden increase of SGNA, the sudden increase occurred bilaterally in 90% of the episodes. Both heart rate and SGNA showed statistically significant (P <.01) circadian variation. Nadolol (20 mg/day for 5 days) reduced average heart rate from 99 +/- 8 bpm at baseline to 88 +/- 9 bpm (N = 6, P = .001) but did not significantly alter SGNA. Immunohistochemical staining of the stellate ganglia showed tyrosine hydroxylase-positive ganglion cells and nerves at the recording site. CONCLUSION: There is a circadian variation in sympathetic outflow from canine stellate ganglia. Circadian variation of SGNA is an important cause of circadian variations of cardiac sympathetic tone.     

Impaired baroreflex as a cause of chronotropic incompetence during exercise via autonomic mechanism in patients with heart disease

PURPOSE: It is thought that the mechanisms responsible for impaired chronotropic response to exercise are related to disturbance of cardiovascular autonomic regulation such as the baroreflex. However, it is still unclear whether the baroreflex mechanisms modulate heart rate (HR) responses to exercise via vagal and/or sympathetic alteration. We therefore investigated the effects of baroreflex sensitivity (BRS) on the exercise HR response in the early phase of vagal deactivation and in the later phase of sympathetic excitation via metabolic stimulation. METHODS: Twenty-four patients (18 males and 6 females, age 59+/-9 years) with heart disease underwent symptom-limited treadmill exercise testing according to the Bruce protocol, and BRS was measured utilizing the phenylephrine method. Subjects were grouped by their resting BRS value: 12 with normal BRS (> or =6 ms/mmHg) and 12 with depressed BRS (<6 ms/mmHg). The HR response to exercise was assessed using two parameters: the increment in HR during exercise and the ratio of HR response to the metabolic reserve (chronotropic index). RESULTS: (1) In the patients with depressed BRS, the HR responses within 1 min after the start of exercise and from 1 min to peak exercise were attenuated compared with those having a normal BRS (15+/-8 vs. 24+/-8 bpm and 36+/-9 vs. 47+/-15 bpm, respectively). (2) The chronotropic index in the patients with depressed BRS was lower than in those with normal BRS (0.50+/-0.14 vs. 0.64+/-0.08). CONCLUSION: These findings suggest that impaired BRS modulates both the parasympathetic influence in early exercise and sympathetic effects in the later phase on HR response to exercise.     

Effects of unilateral stellate ganglion block on the spectral characteristics of heart rate variability

The effect of unilateral stellate ganglion block on cardiovascular regulation remains controversial, so the present study used power spectral analysis of heart rate variability to investigate its effect on the autonomic neural control of the heart. In 20 young healthy volunteers (mean age: 25 years), heart rate variability was determined before and after unilateral stellate ganglion block (right side 11, left side 9) using 8 ml of 1% mepivacaine during supine rest. Using autoregressive spectrum analysis, power spectra were quantified by measuring the area in 3 frequency bands: high-frequency power (lnHF, parasympathetic influence) from 0.15 to 0.40 Hz, low-frequency power (lnLF, predominantly sympathetic influence) from 0.04 to 0.15 Hz, and total-frequency power (lnTF) less than 0.40 Hz. Right stellate ganglion block decreased not only the lnLF component from 6.55+/-0.84 to 5.77+/-0.47 but also the lnHF component from 4.40+/-0.95 to 3.42+/-1.12 (p<0.05). In contrast, left stellate ganglion block changed neither the lnLF nor the lnHF component. The lnTF component was also decreased significantly by right stellate ganglion block from 7.80+/-0.95 to 7.01+/-0.36 (p<0.05), but was unchanged following left stellate ganglion block. Neither right nor left stellate ganglion block induced any significant change in both the RR and corrected QT intervals. However, changes in the RR interval induced by right stellate ganglion block showed significant positive correlation with changes in lnHF (p<0.005) and lnTF (p<0.05). These results suggest that (1) autonomic innervation to the sinus node is mainly through the right-sided stellate ganglion, (2) pharmacological right-sided stellate ganglion block may attenuate not only sympathetic but also parasympathetic activity and (3) following right stellate ganglion block the decrease in both the sympathetic and parasympathetic influence on the sinus node may inconsistently counterbalance and change the RR interval.     

