MORPHOGENIC EFFECTS OF ENDOTHELIN-1 ON VASCULAR SMOOTH-MUSCLE CELLS

Endothelin-1 (ET-1), a vasoconstrictor peptide produced by endothelial and vascular smooth muscle cells (VSMC) might play a role in vascular remodelling. To investigate the proposed 'mitogenic' potential of ET- 1, we examined the effects of chronic exposure of VSMC to ET-1 on cell cycle, growth/proliferation and differentiation under essentially mit ogen-free culture conditions. Bulk cultures of thoracic aortic VSMC of spontaneously hypertensive (SHR) and normotensive Wistar Kyoto (WKY) rats, although exhibiting genetically determined differences in growth /proliferation (due to shortened G1 and G2 phases in SHR VSMC), respon d in a similar manner to ET-1 exposure: long-term exposure (12-15 days ) of VSMC from both sources to ET-1 in nonmitogenic medium did not pro mote cycling of cells. On the contrary, ET-1 attenuated the cycling of VSMC which had already cycled beyond the S phase. For cells which had not cycled beyond the S phase, ET-1 interrupted progression through t he cell cycle at the late G1/early S phase. The specific ability of SH R VSMC to grow in mitogen-free medium was abolished by ET-1, most like ly via down-regulation of platelet-derived growth factor (PDGF)-alpha receptors. Subsequent to ET-1 exposure, VSMC expressed increased level s of mRNA and protein for smooth-muscle-specific alpha-actin. However, expression of smooth muscle alpha-actin did not predominate over beta -actin as observed for adult contractile VSMC in vivo. The ET-1-induce d expression of smooth-muscle-specific alpha-actin mRNA was dose depen dent (EC50 approx. 2 x 10(-9) M), and alpha-actin protein expressed wa s associated with organized actin fibers.