ENDOTHELIN-1 CONVERSION AND RECEPTOR CHARACTERIZATION IN HUMAN PLACENTAL ARTERIES

Endothelin-1-( 1--21), a potent presser peptide, is transcribed as big endothelin-(1-38) and converted to active peptide by endothelin-conve rting enzyme. The current investigation tested the hypothesis that hum an fetoplacental blood vessels convert big endothelin-1 to active pept ide and that fetoplacental blood vessels respond to endothelin-1 by bi nding of the peptide to specific receptor sites. In the isolated perfu sed placental cotyledon the addition of big endothelin-1 to the perfus ate caused a time-dependent increase in perfusion pressure that corres ponded to the appearance of endothelin-1 in the perfusate. The propert ies of human placental endothelin-1 receptors were defined in binding studies performed on a plasma membrane fraction of small arteries (<1. 0 mm dissected from the placenta. Binding was saturable, reached stead y state by 3 h at 25 degrees C, and was linear with protein concentrat ion. Scatchard analysis of binding data indicated a single class of hi gh-affinity binding sites with a dissociation constant of 27.6 +/- 2.3 pM and a density of 856 +/- 119 fmol/mg protein (n = 5). The potency order for competitive inhibition of the binding of I-125-labeled endot helin-1 [endothelin-1 = endothelin-2 > endothelin-3 = sarafotoxin S6b > > big endothelin-1 (human) = big endothelin-1 (porcine)] is most con sistent with a type A endothelin receptor subtype. Phenylephrine, brad ykinin, norepinephrine, atrial natriuretic factor, diltiazem, U-46619, and angiotensin II did not displace I-125-endothelin-1 binding. Endot helin receptors were shown to have an approximate molecular weight of 36,600 by polyacrylamide gel electrophoresis. These experiments suppor t the hypothesis that big endothelin transcribed in the placenta can m odulate fetoplacental vascular resistance by local conversion to the a ctive peptide, which then acts on specific receptor sites in placental blood vessels.