Pre-synaptic NO-cGMP pathway modulates vagal control of heart rate in isolated adult guinea pig atria

The role of nitric oxide (NO) in the vagal modulation of heart rate (HR) is controversial. We tested the hypothesis that NO acts via a pre-synaptic, guanylyl cyclase (GC) dependent pathway. The effects of inhibiting NO synthase (NOS) and GC were evaluated in isolated atrial/right vagal nerve preparations from adult (550-750 g) and young (150-250 g) female guinea pigs. Levels of NOS protein were quantified in right atria using Western blotting and densitometry. The non-specific NOS inhibitor N- omega -nitro- L -arginine (L -NA, 100 microM, n=5) significantly reduced the negative chronotropic response to vagal nerve stimulation (VNS) at 3 and 5 Hz in the adult guinea pig. This effect was reversed with 1 m ML -arginine. Similar results were observed with the specific neuronal NOS inhibitor vinyl-N5-(1-imino-3-butenyl)- L -ornithine (L -VNIO, 100 microM, n=7). Inhibition of GC with 1H-(1,2,4)-oxadiazolo-(4, 3-a)-quinoxalin-1-one (ODQ, 10 microM, n=7) also significantly reduced the negative chronotropic response to VNS at 3 and 5 Hz in adult guinea pigs. Neither L -NA (n=6), L -VNIO (n=5) nor ODQ (n=6) changed the HR response to cumulative doses of carbamylcholine in adult guinea pig atria suggesting that the action of NO is pre-synaptic. The HR response to VNS was unaffected by L -NA (n=7) or ODQ (n=7) in young guinea pigs and Western blot analysis showed significantly lower levels of nNOS protein in right atria from young animals. These results suggest a pre-synaptic NO-cGMP pathway modulates cardiac cholinergic transmission, although this may depend on the developmental stage of the guinea pig.     

Physiological and catecholamine response to sympathetic stimulation in turner syndrome

OBJECTIVE: Women with Turner syndrome have increased heart rate and high blood pressure (BP), and have been described as having high tolerance for emotional stress. We hypothesized that women with Turner syndrome have reduced catecholaminergic and physiological response to sympathetic stimulation, and that changes in BP and heart rate are related to their catecholamine response to sympathetic stimulation. DESIGN AND PATIENTS: Ten young women with Turner syndrome, age 17-34 years were the subjects of this study. Their response to sympathetic stimulation was compared to a group of 10 age-matched healthy women. MEASUREMENTS: After a period of 30 min resting, subjects and controls were subjected to an escalating series of sympathetic stimulation: orthostatic, cold pressor and exercise, and their plasma catecholamines and haemodynamic response were monitored and compared to resting levels. RESULTS: Resting heart rate was higher in Turner syndrome patients at 83 +/- 9 beats per min (bpm, mean +/- SD), as compared to controls (74 +/- 10 bpm, P < 0.05). Their supine BP was also higher at 122 +/- 9/84 +/- 6 vs. 106 +/- 11/70 +/- 9 mmHg (P < 0.02/< 0.02). The corresponding resting norepinephrine, but not epinephrine, was higher in Turner syndrome patients (2.54 +/- 1.09 nmol/l) as compared to controls (1.69 +/- 0.55 nmol/l, P < 0.02). In response to orthostatic stimulation and cold pressor test the systolic, but not the diastolic BP or heart rate, increased in Turner syndrome patients but not in the control group (P < 0.01). The change in blood catecholamine levels was comparable in both groups. Their physiological response to exercise was normal. Yet, the exercise-induced surge of norepinephrine and epinephrine in Turner syndrome patients was lower (P < 0.02). CONCLUSIONS: Turner syndrome is associated with dysregulation of the sympathetic nervous system (SNS), leading to tachycardia and high BP, increased resting norepinephrine levels, and a greater tolerance of the cathecholamine response to exercise.     

Pravastatin normalises peripheral cardiac sympathetic hyperactivity in the spontaneously hypertensive rat

Hypertension is associated with heightened cardiac sympathetic drive whilst statins reduce angiotensin II (ATII) signalling, superoxide anion production and increase nitric oxide bioavailability, events that can potentially reduce peripheral cardiac sympathetic neurotransmission. We therefore investigated whether pravastatin alters peripheral cardiac sympathetic control in the spontaneously hypertensive rat (SHR). SHRs (16-18 weeks) had significantly (p < 0.05) enhanced atrial ³H-norepinephrine (³H-NE) release to field stimulation compared to normotensive WKYs. 2-week pravastatin supplementation significantly reduced ³H-NE release to levels observed in the WKY. In-vivo, pravastatin lowered resting heart rate (HR) in the SHR despite not affecting arterial blood pressure or serum cholesterol. In SHR atria/right stellate ganglion preparations, the HR response to stellate stimulation (1, 3, and 5 Hz) was also significantly reduced by pravastatin whilst the HR response to exogenous NE (0.025-5 μmol) remained similar. The nitric oxide synthase (NOS) inhibitor l-NAME (1 mmol/l) increased ³H-NE release by similar amounts in atria from supplemented and non-supplemented SHRs, whilst Western blotting showed no difference in protein levels of nNOS, eNOS, guanylyl cyclase, or the NADPH oxidase subunits Gp91 and P40phox. Pravastatin significantly reduced cardiac ATII levels and angiotensin converting enzyme 1 and 2 expressions whilst protein levels of the ATII receptor (ATR₁) remained unchanged in the SHR. Immunohistochemistry co-localised ATR₁ with tyrosine hydroxylase positive neurons in the stellate ganglion. The ATR₁ antagonist Losartan (5 μmol) equalised release of ³H-NE to comparable levels in supplemented and non-supplemented SHRs. These results suggest 2-week pravastatin treatment reduces cardiac ATII, and prevents its facilitatory effect on NE release thus normalising cardiac sympathetic hyper-responsiveness in SHRs.     

