PLASMA NORADRENALINE LEVELS AND CARDIOVASCULAR SYMPATHETIC ACTIVITY A thesis presented for the degree of DOCTOR OF MEDICINE in the Faculty of Medicine of the University of London by Anthony James Ivor Scriven BSc (King's) MB BS (Westminster) MRCP Work carried out at the Royal Postgraduate Medical School, London W12 and the Centre Hospitallier Universitaire Henri-Mondor, Paris 1 ABSTRACT The validity of plasma noradrenaline as a biochemical marker of cardiovascular sympathetic activity was assessed. The ability of peripheral venous plasma noradrenaline to reflect small generalised increases in sympathetic outflow was investigated using tyramine to release endogenous noradrenaline from sympathetic neurones. This showed that the relationship between noradrenaline release and plasma noradrenaline was linear, but the increase in plasma noradrenaline associated with a 10-15 mmHg pressure rise was small, and not easily detected. There was considerable variabilty between individuals' noradrenaline clearance, resting blood pressure, and pressor responsiveness; but no association between these factors and plasma noradrenaline levels. This suggested that plasma noradrenaline could detect short-term changes in sympathetic activity within a group, but was unsuitable for comparisons between individuals. Further studies showed that tyramine raised blood pressure by stimulating cardiac contractility. Therefore, to assess the significance of plasma noradrenaline during regional changes in noradrenaline release, the effects of tyramine were compared with cold stress, which causes peripheral sympathetic activation. The results showed that the increment of plasma noradrenaline associated with a given rise in blood pressure was not constant, but appeared to vary with different regional 2 contributions to the pressor response. It was concluded that the pattern of regional sympathetic activity cannot be assessed by peripheral plasma noradrenaline. Furthermore, even within an individual, the ability of plasma noradrenaline to compare different sympathetic responses is in doubt. A new method was devised to assess cardiac sympathetic responses by estimation of intracardiac noradrenaline turnover. This was measured at rest, and gave results which have been confirmed by others, using different methods. The technique of coupled pacing produced a marked increase in cardiac noradrenaline release; this was accompanied by haemodynamic changes consistent with increased cardiac sympathetic activity. Paradoxically, propranolol also caused increased cardiac noradrenaline release for reasons which were not clear. It was concluded that this method may be useful for more detailed assessment of cardiac sympathetic responses. 3 CONTENTS Abstract Chapter 1 Introduction Chapter 2 The structure and function of the sympathetic nervous system Chapter 3 The role of the sympathetic nervous system in hypertension Chapter 4 The relationship between sympathetic nervous activity and the plasma noradrenaline concentration Chapter 5 General methodology Chapter 6 The relationship between blood pressure and plasma noradrenaline levels after release of endogenous neuronal noradrenaline by tyramine Chapter 7 The effects of propranolol on the pressor response to tyramine Chapter 8 The effects of tyramine on cardiac systolic time intervals before and after propranolol 4 Chapter 9 Tyramine and cold stress Chapter 10 A method for the estimation of intracardiac noradrenaline kinetics Chapter 11 Cardiac noradrenaline release at rest and during coupled right ventricular pacing Chapter 12 The effects of propranolol on cardiac noradrenaline release Chapter 13 Conclusions Acknowledgements Appendix Assay instruction sheet Bibliography 5 CHAPTER 1 INTRODUCTION 6 1.1 INTRODUCTION The original and widely-held view of the sympathetic nervous system was that of the "fight or flight" reaction initiated by a sudden and massive activation of the adrenomedullary system leading to an outpouring of adrenaline from the adrenal medulla. The effects - a pounding tachycardia, deep rapid respiration, widely dilated pupils, piloerection, sweating and pallor -are well known to many. This dramatic defence reaction seemed to have little to do with everyday regulation of the arterial blood pressure. However, the physiological role of the sympathetic nervous is now better understood, and it is clear that it is probably the dominant force in the control of the circulation. 