Heterogeneity of cell surface endothelin receptors

Two distinct cell surface endothelin receptors were identified, namely a 73-kDa protein referred to as ET-R1 and a 60-kDa protein named ET-R2. ET-R1 was expressed as the sole endothelin receptor on rat A10 vascular smooth muscle cells and C6 glial cells. Binding of 125I-ET-1 to these cells was inhibited by 50-200 pM endothelin-1 and -2, whereas endothelin-3 did not compete for this receptor subtype. Binding of 125I-ET-1 to intact A10 and C6 cells was reversible, indicating that ET-R1 is located on the cell surface. Affinity labelling of a single 73-kDa band on sodium dodecyl sulfate-polyacrylamide gels by 125I-ET-1 in A10 and C6 cells was inhibited by endothelin-1 but not by endothelin-3. In A10 cells, endothelin-1 but not endothelin-3 elicited a concentration-dependent increase in intracellular inositol trisphosphate levels. ET-R1 was also expressed in cultured rat glomerular mesangial cells based on findings of a subset of receptors with an apparent molecular mass of 73 kDa that bound 125I-ET-1 displacable by endothelin-1 and endothelin-2 but not by endothelin-3. These cells also expressed the ET-R2 receptor subtype, based on findings of a 60-kDa binding site that could be labeled by both 125I-ET-1 and 125I-ET-3. Labeling of ET-R2 by the radioactive endothelins-1 and -3 was inhibited competitively by endothelins-1, -2, and -3. Furthermore, ET-R2 was shown to be a functional receptor, as endothelin-3 caused inositol trisphosphate levels to rise in mesangial cells. An endothelin binding site with high affinity for endothelin-3 was also identified on rat PC12 pheochromocytoma cells, although the apparent molecular mass of this receptor could not be verified by cross-linking studies. Since endothelin-1 or -3 failed to augment inositol trisphosphate levels in these cells, this binding site could represent a third endothelin receptor subtype. Thus, two distinct functional receptors for endothelins were identified on rat cells, namely the 73-kDa ET-R1 which has an exceedingly low affinity for endothelin-3 and the 60-kDa ET-R2 which binds endothelin-3 with high affinity. Whether an additional endothelin receptor subtype exists in PC12 cells remains to be shown with certainty.