The effects of the intracerebroventricular administration of cimetidine on hemodynamics in conscious rats: relation to the sympathetic nervous system

Histamine H2-receptor antagonists administered into the central nervous system have been shown to increase arterial pressure (AP) in anaesthetized animals (Paakkari et al., 1982). Few studies have been reported on the effects of centrally administered cimetidine (CIM), one of the histamine H2-receptor antagonists, in conscious animals. However, the mechanism of the pressor action of histamine H2-receptor antagonists remains unclear. The present study was designed to investigate the hemodynamic effects of intracerebroventricular (i.c.v.) CIM and the interaction between the sympathetic nervous system and histamine receptor system in conscious rats. Male Wistar rats weighing 200 gr were prepared for the experiment under a conscious and minimally restricted state. Five micrograms of i.c.v. saline (SAL-ICV group, n = 5) did not produce significant changes in mean arterial pressure (MAP) or heart rate (HR) (MAP from 85.6 +/- 3.4 to 86.0 +/- 4.3 mmHg and HR from 395.0 +/- 13.9 to 395.2 +/- 8.2 bpm, respectively). Twenty micrograms of i.c.v. phenoxybenzamine (POB-ICV group, n = 6) decreased MAP from 95.8 +/- 4.1 to 85.2 +/- 3.1 mmHg, -10.7 +/- 2.2 mmHg as delta MAP, and increased HR from 392.5 +/- 8.5 to 435.3 +/- 13.9 bpm, +42.8 +/- 6.8 bpm as delta HR. Two-hundred micrograms of intravenous (i.v.) POB (POB-IV group, n = 5) also decreased MAP from 96.0 +/- 4.3 to 71.0 +/- 5.1 mmHg, -25.0 +/- 2.7 mmHg as delta MAP, and increased HR from 395.8 +/- 10.5 to 473.0 +/- 12.4 bpm, +77.2 +/- 7.6 bpm as delta HR. The changes in MAP and HR were much greater in the POB-IV group than those in the other two groups. The subsequent i.c.v. administration of 250 micrograms of CIM induced an increase in MAP (+19.4 +/- 1.7 mmHg as delta MAP) and a decrease in HR (-36.4 +/- 3.1 bpm as delta HR) in the SAL-ICV group, which continued for at least 30 minutes producing peak effects 2 minutes after i.c.v. administration of CIM. However, an elevation of MAP caused by i.c.v. CIM was much more inhibited in the POB-ICV group than in the POB-IV group (+2.5 +/- 0.7 mmHg as delta MAP in the POB-ICV group and +5.0 +/- 1.3 mmHg as delta MAP in the POB-IV group, respectively).(ABSTRACT TRUNCATED AT 400 WORDS)     

Selective increase of cardiac neuronal sympathetic tone: a catheter-based access to modulate left ventricular contractility.

OBJECTIVES: This study was designed to develop a technique to selectively increase the sympathetic tone to the heart by cardiac sympathetic nerve stimulation (SNS). BACKGROUND: Access to the cardiac sympathetic neurons may allow modulating the adrenergic tone of the heart while avoiding systemic side effects. METHODS: Cardiac sympathetic nerves course within neural sleeves along the subclavian artery. Because of this proximity, transvascular SNS was attempted with electrode catheters inside the subclavian artery in 16 pigs. RESULTS: Right/left (R-/L-) SNS (20 Hz) during ventricular pacing at 200/min evoked a >100% increase of left ventricular systolic pressure (baseline: 51 +/- 1 mm Hg; L-SNS: 118 +/- 26 mm Hg; R-SNS: 116 +/- 33 mm Hg; p < 0.001) while systemic vascular resistance remained unchanged. There was a sigmoid dose-response curve with rapid on- and offset of the effect during SNS initiation/cessation. Positive inotropic effects persisted for 12 h of continued SNS (n = 4). Besides positive dromotropic effects, L-SNS/R-SNS yielded a 41% and 77% sinus rate increase, respectively. CONCLUSIONS: The neural adrenergic tone to the heart can be selectively increased by catheter stimulation of cardiac efferent sympathetic nerves.     