'The primary function is to maintain tissue perfusion within set limits. This is achieved by control of cardiac output and systemic resistance, and requires maintainance of a fairly constant level of arterial blood pressure in the face of frequent changes of posture, physical activity and metabolic activity. Appreciation of the normal role of the sympathetic nervous system in blood pressure control led to interest in what its role might be in essential hypertension. Certain patients, usually in the earlier stages of hypertension, may have haemodynamic and sometimes somatic features suggestive of 7 excessive sympathetic nervous activity; thus, a 'neurogenic' hypothesis of essential hypertension held that excessive activity of the sympathetic nervous system might have a primary role in the initiation or maintenance of elevated blood pressure in essential hypertension. In the early 1970s, the concept of neurogenic essential hypertension driven by a primary increase in sympathetic activity received considerable support from several reports that hypertensive patients had elevated levels of plasma noradrenaline compared with normotensive controls. These observations, which were highly controversial, stimulated an enormous research effort devoted to the measurement and significance of plasma noradrenaline in populations of hypertensive and normotensive subjects. The central assumption common to all of these studies was that plasma noradrenaline was a valid biochemical marker of cardiovascular sympathetic activity. This assumption was based on very incomplete data. A small number of isolated organ studies had shown that the release of sympathetic transmitter was proportional to the intensity of nerve stimulation. Subsequent work showed that a variety of physiological and psychological stimuli known to be associated with increased sympathetic nervous activity consistently produced rises in plasma noradrenaline concentrations. Conversely, reduction of sympathetic activity by drugs, nerve section or neurological disease resulted diminished the plasma levels of noradrenaline. 8 Although it cannot be doubted that manoeuvres that alter sympathetic activity are associated with changes of plasma noradrenaline in the appropriate direction, it is by no means certain that the results of these rather disparate sympathetic stimuli can be extrapolated to a general proposition that plasma noradrenaline levels are a valid and quantitative marker of sympathetic activity. There are three main areas of difficulty. First, the plasma noradrenaline level is not determined solely by the rate of release from sympathetic neurones. The other major determinant is the plasma metabolic clearance rate, which may vary several-fold from one individual to the next without any apparent relationship to prevailing sympathetic activity. Thus, for any given input of noradrenaline into the circulation, a high clearance rate will result in a low plasma level, while a low clearance rate produces a high plasma level. Other factors which may also influence noradrenaline kinetics include the rate of diffusion from synapse to plasma, the volume of distribution, tissue binding, and uptake by platelets. Second, any increase in sympathetic activity in hypertensive patients would presumably be modest (but sustained). Plasma noradrenaline levels measured in hypertensive patients during supine rest are only 20-40% greater than in normotensive controls. By contrast, the experimental stimuli used to provoke noradrenaline release are often relatively intense all-or-nothing stimuli which involve changes 9 in posture or central haemodynamics; plasma noradrenaline levels may rise by twofold or more. These methods are not well suited to assessment of small changes in sympathetic activity. Thus, evidence that small changes in sympathetic activity can be detected by small increases in plasma noradrenaline is lacking. Third, the data concerning increases in plasma noradrenaline are based on acute studies. It is not known to what extent such increases would persist if the increase in sympathetic acivity were sustained, particularly since other parts of the sympathetic nervous system are capable of adapting to long-term changes in activity, eg receptor regulation. In conclusion, the extent to which plasma noradrenaline adequately reflects sympathetic activity, and particularly small changes in sympathetic activity, is not well defined. 10 1.2 AIMS OF THIS STUDY The overall purpose of the work reported in his thesis was to assess the usefulness, and to define the limitations of plasma noradrenaline levels as a biochemical index of sympathetic nervous system activity. 1.2.1 Assessment of generalised sympathetic activity. The data is presented in two parts. The first part concerns the assessment of peripheral venous plasma noradrenaline in relation to 'generalised' sympathetic activity. Specific aims were as follows. 1. Several related studies were performed to determine whether plasma noradrenaline levels were sensitive to small changes in sympathetic activity. 2. A further aim was to determine over a broader range