Nitric oxide donors can increase heart rate independent of autonomic activation

Cyclic nucleotide signaling and pacemaker activity

The sinoatrial node (SAN) is the natural pacemaker of the heart, producing the electrical impulse that initiates every heart beat. Its activity is tightly controlled by the autonomic nervous system, and by circulating and locally released factors. Neurohumoral regulation of heart rate plays a crucial role in the integration of vital functions and influences behavior and ability to respond to changing environmental conditions. At the cellular level, modulation of SAN activity occurs through intracellular signaling pathways involving cyclic nucleotides: cyclic AMP (cAMP) and cyclic GMP (cGMP). In this Review, dedicated to Professor Dario DiFrancesco and his accomplishements in the field of cardiac pacemaking, we summarize all findings on the role of cyclic nucleotides signaling in regulating the key actors of cardiac automatism, and we provide an up-to-date review on cAMP- and cGMP-phosphodiesterases (PDEs), compellingly involved in this modulation.

Isoform-specific regulation of HCN4 channels by a family of endoplasmic reticulum proteins

Ion channels in excitable cells function in macromolecular complexes in which auxiliary proteins modulate the biophysical properties of the pore-forming subunits. Hyperpolarization-activated, cyclic nucleotide-sensitive HCN4 channels are critical determinants of membrane excitability in cells throughout the body, including thalamocortical neurons and cardiac pacemaker cells. We previously showed that the properties of HCN4 channels differ dramatically in different cell types, possibly due to the endogenous expression of auxiliary proteins. Here, we report the discovery of a family of endoplasmic reticulum (ER) transmembrane proteins that associate with and modulate HCN4. Lymphoid-restricted membrane protein (LRMP, Jaw1) and inositol trisphosphate receptor-associated guanylate kinase substrate (IRAG, Mrvi1, and Jaw1L) are homologous proteins with small ER luminal domains and large cytoplasmic domains. Despite their homology, LRMP and IRAG have distinct effects on HCN4. LRMP is a loss-of-function modulator that inhibits the canonical depolarizing shift in the voltage dependence of HCN4 in response to the binding of cAMP. In contrast, IRAG causes a gain of HCN4 function by depolarizing the basal voltage dependence in the absence of cAMP. The mechanisms of action of LRMP and IRAG are independent of trafficking and cAMP binding, and they are specific to the HCN4 isoform. We also found that IRAG is highly expressed in the mouse sinoatrial node where computer modeling predicts that its presence increases HCN4 current. Our results suggest important roles for LRMP and IRAG in the regulation of cellular excitability, as tools for advancing mechanistic understanding of HCN4 channel function, and as possible scaffolds for coordination of signaling pathways.

Decreased excitability and voltage-gated sodium currents in aortic baroreceptor neurons contribute to the impairment of arterial baroreflex in cirrhotic rats

Cardiovascular autonomic dysfunction, which is manifested by an impairment of the arterial baroreflex, is prevalent irrespective of etiology and contributes to the increased morbidity and mortality in cirrhotic patients. However, the cellular mechanisms that underlie the cirrhosis-impaired arterial baroreflex remain unknown. In the present study, we examined whether the cirrhosis-impaired arterial baroreflex is attributable to the dysfunction of aortic baroreceptor (AB) neurons. Biliary and nonbiliary cirrhotic rats were generated via common bile duct ligation (CBDL) and intraperitoneal injections of thioacetamide (TAA), respectively. Histological and molecular biological examinations confirmed the development of fibrosis in the livers of both cirrhotic rat models. The heart rate changes during phenylephrine-induced baroreceptor activation indicated that baroreflex sensitivity was blunted in the CBDL and TAA rats. Under the current-clamp mode of the patch-clamp technique, cell excitability was recorded in DiI-labeled AB neurons. The number of action potential discharges in the A- and C-type AB neurons was significantly decreased because of the increased rheobase and threshold potential in the CBDL and TAA rats compared with sham-operated rats. Real-time PCR and Western blotting indicated that the NaV1.7, NaV1.8, and NaV1.9 transcripts and proteins were significantly downregulated in the nodose ganglion neurons from the CBDL and TAA rats compared with the sham-operated rats. Consistent with these molecular data, the tetrodotoxin-sensitive NaV currents and the tetrodotoxin-resistant NaV currents were significantly decreased in A- and C-type AB neurons, respectively, from the CBDL and TAA rats compared with the sham-operated rats. Taken together, these findings implicate a key cellular mechanism in the cirrhosis-impaired arterial baroreflex.

Heterogeneous abnormalities of in-vivo left ventricular calcium influx and function in mouse models of muscular dystrophy cardiomyopathy

BACKGROUND

Manganese-enhanced cardiovascular magnetic resonance (MECMR) can non-invasively assess myocardial calcium influx, and calcium levels are known to be elevated in muscular dystrophy cardiomyopathy based on cellular studies.

METHODS

Left ventricular functional studies and MECMR were performed in mdx mice (model of Duchenne muscular dystrophy, 24 and 40 weeks) and Sgcd -/- mice (limb girdle muscular dystrophy 2 F, 16 and 32 weeks), compared to wild type controls (C57Bl/10, WT).

