The significant role of endothelin-3 in potentiating electrically stimulated contractions in the rat iris sphincter

Purification of a novel endothelin-converting enzyme specific for big endothelin-3

Endothelin-3 (ET-3), a potent vasoactive peptide, is considered to be produced from big ET-3 by endothelin-converting enzyme (ECE) like the other members of the endothelin family (ET-1 and ET-2). We purified a novel ECE from bovine iris microsomes. The purified enzyme, a 140 kDa protein by SDS-PAGE analysis, converted big ET-3 to ET-3 but not big ET-1, with a Km value of 0.14 microM for big ET-3. The conversion to ET-3 was confirmed with sandwich EIA by monoclonal antibodies, the elution profile of HPLC, and intracellular calcium mobilization in CHO-K1 cells expressing recombinant human ET(B) receptors. The conversion activity was inhibited by an inhibitor of neutral endopeptidase 24.11 (NEP) phosphoramidon. These results show that ECE-3 purified from bovine iris is a novel metalloprotease totally different from ECE-1 or ECE-2, in that the enzyme is highly specific for big ET-3.

Postganglionic sympathetic neurons express endothelin

Endothelin (ET) is a peptide originally identified as an endothelial-derived vasoconstrictor. It is now recognized that ET is produced by and acts on many other tissues including the brain and spinal cord, where it is believed to modulate neurotransmission. The present studies demonstrate that ET is synthesized by and secreted from postganglionic sympathetic neurons. With the use of Northern analysis, ET-1 mRNA was detected in cultures of sympathetic superior cervical ganglion (SCG) neurons isolated from 3- to 5-day old rat pups. ET-1 and ET-3 peptides were also detected in cultured SCG neurons using immunohistochemistry. ET-1 (50 pg/106 cells) and ET-3 (173 pg/106 cells) were detected by radioimmunoassay of media conditioned by cultured SCG. ET-1 (77 pg/mg protein) and ET-3 (30 pg/mg protein) were also detected by radioimmunoassay of extracts of adult SCG.