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 [¹¹C]-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 [¹¹C]-CGP12177. Myocardial blood flow (MBF in mL/min/mL tissue) was measured by imaging [15O]-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 VO_{2 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.