Neil Herring

Nitric oxide‐cGMP pathway facilitates acetylcholine release and bradycardia during vagal nerve stimulation in the guinea‐pig in vitro

1 We tested the hypothesis that nitric oxide (NO) augments vagal neurotransmission and bradycardia via phosphorylation of presynaptic calcium channels to increase vesicular release of acetylcholine. 2 The effects of enzyme inhibitors and calcium channel blockers on the actions of the NO donor sodium nitroprusside (SNP) were evaluated in isolated guinea‐pig atrial‐right vagal nerve preparations. 3 SNP (10 μm) augmented the heart rate response to vagal nerve stimulation but not to the acetylcholine analogue carbamylcholine (100 nm). SNP also increased the release of [3H]acetylcholine in response to field stimulation. No effect of SNP was observed on either the release of [3H] acetylcholine or the HR response to vagal nerve stimulation in the presence of the guanylyl cyclase inhibitor 1H‐(1,2,4)‐oxadiazolo‐(4,3‐a)‐quinoxalin‐1‐one (ODQ, 10 μm). 4 The phosphodiesterase 3 (PDE 3) inhibitor milrinone (1 μm) increased the release of [3H] acetylcholine and the vagal bradycardia and prevented any further increase by SNP. SNP was still able to augment the vagal bradycardia in the presence of the protein kinase G inhibitor KT5823 (1 μm) but not after protein kinase A (PKA) inhibition with H‐89 (0.5 μm) or KT5720 (1 μm) had reduced the HR response to vagal nerve stimulation. Neither milrinone nor H‐89 changed the HR response to carbamylcholine. 5 SNP had no effect on the magnitude of the vagal bradycardia after inhibition of N‐type calcium channels with ω‐conotoxin GVIA (100 nm). 6 These results suggests that NO acts presynaptically to facilitate vagal neurotransmission via a cGMP‐PDE 3‐dependent pathway leading to an increase in cAMP‐PKA‐dependent phosphorylation of presynaptic N‐type calcium channels. This pathway may augment the HR response to vagal nerve stimulation by increasing presynaptic calcium influx and vesicular release of acetylcholine.

Valvular heart disease and the use of cabergoline for the treatment of prolactinoma

Objective  The use of high doses of the ergot‐derived dopamine agonist cabergoline (> 3 mg/day), especially with cumulative doses > 4000 mg, has been associated with an increase in cardiac valvular thickening and significant (moderate to severe) regurgitation. Whether lower doses commonly used in the treatment of prolactinomas (0·25–3 mg/week) are also associated with significant valvulopathy is controversial. The mitral valve tenting area, a subclinical index of leaflet stiffening, has also been correlated with the cumulative dose of cabergoline and severity of valvular regurgitation.

Neuromodulators of peripheral cardiac sympatho-vagal balance.

The traditional model of efferent cardiac noradrenaline and acetylcholine release being driven solely via brainstem integration of circulatory reflex afferent input needs to be modified in the light of the discovery of numerous local cardiac factors that impact on peripheral neuronal neurotransmitter release. These neuromodulators can be intrinsic to sympathetic ganglia or vagal neurons (such as neuronal nitric oxide synthase), act as cotransmitters between these neuronal populations (such as neuropeptide Y) or are released from the myocardium itself to act on neurons in a paracrine manner (such as natriuretic peptides). Both myocardial infarction and congestive heart failure are characterized by enhanced regulation of these neuromodulators. This review will focus on recent evidence that nitric oxide, natriuretic peptides and neuropeptide Y act by converging on neuronal cyclic nucleotide‐dependent pathways to alter the autonomic phenotype in both health and disease.

Cardiac resynchronization therapy delivered via a multipolar left ventricular lead is associated with reduced mortality and elimination of phrenic nerve stimulation: Long-term follow-up from a multicenter registry

Cardiac resynchronization therapy (CRT) using quadripolar left ventricular (LV) leads provides more pacing vectors compared to bipolar leads. This may avoid phrenic nerve stimulation (PNS) and allow optimal lead placement to maximize biventricular pacing. However, a long‐term improvement in patient outcome has yet to be demonstrated.

Translational neurocardiology: preclinical models and cardioneural integrative aspects

Neuronal elements distributed throughout the cardiac nervous system, from the level of the insular cortex to the intrinsic cardiac nervous system, are in constant communication with one another to ensure that cardiac output matches the dynamic process of regional blood flow demand. Neural elements in their various ‘levels’ become differentially recruited in the transduction of sensory inputs arising from the heart, major vessels, other visceral organs and somatic structures to optimize neuronal coordination of regional cardiac function. This White Paper will review the relevant aspects of the structural and functional organization for autonomic control of the heart in normal conditions, how these systems remodel/adapt during cardiac disease, and finally how such knowledge can be leveraged in the evolving realm of autonomic regulation therapy for cardiac therapeutics.

Prioritizing echocardiography in Staphylococcus aureus bacteraemia

OBJECTIVES Infective endocarditis (IE) is a severe complication in Staphylococcus aureus bacteraemia (SAB) and recent guidelines from the BSAC recommend all patients undergo echocardiography. We assessed the use of echocardiography at a major tertiary referral centre and sought to identify those patients most likely to have positive findings. METHODS We retrospectively evaluated all cases of SAB at Oxford University Hospitals NHS Trust between September 2006 and August 2011. RESULTS Three-hundred-and-six out of 668 patients with SAB underwent cardiac imaging on average 9.8 ± 1.3 days from the first culture. Thirty-one patients (10.1%) had echocardiographic evidence of IE. Risk factors for observing evidence of IE on scanning included the presence of prosthetic heart valves (32% versus 4%, P < 0.001) or cardiac rhythm management (CRM) devices (16% versus 3%, P < 0.004). On excluding patients with prosthetic valves or CRM devices from the analysis, no patient with a line-related bacteraemia and only one patient (an intravenous drug user) with no/mild regurgitation on transthoracic echocardiography had echo evidence of IE. CONCLUSIONS We propose that the use of scarce echocardiography resources could be prioritized. Patients with prosthetic heart valves or a CRM device should receive early cardiological input and transoesophageal echocardiography. In patients with a clearly defined line-related bacteraemia who do not have a prosthetic valve or CRM device or clinical features of IE, response to treatment could be closely monitored and imaging deferred. Patients without a line-related infection or prosthetic valve/device could receive a transthoracic echocardiogram as a screening tool.

