Downregulation of endothelin receptors by autocrine production of endothelin-1

The potent vasoconstrictor endothelin-1 (ET-1) is a paracrine, but also autocrine, factor for some types of cells. The goal of our study was to evaluate whether the receptor population in cells expressing endothelin receptor subtype A (rat mesangial cells) or endothelin receptor subtype B (human and rat endothelial cells) was affected by the autocrine production of ET-1. We therefore studied maximal binding capacity of 125I-labeled ET-1 in the presence or absence of the metalloprotease inhibitors phosphoramidon, which blocks the intracellular processing of Big ET-1 to ET-1, and thiorphan, which does not block this conversion. Phosphoramidon inhibited the release of ET-1 by human umbilical vein endothelial cells, rat aortic endothelial cells, and rat mesangial cells, and increased 1.4- to 17-fold the maximal binding capacity in the three types of cells. Thiorphan affected neither ET-1 release nor binding. The increase in receptor binding by phosphoramidon was associated with an increase in the functional effect of ET-1, as measured by arachidonic acid release in rat mesangial cells. We conclude that autocrine production of ET-1 decreases, either by binding or by downregulation, the number of binding sites available for ET-1 of paracrine or systemic sources. This aspect of modulation of the vasoconstrictor effect of endothelin should be considered in pathological situations or after endothelin-converting-enzyme inhibition.