The effects of exercise training on sympathetic neural activation in advanced heart failure: a randomized controlled trial

OBJECTIVES: The goal of this study was to test the hypothesis that exercise training reduces resting sympathetic neural activation in patients with chronic advanced heart failure. BACKGROUND: Exercise training in heart failure has been shown to be beneficial, but its mechanisms of benefit remain unknown. METHODS: Sixteen New York Heart Association class II to III heart failure patients, age 35 to 60 years, ejection fraction < or =40% were divided into two groups: 1) exercise-trained (n = 7), and 2) sedentary control (n = 9). A normal control exercise-trained group was also studied (n = 8). The four-month supervised exercise training program consisted of three 60 min exercise sessions per week, at heart rate levels that corresponded up to 10% below the respiratory compensation point. Muscle sympathetic nerve activity (MSNA) was recorded directly from peroneal nerve using the technique of microneurography. Forearm blood flow was measured by venous plethysmography. RESULTS: Baseline MSNA was greater in heart failure patients compared with normal controls; MSNA was uniformly decreased after exercise training in heart failure patients (60 +/- 3 vs. 38 +/- 3 bursts/100 heart beats), and the mean difference in the change was significantly (p < 0.05) greater than the mean difference in the change in sedentary heart failure or trained normal controls. In fact, resting MSNA in trained heart failure patients was no longer significantly greater than in trained normal controls. In heart failure patients, peak VO(2) and forearm blood flow, but not left ventricular ejection fraction, increased after training. CONCLUSIONS: These findings demonstrate that exercise training in heart failure patients results in dramatic reductions in directly recorded resting sympathetic nerve activity. In fact, MSNA was no longer greater than in trained, healthy controls.     

Evidence for a sympatholytic effect of mibefradil in the pithed rat preparation

OBJECTIVE: The T-type prevalent calcium channel blocker mibefradil (MIB) was shown to possess N-type calcium channel blocking properties. As this particular type of calcium channel is known to be crucially involved in the neuronal release of noradrenaline, we have investigated whether MIB could be a sympatholytic drug. METHODS: To evaluate the sympathoinhibitory action, the effects of 3 and 10 micromol/kg MIB on the tachycardic effect of electrical stimulation of the preganglionic cardioaccelerator nerves in the pithed rat were investigated. The effect of MIB on the dose-response curve of externally applied noradrenaline was also studied. To compare the results with a classic L-type calcium channel blocker, the experiments were repeated with 3 and 10 micromol/kg verapamil (VER). RESULTS: The maximal increase in heart rate in response to electrical nerve stimulation was 96 +/- 7 bpm (control, n = 6), 70 +/- 6 bpm (3 micromol/kg MIB, n = 8), 57 +/- 6 bpm (10 micromol/kg MIB, n = 5), 93 +/- 5 bpm (3 micromol/kg VER, n = 6) and 46 +/- 7 bpm (10 micromol/kg VER, n = 5). The tachycardic response to electrical stimulation at 1, 5 and 10 Hz was completely blocked by 5 mg/kg intravenous guanethidine. The maximal increase in heart rate in response to noradrenaline was 96 +/- 4 bpm (control, n = 6), 103 +/- 6 (3 micromol/kg MIB, n = 6), 42 +/- 9 bpm (10 micromol/kg MIB, n = 5), 73 +/- 5 bpm (3 micromol/kg VER, n = 5) and 40 +/- 7 bpm (10 micromol/kg VER, n = 6). Under control conditions and in the presence of 3 micromol/kg MIB and VER the maximal effect of noradrenaline was reached at 0.1 micromol/kg whereas in the presence of 10 micromol/kg MIB and VER it was reached at a dose of 1 micromol/kg. MIB at a dose of 3 micromol/kg was significantly more effective in reducing the chronotropic response to electrical stimulation compared with externally applied noradrenaline. For VER the opposite holds true. These differences were not observed with doses of 10 micromol/kg MIB and VER. CONCLUSION: Mibefradil, besides its direct effect on cardiac T- and L-type calcium channels, reduces the release of noradrenaline from sympathetic nerve endings, most probably by inhibition of presynaptic N-type calcium channels. In the model used this effect is only observable at relatively low concentrations, most probably because of the direct cardiodepressant action of MIB provoked by L-type channel blockade.     