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.

NO-cGMP pathway increases the hyperpolarisation-activated current, I(f), and heart rate during adrenergic stimulation.

OBJECTIVES The role of the nitric oxide (NO)-cGMP pathway in the autonomic modulation of cardiac pacemaking is controversial and may involve an interplay between the L-type calcium current, I(CaL), and the hyperpolarisation activated current, I(f). We tested the hypothesis that following adrenergic stimulation, the NO-cGMP pathway stimulates phosphodiesterase 2 (PDE2) to reduce cAMP dependent stimulation of I(f) and heart rate (HR). METHODS In the presence of norepinephrine (NE, 1 microM), the effects of the NO donor sodium nitroprusside (SNP) were evaluated in sinoatrial node (SAN)/atria preparations and isolated SAN cells from adult guinea pigs. RESULTS Contrary to our hypothesis, SNP (10 and 100 microM, n=5) or the membrane permeable cGMP analogue, 8Br-cGMP (0.5 mM, n=6) transiently increased HR by 5+/-1, 12+/-1 and 12+/-2 beats/min, respectively. The guanylyl cyclase inhibitor 1H-(1,2,4)-oxadiazolo-(4,3-a)-quinoxalin-1-one (ODQ, 10 microM, n=5) abolished the increase in HR to SNP (100 microM) as did the I(f) blockers caesium chloride (2 mM, n=7) and 4-(N-ethyl-N-phenylamino)-1,2-dimethyl-6-(methylamino)-pyrimidinium chloride (ZD7288, 1 microM, n=7). Addition of SNP (10 microM) also transiently increased I(f) in SAN cells (n=5). After inhibition of PDE2 with erythro-9-(2-hydroxy-3-nonyl)-adenine (EHNA, 10 microM, n=5), the increase in HR to SNP in the presence of NE was significantly augmented and maintained. RT-PCR analysis confirmed the presence of PDE2 in addition to cGMP inhibited PDE3 mRNA in central SAN tissue. CONCLUSIONS These results suggest that during adrenergic stimulation, activation of the NO-cGMP pathway does not decrease HR, but has a transient stimulatory effect that is I(f) dependent, and is limited in magnitude and duration by stimulation of PDE2.

Peripheral pre-synaptic pathway reduces the heart rate response to sympathetic activation following exercise training: role of NO

OBJECTIVES We tested the hypothesis that the attenuated heart rate (HR) response to sympathetic activation following swim training in the guinea pig (Cavia porcellus) results from a peripheral modulation of pacemaking by nitric oxide (NO). METHODS Nitric oxide synthase (NOS) inhibition on the increase in heart rate with sympathetic nerve stimulation (SNS) was investigated in the isolated guinea pig double atrial/right stellate ganglion preparation from exercise trained (6-weeks swimming, n=20) and sedentary animals (n=20). Western blot analysis for neuronal nitric oxide synthase (nNOS) was performed on the stellate ganglion from both groups. RESULTS Relative to the control group, the exercise group demonstrated typical exercise adaptations of increased ventricular weight/body weight ratio, enhanced skeletal muscle citrate synthase activity and higher concentrations of [3H]ouabain binding sites in both skeletal and cardiac tissue (P<0.05). The increase in heart rate (bpm) with SNS significantly decreased in the exercise group (n=16) compared to the sedentary group (n=16) from 30+/-5 to 17+/-3 bpm at 1 Hz; 67+/-7 to 47+/-4 bpm at 3 Hz; 85+/-9 to 63+/-4 bpm at 5 Hz and 101+/-9 to 78+/-5 bpm at 7 Hz stimulation (P<0.05). The increase in heart rate with cumulative doses (0.1-10 microM) or a single dose (0.1 microM) of bath-applied norepinephrine expressed as the effective doses at which the HR response was 50% of the maximum response (EC50) were similar in both exercise (EC50 -6.08+/-0.16 M, n=8) and sedentary groups (EC50 -6.18+/-0.07 M, n=7). Trained animals had significantly more nNOS protein in left stellate ganglion compared to the sedentary group. In the exercise group, the non-isoform selective NOS inhibitor, N-omega nitro-L-arginine (L-NA, 100 microM) caused a small but significant increase in the heart rate response to SNS. However, the positive chronotropic response to sympathetic nerve stimulation remained significantly attenuated in the exercise group compared to the sedentary group during NOS inhibition (P<0.05). CONCLUSIONS Our results indicate that there is a significant peripheral pre-synaptic component reducing the HR response to sympathetic activation following training, although NO does not play a dominant role in this response.

Autonomic control of the heart: going beyond the classical neurotransmitters

What is the topic of this review? This symposium report discusses the evidence for release of cardiac sympathetic cotransmitters in addition to noradrenaline. What advances does it highlight? It highlights the potential role of neuropeptide Y in reducing vagal neurotransmission and directly influencing ventricular myocyte excitability in the presence of β‐receptor blockade.