RESULTS

Both models had left ventricular hypertrophy at the later age compared to WT, though the mdx mice had reduced stroke volumes and the Sgcd -/- mice increased heart rate and cardiac index. Especially at the younger ages, MECMR was significantly elevated in both models (both P < 0.05 versus WT). The L-type calcium channel inhibitor diltiazem (5 mg/kg i.p.) significantly reduced MECMR in the mdx mice (P < 0.01), though only with a higher dose (10 mg/kg i.p.) in the Sgcd -/- mice (P < 0.05). As the Sgcd -/- mice had increased heart rates, to determine the role of heart rate in MECMR we studied the hyperpolarization-activated cyclic nucleotide-gated channel inhibitor ZD 7288 which selectively reduces heart rate. This reduced heart rate and MECMR in all mouse groups. However, when looking at the time course of reduction of MECMR in the Sgcd -/- mice at up to 5 minutes of the manganese infusion when heart rates were matched to the WT mice, MECMR was still significantly elevated in the Sgcd -/- mice (P < 0.01) indicating that heart rate alone could not account for all the increased MECMR.

CONCLUSIONS

Despite both mouse models exhibiting increased in-vivo calcium influx at an early stage in the development of the cardiomyopathy before left ventricular hypertrophy, there are distinct phenotypical differences between the 2 models in terms of heart rates, hemodynamics and responses to calcium channel inhibitors.

Baroreflex physiology studied in healthy subjects with very infrequent muscle sympathetic bursts

Because it is likely that, in healthy human subjects, baroreflex mechanisms operate continuously, independent of experimental interventions, we asked the question, In what ways might study of unprovoked, very infrequent muscle sympathetic bursts inform baroreflex physiology? We closely examined arterial pressure and R-R interval responses of 11 supine healthy young subjects to arterial pressure ramps triggered by large isolated muscle sympathetic bursts. We triggered data collection sweeps on the beginnings of sympathetic bursts and plotted changes of arterial pressure (finger volume clamp or intra-arterial) and R-R intervals occurring before as well as after the sympathetic triggers. We estimated baroreflex gain from regression of R-R intervals on systolic pressures after sympathetic bursts and from the transfer function between cross-spectra of systolic pressure and R-R intervals at low frequencies. Isolated muscle sympathetic bursts were preceded by arterial pressure reductions. Baroreflex gain, calculated with linear regression of R-R intervals on systolic pressures after bursts, was virtually identical to baroreflex gain, calculated with the cross-spectral modulus [mean and (range): 24 (7-43) vs. 24 (8-45) ms/mmHg], and highly significant, according to linear regression (r(2) = 0.91, P = 0.001). Our results indicate that 1) since infrequent human muscle sympathetic bursts are almost deterministically preceded by arterial pressure reductions, their occurrence likely reflects simple baroreflex physiology, and 2) the noninvasive low-frequency modulus reliably reproduces gains derived from R-R interval responses to arterial pressure ramps triggered by infrequent muscle sympathetic bursts.

Exogenous intravascular nitric oxide enhances ventricular function after hemodilution with plasma expander

AIMS

This study evaluated the hypothesis that exogenous nitric oxide (NO) supplementation during acute hemodilution with plasma expander (PE) provides beneficial effects on cardiac function.

MAIN METHODS

Acute hemodilution in golden Syrian hamsters was induced by a 40% of blood volume exchange with dextran 70 kDa. Intravascular NO supplementation after hemodilution was accomplished with a NO donor, diethylenetriamine NONOate (DETA NONOate). The test group was treated with DETA NONOate, while the control group received only vehicle. Left ventricular cardiac function was studied using pressure-volume measurements obtained with a miniaturized conductance catheter.

KEY FINDINGS

Cardiac output increased to 122±5% and 107±1% of the baseline in the group treated with NO donor and the vehicle group, respectively. Stroke work per stroke volume (SW/SV) after hemodilution reduced to 90% of the baseline and the NO donor significantly reduced SW/SV compared to the vehicle. The minimum rate of pressure change (dP/dt(min)) was significantly lower in animals treated with the NO donor compared to vehicle treated animals. Systemic vascular resistance (SVR) decreased to 62±5% of the baseline in the NO donor group whereas the vehicle group SVR decreased to 83±5% of the baseline. Using intravital microscopy analysis of microvessel in the dorsal skinfold window chamber, we established that the NO donor group induced significant vasodilation compared to the vehicle group.

SIGNIFICANCE

NO supplementation in an acute hemodilution with PE has beneficial effects on cardiac performance. However, the NO supplementation effects with a NO donor are dose-independent and short-lasting.

A comparison of pharmacologic and spontaneous baroreflex methods in aging and hypertension

BACKGROUND

Phenylephrine bolus injection is an established technique to measure baroreflex sensitivity (BRS). This study quantified the relationship between the phenylephrine method and noninvasive measures of BRS and examined the effects of aging and hypertension on BRS. We also examined whether heart rate variability (HRV) provides as much information as does BRS.

METHODS

BRS was determined by phenylephrine bolus (BRSphe), amyl nitrite inhalation (BRSamyl), Valsalva maneuver (BRSVals) and by time (BRS(+)) and spectral domain analysis (BRS(LFalpha), 004-015 Hz) of spontaneous blood pressure and R-R interval changes over the 5-min time period.

