Non-invasive assessment of the effect of cardiac sympathetic innervation on metabolism of the human heart

The role of cardiac sympathetic nerves in the regulation of myocardial metabolism is not well defined. Owing to the presence of incomplete reinnervation, heart transplant recipients provide a unique model to study the effects of efferent sympathetic innervation. Using this model, we sought to determine the influence of cardiac sympathetic signals on substrate utilisation and overall oxidative metabolism. In 21 transplant recipients, positron emission tomography was applied to determine sympathetic innervation with the noradrenaline analogue carbon-11 hydroxyephedrine, oxidative metabolism with carbon-11 acetate (n= 14), and glucose utilisation with fluorine- 18 fluorodeoxyglucose (n=7). The reinnervated area comprised 22%±20% of the left ventricle. Oxidative metabolism was similar in denervated and reinnervated myocardium [0.06±0.01 vs 0.06±0.01/min for k(mono)], while glucose uptake was significantly higher in denervated myocardium (6.9±6.6 vs 6.0±6.2 μmol/min/100 g; P=0.03). Reinnervation mainly occurred in the territory of the left anterior descending artery, where retention of ¹¹C-hydroxyephedrine (6.8±2.7%/min) was higher compared with territories of the left circumflex (4.1±l.7%/min; P<0.01) and right coronary (3.8±1.1%/min; P<0.01) arteries. Oxidative metabolism was similar in all three territories, but compared with the reinnervated territory of the left anterior descending artery (53%±16% of maximum), relative FDG uptake was higher in territories of the left circumflex (76%±6%, P<0.01) and right coronary (67%±10%, P<0.05) arteries. Similar degrees of regional heterogeneity were not observed in normals. Thus, while overall energy production through oxidative metabolism remains unaffected, cardiac utilisation of glucose in the fasting state is increased in the absence of catecholamine uptake sites. Innervated myocardium, however, may preferentially utilise free fatty acids, suggesting a role for sympathetic tone in substrate utilisation.