Diurnal blood pressure variation in progressive autonomic failure

To investigate the role of the autonomic nervous system (ANS) in the generation of the circadian blood pressure (BP) variation, the degree of impairment of the ANS was related to the results of ambulatory BP recordings in 212 patients with progressive autonomic failure due to familial amyloid polyneuropathy. On the basis of BP and/or heart rate (HR) responses to the Valsalva maneuver, 60 degrees head-up tilting, deep-breathing tests, and plasma norepinephrine levels, 4 groups of patients were distinguished. In all patients and in 38 age-matched control subjects, ambulatory BP was monitored. Patients of group I (n=40, aged 32+/-3 y), with no evidence yet of impairment of their ANS, had circadian BP and HR variations indistinguishable from controls. Patients of group II (n=41, aged 34+/-5 y) had a variable degree of impairment of their parasympathetic ANS, but their sympathetic ANS was still intact. Twenty-four-hour HR was higher in these patients than in controls (88+/-11 versus 78+/-7 bpm, P<0.01). Their circadian HR variation was maintained, but their circadian BP variation was diminished (10+/-6/11+/-4 versus 17+/-6/16+/-4 mm Hg in controls, P<0.01) because of an attenuation of the nocturnal BP decline. Patients of group III (n=69, aged 36+/-6 y), with parasympathetic failure and intermediate sympathetic dysfunction, had a blunted diurnal BP variation, whereas patients of group IV (n=62, aged 38+/-6 y), with parasympathetic failure and severe sympathetic dysfunction, had an absent diurnal BP variation. In patients of groups III and IV, a decrease in daytime BP accounted for the blunted circadian BP variation. This extensive study in progressive autonomic failure confirms the important role of the ANS in the generation of circadian BP variation. For the maintenance of a normal circadian BP pattern, not only an intact sympathetic but also an intact afferent parasympathetic ANS is a prerequisite.     

Exercise intensity prescription after myocardial infarction in patients treated with beta-blockers

PURPOSE: The aim of our study was to answer the following questions: (1) Is it possible to estimate the exercise training intensity according to heart rate in patients treated with beta-blockers after myocardial infarction? and (2) Are there any other appropriate alternate possibilities to estimate the training intensity? METHODS: This study involved 112 men (60.2 +/- 8.6 years) with a previous myocardial infarction treated with beta-blockers. Patients underwent exercise echocardiography and also completed a symptom-limited cardiopulmonary ramp test to determine peak exercise capacity, maximal heart rate, heart rate (HR) at the anaerobic threshold (AT), peak oxygen uptake (VO2peak) VO2 consumption at AT, and exercise capacity at AT. RESULTS: The mean value of HR at AT was 104.7 +/- 13.3 bpm, corresponding to 81.0% +/- 8% of VO2peak and 87.9% +/- 5.6% of HRpeak. The mean HR at 80% HRpeak was 96 +/- 13.7 bpm, at 70% heart rate reserve (HRR) 103.3 +/- 13.1 bpm and at 80% HRR 108 +/- 14.4 bpm. A close correlation was observed between HR at AT and values at 80% HRpeak (r = 0.86, P < .01). A similar correlation was found also for 70% and 80% HRR (r = 0.87 and 0.88, respectively, P < .01). Exercise intensity at AT occurred close to the value of 1 W/kg(bodyweight). CONCLUSIONS: As an upper limit in determining training intensities, the assessment of AT is the gold standard. However, findings suggest that %HRpeak, %HRR, and %VO2peak can be used alternatively. The use of workload expressed as in W/kg also appears useful.     

Activation of the renin-angiotensin system and blood pressure variability in rats

This study was designed to assay, using spectral analysis, the influence of the renin-angiotensin system activation on the blood pressure variability. Rats were surgically prepared with a supra-renal catheter inserted via the left carotid artery to perform local infusions and with a femoral artery catheter to measure blood pressure (BP) and heart rate (HR). The beta-adrenoceptors stimulation by isoprenaline was used to increase the plasma renin activity (PRA). A first group (n = 8) was infused with isoprenaline (0, 0.003, 10, 100, 300 ng/kg/min) at a rate of 20 microL/min. A second group (n = 8) received a bolus of the angiotensin II (AII) AT1 receptor-antagonist valsartan (2 mg/kg/mL, i.a.) prior to isoprenaline infusions. Five groups were used for blood sampling (one group infused with one concentration of isoprenaline) to assay PRA and catecholamines (CA). BP recordings were analysed using the fast Fourier transforms (FFT) on 2048 points time series (204.8 s). Isoprenaline from the concentration of 10 ng/kg/min increased PRA with a maximum effect of 8.5 fold with the highest concentration (300 ng/kg/min, p < 0.05); CA were not modified. Isoprenaline amplified the low-frequency (LF: 0.02-0.20 Hz) component of the systolic BP (SBP) variability (10 ng/kg/min: 4.16 +/- 0.62 mmHg2 versus: 2.90 +/- 0.44 mmHg2 for control value, p < 0.05) even if it did not modify BP and HR levels. Isoprenaline lowered BP and had a tachycardic effect at concentrations > or = 100 ng/kg/mL (at 100 ng/kg/mL: SBP = 115 +/- 3 mmHg, HR = 464 +/- 15 bpm, versus control: SBP = 128 +/- 3 mmHg, HR = 351 +/- 7 bpm, p < 0.05). Valsartan modified neither BP levels nor BP variability but exerted a tachycardic effect (+25 bpm, p < 0.001). Valsartan prevented the amplification of the LF oscillations of SBP induced by isoprenaline (10 ng/kg/min: 2.53 +/- 0.38 mmHg2 versus: 2.20 +/- 0.25 mmHg2 for control value (valsartan), ns). We conclude that a moderate endogenous production of renin increases SBP variability in the LF range in the conscious rat. This effect which does not affect BP and HR levels is mediated by AII AT1 receptors and does not involve the sympathetic nervous system.     