RESULTS

The phenylephrine method significantly correlated with other methods (BRS(LFalpha) R = 0.54, BRS(+) R = 0.55, BRSVals R = 0.43 and BRSamyl R = 0.39; P < or = 0.001). Each method underestimated the BRSphe by the factors 0.62, 0.64, 0.59 and 0.33, respectively; P value less than 0.001. Only BRS(LFalpha) was significantly different between normotensive and hypertensive patients in young [24.3 +/- 1.4 (n = 40) vs. 12.2 +/- 2.3 (n = 7)] and middle-aged [16.5 +/- 1.1 (n = 71) vs. 10.8 +/- 1.1 (n = 31) groups, respectively]. HRV in the high frequency band (0.15-0.40 Hz) was significantly lower in young hypertensive patients than in normal controls (26 +/- 6.0 vs. 50 +/- 2.4, P < 0.05).

CONCLUSION

Although all methods correlated with the phenylephrine technique, none of them could be used interchangeably with that technique. BRS(LFalpha) detected the baroreflex loss of hypertension most clearly, and BRSamyl did not differ among groups.

Boveris A, T. Arranz C, Balaszczuk AM. Cardiac mitochondrial nitric oxide: a regulator of heart rate?

Alterations in autonomic control and myocardial nitric-oxide (NO) production are likely linked to the development and progression of heart dysfunction. By focusing on heart rate, the complexity of the actions of NO at distinct levels throughout the autonomic nervous system and its relationship with other regulators can be demonstrated. Given the multiple and opposing actions of NO on cardiac control, it is difficult to interpret a response after a global intervention in the NO system. The diversity of intracellular pathways activated by NO, and their differing sensitivities to different levels of NO, might account for some aspects of reported specific but opposite effects. We discuss factors that might contribute to this diversity of actions. A proper elucidation of the effects of NO on metabolic pathways and on energy generation could lead to novel therapeutic strategies aimed at the early treatment of heart dysfunction.

Mechanical and neural contributions to hysteresis in the cardiac vagal limb of the arterial baroreflex

According to conventional wisdom, hysteresis in cardiac vagal baroreflex function exhibits a specific pattern: pressure falls are associated with longer heart periods and a smaller linear gain. A similar pattern occurs in the pressure-diameter relationship of barosensory vessels, and therefore it has been suggested that baroreflex hysteresis derives solely from vascular behaviour. However, we hypothesized that mechanical and neural baroreflex components contribute equally to baroreflex hysteresis. Blood pressure, carotid diameter and the electrocardiogram were recorded continuously during two trials of sequential bolus injections of nitroprusside and phenylephrine in 14 young healthy subjects. Baroreflex gain and its mechanical and neural components were estimated for falls and rises in pressure and diameter. The position or set point of the relations was quantified at the mean pressure and mean diameter. Gains were determined via piecewise linear regression. Set points and gains for falls versus rises in pressure and diameter were compared with the Chow test. Hysteresis was observed in all individuals, but not in every trial. In most, but not all, trials pressure falls were associated with longer heart periods and smaller linear gain, as conventional wisdom would predict. However, the pattern of hysteresis derived from the interaction of both mechanical and neural components. The two components most often acted in opposition to determine differences in set point, but in conjunction to determine differences in baroreflex gain. Therefore, we conclude that hysteresis is not solely determined by barosensory vessel behaviour but by the complex interaction of mechanical and neural aspects of the arterial baroreflex.

NO Differentially Regulates Neurotransmission to Premotor Cardiac Vagal Neurons in the Nucleus Ambiguus

NO is involved in the neural control of heart rate, and NO synthase expressing neurons and terminals have been localized in the nucleus ambiguus where parasympathetic cardiac vagal preganglionic neurons are located; however, little is known about the mechanisms by which NO alters the activity of premotor cardiac vagal neurons. This study examines whether the NO donor sodium nitroprusside ([SNP] 100 μmol/L) and precursor, l -arginine (10 mmol/L), modulate excitatory and inhibitory synaptic neurotransmission to cardiac vagal preganglionic neurons. Glutamatergic, GABAergic, and glycinergic activity to cardiac vagal neurons was examined using whole-cell patch-clamp recordings in an in vitro brain slice preparation in rats. Both SNP, as well as l -arginine, increased the frequency of GABAergic neurotransmission to cardiac vagal preganglionic neurons but decreased the amplitude of GABAergic inhibitory postsynaptic currents. In contrast, both l -arginine and SNP inhibited the frequency of glutamatergic and glycinergic synaptic events in cardiac vagal preganglionic neurons. SNP and l -arginine also decreased glycinergic inhibitory postsynaptic current amplitude, and this response persisted in the presence of tetrodotoxin. Inclusion of the NO synthase inhibitor 7-nitroindazole (100 μmol/L) prevented the l -arginine–evoked responses. These results demonstrate that NO differentially regulates excitatory and inhibitory neurotransmission, facilitating GABAergic and diminishing glutamatergic and glycinergic neurotransmission to cardiac vagal neurons.

