Human cardiovascular neuroscience, in its various forms is the research interest of the laboratory. Scientists in the laboratory work on four research themes:
- The treatment of severe, drug-resistant hypertension.
- Orthostatic intolerance syndromes, with postural fainting.
- The links between stress, psychological illness and heart disease.
The principal laboratory focus is the sympathetic nervous system, in health and disease. The laboratory scientists are eminent in this field.
Evaluation of a device-based treatment of severe hypertension, specifically the application of a radio frequency renal artery catheter to ablate the renal sympathetic nerves.
Orthostatic intolerance syndromes
Delineating the diverse impaired neural mechanisms leading to low blood pressure and fainting on standing.
Research on the consequences and treatment of obesity.
Psychogenic cardiovascular disease
Delineating the neural mechanisms by which mental stress and psychological illness lead to hypertension and heart disease.
The impact of overweight and obesity in young adults
Excess weight is established as a major risk factor for cardiovascular diseases (CVD) particularly in young individuals. To get a better understanding of the pathophysiology underlying increased CVD risk, we evaluated early signs of organ damage and their possible relationship to the sympathetic nervous activity (SNA). We comprehensively assessed subclinical target organ damage including:
- Assessment of renal function.
- Left ventricular structure and systolic and diastolic function.
- Endothelial function.
Muscle SNA (MSNA) was assessed by microneurography. Participants with excess weight had decreased endothelial function, elevated creatinine clearance, increased left ventricular mass index and left ventricular wall thickness, lower systolic and diastolic function and elevated MSNA compared to lean individuals. In multiple regression analysis, endothelial function was inversely related to MSNA, while creatinine clearance and left ventricular mass index were positively related to MSNA, after adjustment for BMI, sex and blood pressure.
This study demonstrated that excess weight in young individuals is associated with subclinical alterations in renal, endothelial function as well as in the structure of the heart, even in the absence of hypertension. Elevated sympathetic activity appears to be a major driver of cardiovascular and renal alterations observed in these subjects. The consequences of elevated SNA may, in part, explain the increased CVD risk in young individuals with excess weight. Ongoing studies are focused on the targeted reduction of the SNS in order to reverse the march towards clinical end organ disease.
Sympathetic neural responsiveness in metabolic syndrome obesity: inter-relationships with insulin resistance and cortisol
Leader: Dr Nora Straznicky
Metabolic syndrome (MetS) obesity is an increasingly prevalent multidimensional risk factor for type 2 diabetes and cardiovascular disease. Neuroadrenergic abnormalities, comprising increased resting sympathetic nervous system (SNS) activity and blunted sympathetic neural responsiveness are recognised features of this condition which contribute importantly to both its pathophysiology and adverse clinical prognosis. The thermic effect of food is defined as the increase in resting metabolic rate after ingestion of a meal. In healthy lean persons approximately one third of the thermic effect of food is mediated by the meal-induced increment in SNS activity (facultative thermogenesis) and accounts for 3-4% (50-120 calories per day) of daily energy expenditure. Blunted nutritional sympathetic and thermogenic responsiveness are recognised characteristics of MetS obesity which may play a role in the development and/or maintenance of the obese state.
In this project we are examining whether treatments that specifically target peripheral and central insulin resistance, enhance sympathetic and thermogenic responsiveness in this clinical setting. Efforts to delineate the mechanisms of sympathetic dysfunction and its therapeutic modification are relevant to the prevention of type 2 diabetes and cardiovascular disease in this high risk obese phenotype. Our overarching hypothesis is that improvement in insulin sensitivity in peripheral tissues and the brain will be accompanied by a reversal of blunted nutritional SNS responsiveness and increased facultative thermogenesis in obese insulin resistant MetS subjects.
Depression and the heart — mechanisms of cardiac risk
Group leader: Associate Professor David Barton
There is strong evidence that patients with major depressive disorder (MDD) are at increased risk of developing coronary heart disease. While the mechanism of increased cardiac risk attributable to MDD at this stage is not known, recent work by us, and others, implicates brain serotonin, serotonin transporter genotype (5-HTT) and the sympathetic nervous system as possible candidates involved in generating increased risk.
