Peptides and their interaction with receptors play a major role in normal physiology as well as in pathophysiology. One important group of these receptor systems is the seven-transmembrane G-protein coupled receptor (GPCR), targeted by about 50% of the currently used drugs. In addition, a number of novel peptide receptor systems predicted from the human genome and their cognate ligands have recently been identified. The aim of our work is to investigate if these peptides and their receptor sub-types play a role in the human cardiovascular system (Maguire & Davenport, 2005), and if so to quantify any changes associated with disease and to identify new targets for novel drugs. Positron emission tomography (PET) is the only technique available to image and quantify receptors in vivo with high sensitivity. With the recent development of dedicated PET scanners for small animals such as the microPET, it is now possible to perform functional imaging in rodents with a spatial resolution sufficient to delineate discrete organs and their larger substructures. In this presentation we will illustrate our approach to explore novel peptide receptors using PET with our recent work on the endothelin receptor system (Johnström et al. 2003, 2004, 2005, 2006). Endothelin-1 (ET-1) is a multifunctional peptide in humans and alteration in the ET receptor system has been linked to a number of vascular diseases. ET-1 is produced from its precursor peptide bigET-1 by the action of endothelin converting enzymes (ECE). Two different strategies are currently being pursued to prevent the vasoconstrictor actions of endothelin: selective receptor antagonists or blocking synthesis of the peptide by inhibition of ECE. For this purpose we have synthesised novel subtype selective PET radioligands as well as labelled the endogenous peptide ET-1 and its precursor peptide big ET-1 with ¹⁸F and investigated these radioligands pharmacodynamic and pharmacokinetic properties in vivo in anaesthetised animals with microPET. [¹⁸F]-ET-1 is rapidly cleared from the circulation with a subsequent fast accumulation in ET receptor rich tissue such as lung, kidney and liver, consistent with receptor binding. The in vivo distribution correlated with the anatomical localisation of receptors detected in vitro using [¹²⁵I]-ET-1. However, the receptor density visualised in the heart was unexpectedly low compared with that predicted from the in vitro measurements. Uptake in lung and kidney was significantly blocked when the animal was pre-treated with the ET_B selective antagonist BQ788, confirming that the fast clearance from the circulation was ET_B receptor mediated. In contrast, radioactivity increased in the liver suggesting binding of increased levels of circulating ET-1 to ET_A receptors. Hence the ET_B receptor plays an important role as a clearing receptor removing ET-1 from the circulation and this mechanism may be beneficial in protecting the cardiovascular system from the detrimental effects caused by upregulated ET-1 in disease. Infused [¹⁸F]-big ET-1 showed a localised distribution in lung suggesting tissue specific conversion of big ET-1 to ET-1 and binding to ET_A receptors in the vasculature. This was confirmed by dynamic PET where pre-treatment with the ECE inhibitor phosphoramidon or the ET_A selective antagonist FR139317 reduced binding of ET-1 to ET_A receptors due to inhibition of ECE activity or ET_A receptor blockade. This demonstrate that tissue-specific conversion of big ET-1 in the vasculature may increase levels of circulating ET-1 and subsequent binding to ET_A receptors suggesting that inhibition of ECE activity will be beneficial in limiting the harmful effects of ET-1 in disease. Our results clearly demonstrate that we can study ET receptor pharmacology in vivo in small animals using microPET. Thus we have the potential to obtain pharmacodynamic information of novel drugs, image animal models to monitor disease progression or effect of treatment to further clarify the significance of the ET receptor system in disease. Furthermore, they show the potential for studying other emerging orphan receptor systems facilitating the rapid translation of information from the human genome into function and into the clinic.