Autonomic regulation of pacemaker activity: role of heart nitric oxide synthases

In autonomic-blocked rats treated with NG-nitro-L-arginine methyl ester (L-NAME, 7.5 mg/kg), heart rate increased 18% and mean arterial pressure increased 48%. Thyroidectomy, along with autonomic blockade, hampered the chronotropic response but did not modify the effect on blood pressure. After 150 min of autonomic blockade, the experimental end point, total nitric oxide (NO) production by heart NO synthases (NOS) decreased 61%: from 54 to 21 nmol NO.min-1.g heart-1. Mitochondrial NOS (mtNOS) and sarcoplasmic reticulum endothelial NOS activities decreased 74% and 52%, respectively. Mitochondria isolated from whole heart showed a well-coupled oxidative phosphorylation with high respiratory control and ADP-to-O ratios, decreased mtNOS activity (55-60%), and decreased mtNOS protein expression (70%). Immunohistochemistry with anti-inducible NOS antibody linked to gold particles localized mtNOS at the inner mitochondrial membranes. Histochemical right atrial NOS (NADPH-diaphorase) decreased 55% after heart denervation. The effects of autonomic denervation on the NO system were partially prevented by thyroidectomy performed simultaneously with autonomic blockade. Western blot analysis indicated a very rapid mtNOS protein turnover (half time=120 min) with a process of protein expression that was upregulated by thyroidectomy and a degradation process that was downregulated by the autonomic nervous system. The observations suggest that NO-mediated pathways contribute to pacemaker heart activity, likely through the NO steady-state levels in the right atrium and the whole heart.

Development of baroreflex function and hind limb vascular reactivity in the horse fetus

This study investigated, in vivo, the mechanisms underlying the development of cardiovascular function in the horse fetus, with particular relevance to baroreflex function and hind limb vascular arterial reactivity to constrictor agonists. Under general anaesthesia, vascular catheters were inserted and a Transonic flow probe was implanted around one of the metatarsal arteries of 13 horse fetuses, either at 0.6 of gestation (n= 6) or at 0.9 of gestation (n= 7, term approximately 335 days). At least 5 days after surgery, pressor, vasoconstrictor and cardiac chronotropic responses to exogenous bolus doses of phenylephrine, angiotensin II and arginine vasopressin were recorded. Fetal cardiac baroreflex slopes were obtained using the peak pressor and heart rate responses to increasing doses of phenylephrine. Fetal treatment with phenylephrine, angiotensin II and vasopressin produced significant changes in arterial blood pressure, hind limb vascular resistance and heart rate. Pressor and vasopressor responses to all agonists were greater at 0.9 than at 0.6 of gestation; however, fetal cardiac baroreflex sensitivity decreased with advancing gestational age. Correlation analysis revealed that fetal plasma cortisol rather than gestational age was a greater determinant of pressor and vasopressor reactivity. In contrast, gestational age rather than cortisol better determined heart rate and baroreflex responsiveness in the equine fetus. The data show that development of cardiovascular function in the equine fetus occurs via cortisol-dependent and -independent pathways.

Relating drug-induced changes in carotid artery mechanics to cardiovagal and sympathetic baroreflex control

Previous evidence indicates that sensitivity of the baroreflex cardiovagal and sympathetic arms is dissociated. In addition, pharmacologic assessment of baroreflex sensitivity (BRS) has revealed that cardiovagal, but not sympathetic, BRS is greater when blood pressure is increasing versus falling. The origin of this hysteresis is unknown. In this study, carotid artery distensibility and absolute distension (diameter) were assessed to test the hypothesis that vessel mechanics in barosensitive regions affect the BRS of cardiovagal, but not sympathetic, outflow. R-R interval (i.e. time between successive R waves), finger arterial blood pressure, muscle sympathetic nerve activity, and carotid artery dimensions (B-mode imaging) were measured during sequential infusions of sodium nitroprusside (SNP) and phenylephrine (PHE). Systolic and diastolic common carotid artery diameters and pulse pressure were recorded to calculate distensibility of this vessel under each drug condition. Cardiovagal BRS was greater when blood pressure was increasing versus decreasing (p < 0.01). Sympathetic BRS was not affected by direction of pressure change. Distensibility did not differ between SNP and PHE injections. However, compared with SNP, infusion of PHE resulted in larger absolute systolic and diastolic carotid diameters (p < 0.001). Therefore, cardiovagal reflex hysteresis was related to drug-induced changes in common carotid artery diameter but not distensibility. The lack of sympathetic hysteresis in this model suggests a relative insensitivity of this baroreflex component to carotid artery dimensions and provides a possible mechanism for the dissociation between cardiovagal and sympathetic BRS.

Development of baroreflex and endocrine responses to hypotensive stress in newborn foals and lambs

OBJECTIVE

The aims of this study were to compare and contrast the development of the cardiac baroreflex and endocrine responses to acute hypotensive stress in healthy newborn pony foals and lambs during the first two weeks of postnatal life.

METHODS

Under general anaesthesia, seven Welsh pony foals and six Welsh Mountain lambs were catheterised with hind limb artery and vein catheters. Following post-surgical recovery, at 1 week and 2 weeks of age, blood pressures of the animals were raised and lowered acutely by intravenous infusion of phenylephrine and sodium nitroprusside, respectively. During hypotension, blood samples were taken for measurement of plasma hormones associated with activation of the stress axis.

