PET imaging of myocardial beta-adrenergic receptors with fluorocarazolol: lack of interference by endogenous catecholamines.

beta-Adrenergic receptor (beta-AR) concentration can be measured in vivo using positron emission tomography (PET) and the high-affinity antagonist [18F]-(S)-fluorocarazolol {[18F]-(S)-FCZ}. However, the influence of endogenous catecholamines on the in vivo binding properties of [18F]-(S)-FCZ should be measured to aid in selection of the model used to estimate receptor concentration based on PET data. Herein we addressed the questions "What is the influence of endogenous catecholamines on the [18F]-(S)-FCZ binding in the heart?" and "In what range are the in vivo concentrations of endogenous beta-AR ligands?" In PET studies, 3 drug regimens were used to manipulate the levels of endogenous catecholamines. The time courses of myocardial concentration of [18F]-(S)-FCZ were compared before and after drug administration. In vitro binding assays and computer simulations were performed to complement the in vivo studies. Despite the large changes of endogenous catecholamines, no significant changes were observed in the [18F]-(S)-FCZ myocardial concentration. In vitro assays showed that (S)-FCZ has an affinity for beta-receptors that is 3900 and 9500 times higher than those of norepinephrine (NE) and epinephrine (EPI), respectively. Computer simulations support the hypothesis that the binding affinities relative to ligand concentrations in vivo are sufficient to explain the apparent lack of effect of endogenous catecholamines on [18F]-(S)-FCZ myocardial concentration. Increased levels of catecholamines in the physiological range do not affect the myocardial concentration of [18F]-(S)-FCZ as measured by PET. This lack of effect suggests that the myocardial concentration of NE at the synaptic sites cannot be higher than 300 nM.