Using high internal jugular vein blood sampling we demonstrated that brain serotonin turnover was elevated in patients with MDD and was influenced by the 5-HTT genotype, with carriage of the short allele being associated with an over 2-fold increase in brain serotonin turnover. In parallel, using direct cardiac catheterisation techniques we also showed that whole body and cardiac sympathetic nervous activity in patients with MDD follows a bimodal distribution, with values in some patients being extraordinarily high and others being marginally lower than those found in healthy subjects. Further analysis indicated a link between carriage of the short allele of the 5-HTT gene and elevated sympathetic nervous activity in MDD. While at this stage MDD patients with high sympathetic activity do not present with any evidence of current clinical cardiac disease, we, and others, have provided a growing body of evidence linking elevated sympathetic activity and exaggerated sympathetic responses to stress to early stages of end organ dysfunction and markers of disease development. Of particular note is the association of chronic sympathetic nervous activation to:
- The development of left ventricular hypertrophy (LVH) and diastolic dysfunction.
- Abnormal blood pressure regulation.
- The development of insulin resistance.
In this project we will determine the physiological consequences of sympathetic activation in patients with MDD.
Understanding Fontan physiology
Since its conception in 1971, the Fontan procedure has been used as the final stage of palliation for all children born with congenital heart disease who could not be offered a two-ventricle repair. This procedure comprises the redirecting of the systemic veins to drain directly into the pulmonary arteries, without passing through the heart. While patient selection and improved surgical techniques have improved survival, Fontan patients do develop heart failure and left ventricular dysfunction. Heart transplantation is the ultimate and only definitive treatment for Fontan failure. The mechanisms underpinning Fontan failure remain poorly understood, thereby making therapy difficult to optimise.
In this study, conducted in collaboration with Yves d'Udekem and Michael Chang of the Royal Children's Hospital, we are examining sympathetic nervous system function in Fontan patients using direct nerve recording techniques. The study aims to determine the relationship between elevated sympathetic activity in Fontan patients and:
- The presence and degree of diastolic dysfunction.
- Endothelial function.
- Arterial compliance.
Knowledge of the mechanisms at play in Fontan patients may pave the way for the development of new therapies in order to improve morbidity and mortality in this vulnerable patient group.
Sympathetic activation in polycystic ovary disease — a target for therapy?
Polycystic ovary syndrome (PCOS) affects approximately 10% of women of reproductive age, rendering it a major public health issue. Women suffering from PCOS not only experience despair and dejection, but they are also at increased risk of developing cardiovascular disease through hyperinsulinemia, insulin resistance and abdominal obesity, which are metabolic abnormalities often observed in the syndrome. The etiology of PCOS remains unknown, although the metabolic and hemodynamic profile suggests some involvement of the sympathetic nervous system.
We aim to elucidate the role of the sympathetic nervous system in the etiology of PCOS, and to examine the effect of the centrally-acting sympatholytic, moxonidine, on sympathetic nervous activation and the downstream metabolic abnormalities that are regularly observed in women with PCOS. We anticipate that moxonidine treatment will decrease sympathetic nervous system activity and have a beneficial effect on downstream metabolic abnormalities. We will achieve this by conducting a double-blind, placebo-controlled, cross-over trial in 25 women of reproductive age who suffer from PCOS. Treatment will be for a duration of three months, with a two-week wash-out period between phases. Sympathetic activity will be assessed using the technique of muscle sympathetic nerve activity, and we will also examine anthropometric measures, endothelial function as assessed by EndoPat, ambulatory blood pressure and glucose tolerance test; all downstream abnormalities that can result from elevated activation of the sympathetic nervous system.
Associate Professor David Barton
Dr Arup Dhar
Dr Ling Guo
Carolina Ika Sari
Dr Nora Straznicky
Dr Susan Corcoran