RESULTS

Basal arterial blood pressure increased significantly (P<0.05) between week 1 and week 2 in the absence of any significant change in basal heart rate in foals and with a significant reduction in basal heart rate in lambs. In foals, the slope of the heart rate-blood pressure relationship decreased in response to acute hypertension, and it increased in response to acute hypotension, from week 1 to week 2 (all P<0.05). In contrast, in lambs, the slope of the heart rate-blood pressure relationship decreased with both acute hypertension and acute hypotension from week 1 to week 2 (all P<0.05). In foals, there were significant increases in plasma concentrations of noradrenaline, neuropeptide Y (NPY), vasopressin, adrenocorticotrophic hormone (ACTH) and cortisol in response to hypotension (P<0.05). In lambs, there were also significant increases in plasma concentrations of ACTH and cortisol during hypotension. Plasma concentrations of noradrenaline, NPY and vasopressin were not measured during hypotension in lambs. In foals, although the magnitude of the ACTH response to hypotension was smaller at week 2 than week 1, the increment in plasma cortisol was similar in the two age groups. In contrast, in lambs, the profile of both the ACTH and cortisol responses was similar at week 1 and week 2.

CONCLUSION

These data suggest that the increase in basal arterial blood pressure in the foal and the lamb during the first 2 weeks of postnatal life is accompanied by differential maturational changes in the vagal and sympathetic components of the cardiac baroreflex between the two species. These developmental cardiac baroreflex changes occur together with increased adrenocortical responsiveness to acute hypotensive stress, which appears comparatively more mature in lambs than in foals.

Nitric oxide control of cardiac function: is neuronal nitric oxide synthase a key component?

Nitric oxide (NO) has been shown to regulate cardiac function, both in physiological conditions and in disease states. However, several aspects of NO signalling in the myocardium remain poorly understood. It is becoming increasingly apparent that the disparate functions ascribed to NO result from its generation by different isoforms of the NO synthase (NOS) enzyme, the varying subcellular localization and regulation of NOS isoforms and their effector proteins. Some apparently contrasting findings may have arisen from the use of non-isoform-specific inhibitors of NOS, and from the assumption that NO donors may be able to mimic the actions of endogenously produced NO. In recent years an at least partial explanation for some of the disagreements, although by no means all, may be found from studies that have focused on the role of the neuronal NOS (nNOS) isoform. These data have shown a key role for nNOS in the control of basal and adrenergically stimulated cardiac contractility and in the autonomic control of heart rate. Whether or not the role of nNOS carries implications for cardiovascular disease remains an intriguing possibility requiring future study.

Nitric oxide synthase inhibition does not affect regulation of muscle sympathetic nerve activity during head-up tilt

To test the hypothesis that systemic inhibition of nitric oxide (NO) synthase does not alter the regulation of sympathetic outflow during head-up tilt in humans, in eight healthy subjects NO synthase was blocked by intravenous infusion of NG-monomethyl-L-arginine (L-NMMA). Blood pressure, heart rate, cardiac output, total peripheral resistance (TPR), and muscle sympathetic nerve activity (MSNA) were recorded in the supine position and during 60 degrees head-up tilt. In the supine position, infusion of L-NMMA increased blood pressure, via increased TPR, and inhibited MSNA. However, the increase in MSNA evoked by head-up tilt during L-NMMA infusion (change in burst rate: 24 +/- 4 bursts/min; change in total activity: 209 +/- 36 U/min) was similar to that during head-up tilt without L-NMMA (change in burst rate: 23 +/- 4 bursts/min; change in total activity: 251 +/- 52 U/min, n = 6, all P > 0.05). Moreover, changes in TPR and heart rate during head-up tilt were virtually identical between the two conditions. These results suggest that systemic inhibition of NO synthase with L-NMMA does not affect the regulation of sympathetic outflow and vascular resistance during head-up tilt in humans.

Spontaneous indices are inconsistent with arterial baroreflex gain

Spontaneously occurring, parallel fluctuations in arterial pressure and heart period are frequently used as indices of baroreflex function. Despite the convenience of spontaneous indices, their relation to the arterial baroreflex remains unclear. Therefore, in 97 volunteers, we derived 5 proposed indices (sequence method, alpha-index, transfer function, low-frequency transfer function, and impulse response function), compared them with arterial baroreflex gain (by the modified Oxford pharmacologic technique), and examined their relation to carotid distensibility and respiratory sinus arrhythmia. The subjects comprised men and women (n=41) aged 25 to 86 years, 30% of whom had established coronary artery disease. Generally, the indices were correlated with each other (except alpha-index and low-frequency transfer function) and with baroreflex gain. However, the Bland-Altman method demonstrated that the spontaneous indices had limits of agreement as large as the baroreflex gain itself. Even in individuals within the lowest tertile of baroreflex gain for whom baroreflex gain appears to be the most clinically relevant, spontaneous indices failed to relate to baroreflex gain. In fact, for these individuals, there was no correlation between any index and baroreflex gain. Forward stepwise linear regression showed that all spontaneous indices and baroreflex gain were related to respiratory sinus arrhythmia, but only baroreflex gain was related to carotid distensibility. Therefore, these data suggest that spontaneous indices are inadequate estimates of gain and are inconsistent with arterial baroreflex function.

