Endothelin ETA receptor upregulated but ETB downregulated in human pulmonary arterial hypertension

Myrna Carlebur1, Rhoda Kuc1, Janet Maguire1, Mark Toshner2, Nicholas Morrell2, Neil Davie3, Anthony Davenport1. 1Clinical Pharmacology Unit, University of Cambridge, Cambridge, United Kingdom, 2Division of Respiratory Medicine, University of Cambridge, Cambridge, United Kingdom, 3Pulmonary Vasculature Business Unit, Pfizer Ltd, Surrey, United Kingdom.

Pulmonary arterial hypertension (PAH) involves injury to pulmonary vasculature elevating pulmonary arterial pressure. Chronic pressure and volume overload causes hypertrophy and dilatation of right ventricle (RV), leading to RV heart failure - the major cause of death. Endothelin (ET) has been implicated in disease progression and clinically, ET antagonists (ETA selective and ETA/ETB non-selective) are efficacious. In humans, ETA receptors are the principal subtype in medial vascular smooth muscle where they mediate vasoconstriction. ETB receptors localise to endothelium and cause beneficial vasodilatation by release of endothelium-derived relaxing factors opposing vasoconstrictor tone. Organs such as lung are rich in ETB, where this subtype functions to clear ET from plasma. The precise molecular mechanism whereby ET antagonists produce benefit in PAH is not established, and contribution of ETB to the development of the condition and the need to block this sub-type as well as ETA is unclear. The aim was to compare the density of both ET receptor sub-types in RV, left ventricle (LV) and lung in normal and PAH heart and lung tissues.

Human tissues were obtained with local ethical approval and consent from patients with and without PAH undergoing transplantation or lobectomy. In competition binding assays tissue sections were incubated with 0.1nM [125I]-ET-1 and increasing concentrations of FR139317, an ETA selective antagonist (Peter and Davenport, 1996). Data were analysed using non-linear iterative curve fitting programmes to determine KD (receptor affinity) and BMAX (receptor density). n-Values were the number of individuals from whom tissue was obtained.

An increase in ETA and decrease in ETB densities was observed in LV and RV of PAH hearts compared to control. In normal RV, ETA:ETB was 52:48 (n = 12). In failing RV in PAH, there was a significant shift in ETA:ETB to 69:31 (n = 9) (Student’s t-test P<0.05). In LV, no significant difference in ETA:ETB was observed. These changes observed in the RV of PAH patients are consistent with the disease profile and efficacy of ET antagonists.

Competition analysis did not identify changes in ETA and ETB densities or ETA:ETB subtype ratio in PAH compared to normal lung. However, autoradiographical analysis indicated tissue specific upregulation of ETA and ETB densities in parenchyma and small blood vessels that warrants further investigation to elucidate precise cellular localisation and potential role of ET in these small vessels.

Peter MG, Davenport AP. Br J Pharmacol 1996; 117:455-462.