Salt supresses baseline muscle sympathetic nerve activity in salt-sensitive and salt-resistant hypertensives

The objective of our study was to evaluate the role of the baroreflex control of peripheral sympathetic nervous system on the increase of muscle sympathetic nerve activity (MSNA) in salt-sensitive (SS) and salt-resistant (SR) hypertensives under low salt diet. In phase I mild-to-moderate hypertensive patients (n=5) received three diet periods: a first regular salt (RS1), a low salt (LS=20 meq Na+/day), followed by a second regular salt diet (RS2) with a 7-day duration of each. At the end of each period, sympathetic and heart rate baroreflex control were recorded. Baseline MSNA varied (P<0.005) from 18+/-8 (RS1) to 32+/-9 (LS) and to 14+/-9 (RS2) bursts per minute (bpm). In phase II additional patients (n=6) were included to have baseline MSNA, sympathetic and heart rate baroreflex control evaluated at the end of the LS and RS2. For all patients (n=11), there was a significant decrease of MSNA from 36+/-4 to 20+/-8 bpm on day 7 of LS to RS2 (P<0.05). The response of MSNA to a salt restriction was similar for SS and SR patients, who showed a change from 32+/-6 to 18+/-11 and from 36+/-9 to 17+/-7 bpm for SS and SR on day 7 of LS and RS2 diets, respectively (P<0.05). MSNA baroreflex gain was similar during phenylephrine infusions at day 7 of LS and RS2 (5.1+/-1.6 and 6.1+/-2.9 bpm/mmHg), but it was reduced under LS during sodium nitroprusside infusion (19.5+/-4.9 vs 8.9+/-0.7 bpm/mmHg) (P<0.05) for the whole group. Baroreflex control of MSNA was also similar during phenylephrine infusions under LS and RS2 diets for SS (4.0+/-0.9 and 3.3+/-0.2 bpm/mmHg) and for SR patients (10.1+/-2.5 and 5.6+/-1.5 bpm/mmHg). During nitroprusside infusion, baroreflex gain was significantly greater under RS2 for SR patients (19.5+/-2.6 bpm/mmHg) when compared to LS (11.2+/-5.2 bpm/mmHg) and the same significant difference was observed among SS patients (14.4+/-4.7 and 9.1+/-3.6 bpm/mmHg under RS2 and LS diets, respectively). There was no difference in heart rate baroreflex gain between LS and RS2 diets. Data support the hypotheses that (1) sodium supresses baseline MSNA in SS and SR hypertensives and (2) sodium restriction may impair baroreflex control of MSNA in SR and SS mild-to-moderate hypertensive patients during blood pressure reductions.     

Exercise testing in patients with the sick sinus syndrome

Heart rate response to exercise was compared in three groups of subjects; 22 patients (mean age 63.7 years) with sick sinus syndrome and no other significant heart disease (Group I); 10 subjects of the same age with stable, asymptomatic sinus bradycardia at rest (Group II); 29 age-matched controls (Group III). All subjects underwent maximal, symptom-limited exercise testing and the maximal heart rate (HR max), the ratio between HR max and the theoretical maximal heart rate (HR%), exercise capacity (EC) and the ratio between heart rate % and exercise capacity (HR%/EC) % of the three groups were compared. Maximal heart rate, heart rate % and (HR%/EC) % in Group I patients were significantly lower than in Group III subjects (119.1 +/- 24.0 vs 139.0 +/- 18.2; 76.0 +/- 13.9 vs 87.9 +/- 10.8 and 83.0 +/- 19.3 vs 97.5 +/- 15.1 respectively); (HR%/EC) % was significantly lower in Group I patients compared to Group II subjects (83.0 +/- 19.3 vs 101.5 +/- 28.6). Heart rate response was the same in Group I and Group II patients and exercise capacity did not differ in the three groups. Maximal heart rate, heart rate % and (HR%/EC) % were similar in Group II and Group III subjects. The association of (HR%/EC) % less than or equal to 85% with either HR max less than or equal to 110/min or HR% less than or equal to 70% at the end of maximal exercise testing may be suggestive of sick sinus syndrome. The reduced heart rate response during exercise may be helpful in assessing sick sinus syndrome in patients with no other signs of heart disease.     