Chronic blockade of nitric oxide does not produce hypertension in baroreceptor denervated rabbits

Although the vascular action of endothelium-derived nitric oxide in modulating arterial pressure is well established, nitric oxide can also act as a neurotransmitter in the central nervous system. In addition, there is evidence for an interaction between nitric oxide and baroreceptor afferent processing; thus, nitric oxide may regulate blood pressure through central modulation of arterial baroreflexes. To test this possible interaction of nitric oxide and baroreflexes in the long-term regulation of blood pressure, we measured arterial pressure and heart rate responses to nitric oxide blockade by using L-NAME (50 mg/kg per day in drinking water) over 7 days in baroreceptor intact and sinoaortic denervated conscious rabbits. In the baroreceptor intact animals, blockade of nitric oxide leads to a significant increase in mean arterial pressure (from 75+/-2 to 84+/-3 mm Hg) and decrease in heart rate (from 233+/-8 to 195+/-8 bpm) that was sustained over the 7 days of nitric oxide blockade. In the sinoaortic denervated animals, blockade of nitric oxide initially led to a similar increase in arterial pressure (82+/-3 mm Hg on the second day), but in all sinoaortic denervated animals this increase was not sustained and recovered back to pre-L-NAME levels. This finding indicates that baroreflexes play an important role in the long-term control of blood pressure, and, second, that one mediator of this control is nitric oxide.

Asymmetric dimethylarginine causes hypertension and cardiac dysfunction in humans and is actively metabolized by dimethylarginine dimethylaminohydrolase

OBJECTIVE

Plasma levels of an endogenous nitric oxide (NO) synthase inhibitor, asymmetric dimethylarginine (ADMA), are elevated in chronic renal failure, hypertension, and chronic heart failure. In patients with renal failure, plasma ADMA levels are an independent correlate of left ventricular ejection fraction. However, the cardiovascular effects of a systemic increase in ADMA in humans are not known.

METHODS AND RESULTS

In a randomized, double-blind, placebo-controlled study in 12 healthy male volunteers, we compared the effects of intravenous low-dose ADMA and placebo on heart rate, blood pressure, cardiac output, and systemic vascular resistance at rest and during exercise. We also tested the hypothesis that ADMA is metabolized in humans in vivo by dimethylarginine dimethylaminohydrolase (DDAH) enzymes. Low-dose ADMA reduced heart rate by 9.2+/-1.4% from 58.9+/-2.0 bpm (P<0.001) and cardiac output by 14.8+/-1.2% from 4.4+/-0.3 L/min (P<0.001). ADMA also increased mean blood pressure by 6.0+/-1.2% from 88.6+/-3.4 mm Hg (P<0.005) and SVR by 23.7+/-2.1% from 1639.0+/-91.6 dyne. s. cm-5 (P<0.001). Handgrip exercise increased cardiac output in control subjects by 96.8+/-23.3%, but in subjects given ADMA, cardiac output increased by only 35.3+/-10.6% (P<0.05). DDAHs metabolize ADMA to citrulline and dimethylamine. Urinary dimethylamine to creatinine ratios significantly increased from 1.26+/-0.32 to 2.73+/-0.59 after ADMA injection (P<0.01). We estimate that humans generate approximately 300 micromol of ADMA per day, of which approximately 250 micromol is metabolized by DDAHs.

CONCLUSIONS

This study defines the cardiovascular effects of a systemic increase in ADMA in humans. These are similar to changes seen in diseases associated with ADMA accumulation. Finally, our data also indicate that ADMA is metabolized by DDAHs extensively in humans in vivo.

New insights into differential baroreflex control of heart rate in humans

Recent data indicate that bilateral carotid sinus denervation in patients results in a chronic impairment in the rapid reflex control of blood pressure during orthostasis. These findings are inconsistent with previous human experimental investigations indicating a minimal role for the carotid baroreceptor-cardiac reflex in blood pressure control. Therefore, we reexamined arterial baroreflex [carotid (CBR) and aortic baroreflex (ABR)] control of heart rate (HR) using newly developed methodologies. In 10 healthy men, 27 +/- 1 yr old, an abrupt decrease in mean arterial pressure (MAP) was induced nonpharmacologically by releasing a unilateral arterial thigh cuff (300 Torr) after 9 min of resting leg ischemia under two conditions: 1) ABR and CBR deactivation (control) and 2) ABR deactivation. Under control conditions, cuff release decreased MAP by 13 +/- 1 mmHg, whereas HR increased 11 +/- 2 beats/min. During ABR deactivation, neck suction was gradually applied to maintain carotid sinus transmural pressure during the initial 20 s after cuff release (suction). This attenuated the increase in HR (6 +/- 1 beats/min) and caused a greater decrease in MAP (18 +/- 2 mmHg, P < 0.05). Furthermore, estimated cardiac baroreflex responsiveness (DeltaHR/DeltaMAP) was significantly reduced during suction compared with control conditions. These findings suggest that the carotid baroreceptors contribute more importantly to the reflex control of HR than previously reported in healthy individuals.

Nitric oxide and autonomic control of heart rate: a question of specificity

Despite its highly diffusible nature, the gaseous signalling molecule nitric oxide (NO) can exert specific effects within the CNS and PNS. To date, the specificity of the actions of NO remains an unsolved puzzle. There are several plausible mechanisms that might account for this specificity in the context of autonomic regulation of heart rate. NO acts at distinct levels within the autonomic nervous system to control cardiac rate, with opposing effects at different sites. We discuss factors that might contribute to this diversity of action, and conclude that the isoform of enzyme involved in producing NO, the spatial proximity of the NO source to the target, and differences in the intracellular coupling within the target cell are all crucial for encoding the functional action of NO.

