Quantification of Endothelin Receptor Subtypes in Peripheral Tissues Reveals Downregulation of ET A Receptors in ET B -Deficient Mice

Abstract

We have previously shown that in homozygous endothelin (ET)_B^{− /−} deficient mice, ET_A receptor density is significantly downregulated in the brain by 45%. In these mice, plasma ET-1 levels are elevated. Our aim was to use quantitative autoradiography to establish the distribution of ET receptor subtypes in peripheral tissues from wild-type mice and to measure the density of the ET_A subtype in ET_B^{− /−} knockout animals. Our second aim was to test whether deletion of ET_B receptors, which is associated with elevated plasma levels of ET-1, would also reduce ET_A expression in the periphery. In longitudinal sections from wild-type mice, the highest densities of ET_A receptors localized to major organs including the ventricle of the heart, lung, and liver parenchyma. High densities of ET_A receptors were detected in the smooth muscle layer of the vasculature such as intrarenal vessels as well as the smooth muscle layer and epithelial cells of the gastrointestinal tract. In these tissues, the ET_A subtype was more abundant, representing between 60% and 100% of the ET receptors. ET_B receptors predominated in the medulla of kidney, with high densities also localizing to glomeruli within the cortex and to the sinusoids from the liver. Lower densities of ET_B receptors were also present in the lung, heart, liver, and the smooth muscle layer of the gastrointestinal tract. In ET_B^{− /−} knockout mice, ET_B receptors were not detected as expected by either ligand binding or immunocytochemistry. The pattern of ET_A receptor distribution in the ET_B^{− /−} knockout mice was similar to the controls, but the density of ET_A receptors was significantly reduced in the lung by 39%. Diminished responses to the endogenous agonist after repeated stimulation are an important feature of G-protein signaling, preventing potential damage to the overstimulated cell, and it is likely that downregulation occurs in response to higher circulating levels of ET-1.