Nerve sprouting and sudden cardiac death

The factors that contribute to the occurrence of sudden cardiac death (SCD) in patients with chronic myocardial infarction (MI) are not entirely clear. The present study tests the hypothesis that augmented sympathetic nerve regeneration (nerve sprouting) increases the probability of ventricular tachycardia (VT), ventricular fibrillation (VF), and SCD in chronic MI. In dogs with MI and complete atrioventricular (AV) block, we induced cardiac sympathetic nerve sprouting by infusing nerve growth factor (NGF) to the left stellate ganglion (experimental group, n=9). Another 6 dogs with MI and complete AV block but without NGF infusion served as controls (n=6). Immunocytochemical staining revealed a greater magnitude of sympathetic nerve sprouting in the experimental group than in the control group. After MI, all dogs showed spontaneous VT that persisted for 5.8+/-2.0 days (phase 1 VT). Spontaneous VT reappeared 13.1+/-6.0 days after surgery (phase 2 VT). The frequency of phase 2 VT was 10-fold higher in the experimental group (2.0+/-2.0/d) than in the control group (0.2+/-0.2/d, P<0.05). Four dogs in the experimental group but none in the control group died suddenly of spontaneous VF. We conclude that MI results in sympathetic nerve sprouting. NGF infusion to the left stellate ganglion in dogs with chronic MI and AV block augments sympathetic nerve sprouting and creates a high-yield model of spontaneous VT, VF, and SCD. The magnitude of sympathetic nerve sprouting may be an important determinant of SCD in chronic MI.     

Exercise training improves baroreflex sensitivity associated with oxidative stress reduction in ovariectomized rats

The protection from coronary events that young women have is sharply reduced at menopause. Oxidative stress and baroreflex sensitivity impairment of the circulation have been demonstrated to increase cardiovascular risk. On the other hand, exercise training has been indicated as a nonpharmacological treatment for many diseases. The aim of the present study was to test the hypothesis that exercise training can improve baroreflex sensitivity associated with reduction in oxidative stress in ovariectomized rats, an experimental model of menopause. Exercise training was performed on a treadmill for 8 weeks. Arterial pressure and baroreflex sensitivity, which were evaluated by tachycardic and bradycardic responses to changes in arterial pressure, were monitored. Oxidative stress was evaluated by chemiluminescence and superoxide dismutase and catalase antioxidant enzyme activities. Exercise training reduced resting mean arterial pressure (112+/-2 vs 122+/-3 mm Hg in the sedentary group) and heart rate (325+/-4 vs 356+/-12 bpm in the sedentary group) and also improved baroreflex sensitivity (tachycardic response, 63% and bradycardic response, 58%). Myocardium (25%) and gastrocnemius muscle (48%) chemiluminescence were reduced, and myocardial superoxide dismutase (44%) and gastrocnemius catalase (97%) activities were enhanced in trained rats in comparison with sedentary rats. Myocardium chemiluminescence was positively correlated with systolic arterial pressure (r=0.6) and inversely correlated with baroreflex sensitivity (tachycardic response, r=-0.8 and bradycardic response, r=-0.7). These results indicate that exercise training in ovariectomized rats improves resting hemodynamic status and reflex control of the circulation, probably associated with oxidative stress reduction, suggesting a homeostatic role for exercise training in reducing cardiovascular risk in postmenopausal women.     

Characterization of autonomic dysfunction in patients with irritable bowel syndrome by means of heart rate variability studies

