ENDOTHELIN SECRETION IS REGULATED BY CYCLIC-AMP AND PHOSPHATASE 2A INENDOTHELIAL-CELLS

Endothelin is a 21 amino acid peptide secreted by endothelial cells an d is the most potent vasoconstrictor known. The present study examines regulatory mechanisms of endothelin secretion, focusing on the role o f protein phosphorylation. Endothelin secretion was measured by radioi mmunoassay in primary cultures of human umbilical vein endothelial cel ls. While treatment that raised cAMP levels reduced the basal endothel in secretion rate, agents that elevated cGMP had no effect. Downregula tion or inhibition of protein kinase C resulted in decreased endotheli n secretion, suggesting that protein kinase C regulates endothelin sec retion in the opposite direction to cAMP dependent protein kinases. Ok adaic acid, at concentrations that selectively inhibit protein phospha tases 2A, reduced the endothelin secretion and the effects of okadaic acid and db-cAMP were additive. Endothelin production was stimulated b y fetal calf serum and by the protein kinase inhibitor 1-(5-isoquinoli nylsulphonyl)-2-methylpiperazine (H7), but was inhibited by the calmod ulin antagonist trifluoperazine. The present findings that regulators of cAMP-dependent protein kinases, protein kinase C, calmodulin, and p rotein phosphatase 2A all affect endothelin secretion suggest that end othelin secretion is controlled by phosphorylation/dephosphorylation o f as yet unidentified regulatory proteins within the cell. (C) 1994 Wi ley-Liss, Inc.