High speed liquid chromatography of phenylethanolamines for the kinetic analysis of [11C]-meta-hydroxyephedrine and metabolites in plasma

Assessment of the Effects of Age, Gender, and Exercise Training on the Cardiac Sympathetic Nervous System Using Positron Emission Tomography Imaging

BACKGROUND

Using positron emission tomography (PET) imaging, we sought to determine whether normal age or exercise training cause changes in the cardiac sympathetic nervous system function in male or female healthy volunteers.

METHODS

Healthy sedentary participants underwent PET studies before and after 6 months of supervised exercise training. Presynaptic uptake by the norepinephrine transporter-1 function was measured using PET imaging of [(11)C]-meta-hydroxyephedrine, a norepinephrine analog, and expressed as a permeability-surface area product (PSnt in mL/min/mL). Postsynaptic function was measured as β-adrenergic receptor density (β'max in pmol/mL tissue) by imaging the β-receptor antagonist [(11)C]-CGP12177. Myocardial blood flow (MBF in mL/min/mL tissue) was measured by imaging [(15)O]-water.

RESULTS

At baseline, there was no age difference in β'max or MBF but PSnt declined with age (1.12±0.11 young vs 0.87±0.06 old, p = .036). Before training, women had significantly greater MBF (0.87±0.03 vs 0.69±0.03, p < .0001) and PSnt (1.14±0.08 vs 0.75±0.07, p < .001) than men. Training increased VO2 max by 13% (p < .0001), but there were no training effects on β'max, PSnt, or MBF. Greater MBF in females and a trend to increased PSnt post-training persisted.

CONCLUSION

With age, presynaptic uptake as measured by PSnt declines, but there were no differences in β'max. Endurance training significantly increased VO2 max but did not cause any changes in the measures of cardiac sympathetic nervous system function. These findings suggest that significant changes do not occur or that current PET imaging methods may be inadequate to measure small serial differences in a highly reproducible manner.

11C-meta-hydroxyephedrine: a promising PET radiopharmaceutical for imaging the sympathetic nervous system

Dysfunction of the sympathetic nervous system underlies many cardiac diseases and can be assessed by molecular imaging using SPECT tracers as I-metaiodobenzylguanidine (I-MIBG). The norepinephrine analog C-meta-hydroxyephedrine (HED) has been used with PET to map the regional distribution of cardiac sympathetic neurons. Hydroxyephedrine is rapidly transported into sympathetic neurons by the norepinephrine transporter and stored in vesicles. This review describes the mechanism of action, radiosynthesis, and application of HED in the assessment of the cardiac sympathetic nervous system in heart failure, myocardial infarction, and arrhythmias. Noncardiac applications of HED in the clinical setting of sympathetic nervous system tumors and other emerging research applications are described.

Assessment of cardiac autonomic neuronal function using PET imaging

The autonomic nervous system is the primary extrinsic control of cardiac performance, and altered autonomic activity has been recognized as an important factor in the progression of various cardiac pathologies. Molecular imaging techniques have been developed for global and regional interrogation of pre- and postsynaptic targets of the cardiac autonomic nervous system. Building on established work with the guanethidine analogue ¹²³I-metaiodobenzylguanidine (MIBG) for single-photon emission tomography (SPECT), development of radiotracers and protocols for positron emission tomography (PET) investigation of autonomic signaling has expanded. PET is limited in availability and requires specialized centers for radiosynthesis and interpretation, but the higher resolution allows for improved regional analysis and kinetic modeling provides more true quantification than is possible with SPECT. A wider array of radiolabeled catecholamines, analogues of catecholamines, and receptor ligands have been characterized and evaluated. Sympathetic neuronal PET tracers have shown promise in the identification of several cardiac pathologies. In particular, recent studies have elucidated a mechanistic role for heterogeneous sympathetic innervation in the development of lethal ventricular arrhythmias. Evaluation of cardiomyocyte adrenergic receptor expression and the parasympathetic nervous system has been slower to develop, with clinical studies beginning to emerge. This review summarizes the clinical and the experimental PET tracers currently available for autonomic imaging and discusses their application in health and cardiovascular disease, with particular emphasis on the major findings of the last decade.

Evidence for pre- to postsynaptic mismatch of the cardiac sympathetic nervous system in ischemic congestive heart failure

Pre- and postsynaptic cardiac sympathetic function is altered in ischemic congestive heart failure (CHF). Whether there is a presynaptic-to-postsynaptic mismatch or whether mismatch is related to adverse cardiac events is unknown.

METHODS

In 13 patients with ischemic CHF and 25 aged-matched healthy volunteers, presynaptic function was measured by PET of (11)C-meta-hydroxyephedrine ((11)C-mHED), a norepinephrine (NE) analog. Postsynaptic function, beta-adrenergic receptor (BAR) density (B'(max)), was measured by imaging (11)C-CGP12177. Myocardial blood flow (MBF) was measured by imaging (15)O-water. Each heart was analyzed both globally and regionally, excluding infarcted regions, and a mismatch score, defined as the ratio of B'(max) to NE uptake (PS(nt))(,) was used to indicate mismatch of post- and presynaptic function.

RESULTS

Global and regional MBF was not different between CHF and healthy subjects. The global measure of PS(nt) was lower in CHF (0.32 +/- 0.34) than that in healthy subjects (0.81 +/- 0.33, P < 0.0001) and in all 12 regions. Global B'(max) tended to be lower in CHF than that in healthy subjects (10.0 +/- 6.4 pmol/mL vs. 13.4 +/- 4.2, P = 0.056) and in all 12 regions. The global mismatch score (B'(max):PS(nt)) in CHF patients was significantly greater than that in healthy subjects (50.3 +/- 50.7 vs. 19.3 +/- 9.7, P = 0.005) and also greater in 11 of 12 regions. After 1.5 y of follow-up, 4 individuals had an adverse outcome (CHF death, new or recurrent sudden death, or progressive CHF leading to transplantation). Three of the 4 had mismatch scores > 3 times that of the healthy subjects or the CHF patients without an adverse outcome.

CONCLUSION

Mismatch between pre- and postsynaptic left ventricular sympathetic function is present in patients with severe CHF and may be more marked in those with adverse outcomes.

PET measures of pre- and post-synaptic cardiac beta adrenergic function

Positron Emission Tomography was used to measure global and regional cardiac beta-adrenergic function in 19 normal subjects and 9 congestive heart failure patients. [(11)C]-meta-hydroxyephedrine was used to image norepinephrine transporter function as an indicator of pre-synaptic function and [(11)C]-CGP12177 was used to measure cell surface beta-receptor density as an indicator of post-synaptic function. Pre-synaptic, but not post-synaptic, function was significantly different between normals and CHF patients. Pre-synaptic function was well matched to post-synaptic function in the normal hearts but significantly different and poorly matched in the CHF patients studied. This imaging technique can help us understand regional sympathetic function in cardiac disease.