OBJECTIVE: Our aim was to characterize autonomic dysfunction in patients with irritable bowel syndrome (IBS) using heart rate variability (HRV) studies. METHODS: EKG signals were obtained from 35 patients (mean age, 39.1 +/- 9.5 yr, M:F ratio = 2.9:1) and 18 healthy controls (mean age, 38.2 +/- 6.5 yr, M:F ratio = 2:1) in supine, standing, and deep-breathing modes. Fast Fourier transformation and autoregressive techniques were used to analyze the HRV power spectra in very low (VLF, 0.0078-0.04 Hz), low (LF, 0.04-0.14 Hz), and high (HF, 0.14-0.4 Hz) frequency bands. RESULTS: In the supine position, the VLF power spectral density (PSD) in IBS was significantly higher than normal (3 vs 1.3 beats per minute [bpm]2/Hz, p < 0.01). On changing from the supine to standing position, the normals (NC) had raised median PSDs in the VLF (1.3 vs 12.8 bpm2/Hz, p < 0.01) and LF (1.6 vs 6.1 bpm2/Hz, p < 0.01) bands, as a sign of increased sympathetic tone, whereas the median HF PSDs (parasympathetic tone) remained unchanged (1.8 bpm2/Hz each, p = 0.8). Similarly, the IBS patients had increased VLF (3.04 vs 14.93 bpm2/Hz, p < 0.01) and LF (2.8 vs 8.7 bpm2/Hz, p < 0.01) PSDs on standing up, but the HF PSD was also raised (from 2.4 to 5.7 bpm2/Hz, p = 0.04). On changing from standing to the deep-breathing mode, the normals had a significant increase in the HF (from 1.8 to 10.3 bpm2/Hz, p < 0.001) and a significant reduction of the VLF (from 12.8 to 2.2 bpm2/Hz, p < 0.01) PSDs. The reduction of the LF PSD was not significant (from 6.1 to 5.6 bpm2/Hz, p = 0.6). In IBS, HF PSD remained constant (5.7 bpm2/Hz each, p = 0.6), whereas the LF PSD increased from 8.7 to 24.2 bpm2/Hz (p < 0.0001). The VLF PSD was reduced (from 14.9 to 4.1 bpm2/Hz, p < 0.0001). In IBS, the median sympathovagal outflow ratio was significantly lower in the standing position (1.4 vs 2.8, p < 0.02) and higher in the deep-breathing mode (7.33 vs 0.42, p < 0.0001) than normal. CONCLUSIONS: IBS patients have reduced sympathetic influence on the heart period in response to orthostatic stress and diminished parasympathetic modulation during deep breathing.     

The cardiac sympathetic co-transmitter galanin reduces acetylcholine release and vagal bradycardia: implications for neural control of cardiac excitability

The autonomic phenotype of congestive cardiac failure is characterised by high sympathetic drive and impaired vagal tone, which are independent predictors of mortality. We hypothesize that impaired bradycardia to peripheral vagal stimulation following high-level sympathetic drive is due to sympatho-vagal crosstalk by the adrenergic co-transmitters galanin and neuropeptide-Y (NPY). Moreover we hypothesize that galanin acts similarly to NPY by reducing vagal acetylcholine release via a receptor mediated, protein kinase-dependent pathway. Prolonged right stellate ganglion stimulation (10 Hz, 2 min, in the presence of 10 μM metoprolol) in an isolated guinea pig atrial preparation with dual autonomic innervation leads to a significant (p<0.05) reduction in the magnitude of vagal bradycardia (5 Hz) maintained over the subsequent 20 min (n=6). Immunohistochemistry demonstrated the presence of galanin in a small number of tyrosine hydroxylase positive neurons from freshly dissected stellate ganglion tissue sections. Following 3 days of tissue culture however, most stellate neurons expressed galanin. Stellate stimulation caused the release of low levels of galanin and significantly higher levels of NPY into the surrounding perfusate (n=6, using ELISA). The reduction in vagal bradycardia post sympathetic stimulation was partially reversed by the galanin receptor antagonist M40 after 10 min (1 μM, n=5), and completely reversed with the NPY Y(2) receptor antagonist BIIE 0246 at all time points (1 μM, n=6). Exogenous galanin (n=6, 50-500 nM) also reduced the heart rate response to vagal stimulation but had no effect on the response to carbamylcholine that produced similar degrees of bradycardia (n=6). Galanin (500 nM) also significantly attenuated the release of (3)H-acetylcholine from isolated atria during field stimulation (5 Hz, n=5). The effect of galanin on vagal bradycardia could be abolished by the galanin receptor antagonist M40 (n=5). Importantly the GalR(1) receptor was immunofluorescently co-localised with choline acetyl-transferase containing neurons at the sinoatrial node. The protein kinase C inhibitor calphostin (100 nM, n=6) abolished the effect of galanin on vagal bradycardia whilst the protein kinase A inhibitor H89 (500 nM, n=6) had no effect. These results demonstrate that prolonged sympathetic activation releases the slowly diffusing adrenergic co-transmitter galanin in addition to NPY, and that this contributes to the attenuation in vagal bradycardia via a reduction in acetylcholine release. This effect is mediated by GalR(1) receptors on vagal neurons coupled to protein kinase C dependent signalling pathways. The role of galanin may become more important following an acute injury response where galanin expression is increased.