The present study showed evidence that the AT and
The present study showed evidence that the AT1 and the ETA receptors were not expressed near or on the plasma membrane as monomers, but also as possibly constitutive dimers and tetramers. For both receptors, the proportion of monomers, dimers and tetramers were unaffected by their respective agonist binding. Same situation was seen by the selective ETA receptor antagonist sitaxentan.
Conflict of interest
Acknowledgments The authors gratefully acknowledge the National Secretariat for Science, Technology and Innovation (SENACYT) of the Republic of Panama for financial support to Dr. Caballero-George and Ms. Planes through the incentive program of the National Investigation System (SNI). Also to Dr. Caballero-George through grant COL10-070, additionally to Ms. Planes through grant No. 172-2016. Moreover, thanks to IFARHU from the Panamanian government, which jointly with SENACYT gave a scholarship to Ms. Planes. The authors wish to thank Hongtao Chen for the assistance on the TIRF microscope setup and Maria Beatriz Carreira for helping with the statistical analysis. Dr. Gratton and Dr. Digman acknowledge support from Grants NIH P41-GM103540 and NIH P50-GM076516.
Introduction Endothelin-1 (ET-1), a 21-amino PTC-209 formula peptide, was initially identified in 1988 as a potent vasoconstrictor and pressor substance isolated from the culture supernatant of porcine aortic endothelial cells . This peptide has a molecular weight of 2492 with free amino and carboxyl termini, and two intramolecular disulfide bonds. Following the initial isolation, ET-1 has been shown to exist in many mammalian species, including human. Analysis of the human genome revealed that two other endothelin isopeptides were encoded in separate genes. They were named endothelin-2 (ET-2) and endothelin-3 (ET-3)  in addition to ET-1 which was initially found in endothelial cells. These isoforms of ET show a high degree of primary amino acid sequence identity; all are 21-amino acid polypeptides, containing two intramolecular disulfide bonds. The three ET isopeptides share a remarkable resemblance, both in structure and biological activity, to the sarafotoxins, a family of isopeptides isolated from a venom of the snake Atractaspis engaddensis, suggesting a possible common evolutionary origin . Although vascular endothelial cells are the major source of ET-1, the genes encoding the three ET isopeptides are expressed with different patterns in a wide variety of cell types including cardiac myocytes, vascular smooth muscle cells, renal tubular epithelial cells, glomerular mesangial cells, glial cells, pituitary cells, macrophages, mast cells, etc., suggesting that the peptides may participate independently in complex regulatory mechanisms in various organs [2,4]. Also numerous papers have demonstrated that ET has a variety of pharmacological actions not only in cardiovascular system but also in non-cardiovascular systems. Those pharmacological responses can be divided into two groups according to the pharmacological potency of the three peptides . Indeed, two ET-receptors subtypes, ETA and ETB receptors were cloned [6,7], which were distinct in their affinity rank order to endogenous ETs . The discovery of the ET-receptor, in turn, has stimulated the development of ET receptor antagonists. Advances in the development of ET receptor antagonists have accelerated the pace of investigations of the exact physiological and/or pathophysiological roles of endogenous ETs in animals and humans. As a result, the physiological and pathophysiological roles of ET-1 have been revealed. ET-deficient  and ET – receptor - deficient  mice by means of gene targeting has provided us further important information. Unexpectedly they showed that the ETs have crucial roles in normal embryonic development. ET-1 - deficient mice have craniofacial and cardiac abnormities at birth and die of respiratory failure soon after the birth . In contrast, ETB receptor-deficient mice exhibited aganglionic megacolon associated with coat color spotting, resembling a hereditary syndrome of humans, i,e, Hirschsprung\'s disease .