Cyclosporine attenuates the autonomic modulation of reflex chronotropic responses in conscious rats

Cyclosporine A (CyA), an immunosuppressant drug, has been shown to attenuate the baroreflex control of heart rate (HR). This study investigated whether or not the CyA-induced baroreflex dysfunction is due to alterations in the autonomic (sympathetic and parasympathetic) control of the heart. We evaluated the effect of muscarinic or beta-adrenergic blockade by atropine and propranolol, respectively, on reflex HR responses in conscious rats treated with CyA (20 mg·kg–1·day–1 dissolved in sesame oil) for 11–13 days or the vehicle. Baroreflex curves relating changes in HR to increases or decreases in blood pressure (BP) evoked by phenylephrine (PE) and sodium nitroprusside (NP), respectively, were constructed and the slopes of the curves were taken as a measure of baroreflex sensitivity (BRSPE and BRSNP). Intravenous administration of PE and NP produced dose-related increases and decreases in BP, respectively, that were associated with reciprocal changes in HR. CyA caused significant (P < 0.05) reductions in reflex HR responses as indicated by the smaller BRSPE (–0.97 ± 0.07 versus –1.47 ± 0.10 beats·min–1·mmHg–1 (1 mmHg = 133.322 Pa)) and BRSNP (–2.49 ± 0.29 versus –5.23 ± 0.42 beats·min–1·mmHg–1) in CyA-treated versus control rats. Vagal withdrawal evoked by muscarinic blockade elicited significantly lesser attenuation of BRSPE in CyA compared with control rats (40.2 ± 8.0 versus 57.7 ± 4.4%) and abolished the BRSPE difference between the two groups, suggesting that CyA reduces vagal activity. CyA also appears to impair cardiac sympathetic control because blockade of beta-adrenergic receptors by propranolol was less effective in reducing reflex tachycardic responses in CyA compared with control rats (41.6 ± 4.2 versus 59.5 ± 4.5%). These findings confirm earlier reports that CyA attenuates the baroreceptor control of HR. More importantly, the study provides the first pharmacological evidence that CyA atten uates reflex chronotropic responses via impairment of the autonomic modulation of the baroreceptor neural pathways.Key words: cyclosporine A, baroreflex sensitivity, autonomic control, atropine, propranolol.

Chronotropic effects of nitric oxide in the denervated human heart

Nitric oxide synthase is expressed in the sino-atrial node and animal data suggests a direct role for nitric oxide on pacemaker activity. Study of this mechanism in intact humans is complicated by both reflex and direct effects of nitric oxide on cardiac autonomic control. Thus, we have studied the direct effects of nitric oxide on heart rate in human cardiac transplant recipients who possess a denervated donor heart. In nine patients, the chronotropic effects of systemic injection of the nitric oxide synthase inhibitor N(G)-monomethyl-L-arginine (L-NMMA) (3 mg kg(-1)) or increasing bolus doses of the nitric oxide donor, sodium nitroprusside (SNP), were studied. Injection of L-NMMA increased mean arterial pressure by 17 +/- 2 mmHg (mean +/- S.E.M.; P < 0.001) and also had a significant negative chronotropic effect, lengthening the R-R interval by 54 +/- 8 ms (P < 0.001). This bradycardia was not reflex in origin since injection of the non-NO-dependent vasoconstrictor, phenylephrine (100 microg) achieved a similar rise in mean arterial pressure (18 +/- 3 mmHg; P < 0.001) but failed to change R-R interval duration (Delta R-R = -3 +/- 4 ms). Furthermore, no change in levels of circulating adrenaline was observed with L-NMMA. Conversely, injection of sodium nitroprusside resulted in a positive chronotropic effect with a dose-dependent shortening of R-R interval duration, peak Delta R-R = -25 +/- 8 ms with 130 microg (P < 0.01). These findings indicate that nitric oxide exerts a tonic, direct, positive chronotropic influence on the denervated human heart. This is consistent with the results of animal experiments showing that nitric oxide exerts a facilitatory influence on pacemaking currents in the sino-atrial node.

Natriuretic peptides like NO facilitate cardiac vagal neurotransmission and bradycardia via a cGMP pathway

We tested the hypothesis that natriuretic peptide receptors (NPRs) that are coupled to cGMP production act in a similar way to nitric oxide (NO) by enhancing acetylcholine release and vagal-induced bradycardia. The effects of enzyme inhibitors and channel blockers on the action of atrial natriuretic peptide (ANP), brain-derived natriuretic peptide (BNP), and C-type natriuretic peptide (CNP) were evaluated in isolated guinea pig atrial-right vagal nerve preparations. RT-PCR confirmed the presence NPR B and A receptor mRNA in guinea pig sinoatrial node tissue. BNP and CNP significantly (P < 0.05) enhanced the heart rate (HR) response to vagal nerve stimulation. CNP had no effect on the HR response to carbamylcholine and facilitated the release of [(3)H]acetylcholine during atrial field stimulation. The particulate guanylyl cyclase-coupled receptor antagonist HS-142-1, the phosphodiesterase 3 inhibitor milrinone, the protein kinase A inhibitor H89, and the N-type calcium channel blocker omega-conotoxin all blocked the effect of CNP on vagal-induced bradycardia. Like NO, BNP and CNP facilitate vagal neurotransmission and bradycardia. This may occur via a cGMP-PDE3-dependent pathway increasing cAMP-PKA-dependent phosphorylation of presynaptic N-type calcium channels.

Nitric oxide modulates arterial baroreflex control of heart rate in conscious lambs in an age-dependent manner

Experiments were carried out in conscious chronically instrumented lambs aged 1 (n = 6) and 6 wk (n = 5) to evaluate the arterial baroreflex control of heart rate (HR) during postnatal maturation and to investigate any modulatory role of endogenously produced nitric oxide (NO). Before and after intravenous administration of 20 mg/kg of the L-arginine analog N(G)-nitro-L-arginine methyl ester (L-NAME), the arterial baroreflex was assessed by measuring HR responses to increases and decreases in systolic arterial pressure achieved by intravenous administration of phenylephrine and sodium nitroprusside. The HR range over which the baroreflex operates and minimum HR as well as maximum gain were greater at 1 than at 6 wk of age. These age differences were abolished in the presence of L-NAME, which decreased the HR range and gain of the arterial baroreflex control of HR at 1 but not at 6 wk of age. These data provide new information that age-dependent effects of the arterial baroreflex appear to result from effects of endogenously produced NO.

Role of cGMP-inhibited phosphodiesterase and sarcoplasmic calcium in mediating the increase in basal heart rate with nitric oxide donors

Nitric oxide (NO) donors increase heart rate (HR) through a guanylyl cyclase-dependent stimulation of the pacemaker current I(f), without affecting basal I(Ca-L). The activity of I(f)is known to be enhanced by cyclic nucleotides and by an increase in cytosolic Ca(2+). We examined the role of cGMP-dependent signaling pathways and intracellular Ca(2+)stores in mediating the positive chronotropic effect of NO donors. In isolated guinea pig atria, the increase in HR in response to 1-100 micromol/l 3-morpholino-sydnonimine (SIN-1; with superoxide dismutase, n=6) or diethylamine-NO (DEA-NO, n=8) was significantly attenuated by blockers of the cGMP-inhibited phosphodiesterase (PDE3; trequinsin, milrinone or Ro-13-6438, n=22). In addition, the rate response to DEA-NO or sodium nitroprusside (SNP) was significantly reduced following inhibition of PKA (KT5720 or H-89, n=15) but not PKG (KT5728 or Rp-8-pCPT-cGMPs, n=16). Suppression of sarcoplasmic (SR) Ca(2+)release by pretreatment of isolated atria with ryanodine or cyclopiazonic acid (2 micromol/l and 60 micromol/l, n=16) significantly reduced the chronotropic response to 1-100 micromol/l SIN-1 or DEA-NO. Moreover, in isolated guinea pig sinoatrial node cells 5 micromol/l SNP significantly increased diastolic and peak Ca(2+)fluorescence (+13+/-1% and +28+/-1%, n=6, P<0.05). Our findings are consistent with a functionally significant role of cAMP/PKA signaling (via cGMP inhibition of PDE3) and SR Ca(2+)in mediating the positive chronotropic effect of NO donors.

Daily exercise attenuated the sympathetic component of the spontaneous arterial baroreflex control of heart rate in hypertensive rats

The influence of daily spontaneous running on the sympathetic and parasympathetic components of the spontaneous arterial baroreflex control of heart rate was examined in 22 female spontaneously hypertensive rats [12 sedentary and 10 daily spontaneous running]. Following 8 weeks of sedentary control or daily spontaneous running, animals were chronically instrumented with an arterial catheter. Daily spontaneous running resulted in an increased heart weight/body weight ratio (5.2 +/- 0.27 vs 4.3 +/- 0.01 g/kg) and a resting bradycardia (321+/- 8 bpm vs 360 +/- 6). The spontaneous changes in arterial pressure and the reflex responses of heart rate were examined under three experimental conditions: 1) pre-blockade, 2) following beta1-adrenergic receptor blockade, and 3) following muscarinic-cholinergic receptor blockade. Daily spontaneous running attenuated the spontaneous gain of the arterial baroreflex control of heart rate (56%). After muscarinic-cholinergic receptor blockade, the spontaneous gain remained reduced in daily spontaneous running rats (57%). In contrast, after beta1-adrenergic receptor blockade the spontaneous gain was not different between sedentary control and daily spontaneous running animals. Results demonstrate that daily spontaneous running decreased the sympathetic component resulting in an apparently greater influence of the parasympathetic component on the spontaneous arterial baroreflex control of heart rate.

Effect of exogenous nitric oxide on baroreflex function in humans

Nitric oxide (NO) donors inhibit sympathetic neurotransmission and baroreceptor activity and can directly stimulate heart rate (HR) in vitro. To assess whether exogenous NO affects cardiovascular autonomic control in humans, we tested the baroreceptor-cardiac reflex [baroreflex sensitivity (BRS)] and the arterial blood pressure (BP) and HR variability during an infusion of the NO donor sodium nitroprusside (SNP, 2 micrograms . kg(-1). min(-1)) or 5% glucose in 16 healthy subjects. The hypotensive action of SNP was prevented by phenylephrine (PE, 0.9 +/- 0.15 micrograms . kg(-1). min(-1)). The SNP + PE infusion did not affect BRS or HR variability, but it caused a significant reduction in the diastolic and systolic BP low-frequency power. In addition, SNP + PE caused a sustained 12% increase in HR in the absence of changes in brachial and aortic BP. In conclusion, SNP had no effect on the cardiac-vagal limb of the baroreflex in humans but caused a substantial reduction in BP low-frequency power consistent with a decreased baroreflex/sympathetic control of peripheral resistance. The increase in HR in the absence of baroreceptor downloading confirms our previous finding of a direct positive chronotropic effect of NO donors.