ENDOTHELIN-1 ALTERS THE CONTRACTILE PHENOTYPE OF CULTURED EMBRYONIC SMOOTH-MUSCLE CELLS

Smooth muscle tissues may be classified into phasic (fast) or tonic (s low) contractile phenotypes. This study was initiated to examine the s pecification of these phenotypes during development and the role of gr owth factors in this process. We used myosin light chain 17 (MLC17) an d myosin heavy chain transcript splice variants as markers of the toni c (aortic) and phasic (intestinal) smooth muscle phenotypes in chick e mbryos. By reverse transcription-polymerase chain reaction, we determi ned embryonic days 6 to 16 to be a critical period for the establishme nt of these phenotypes. During this period, endothelin-1 is present at 40-fold-higher levers in aortic compared with intestinal tissues. To test the hypothesis that endothelin-1 may be involved in establishing the aortic (tonic) phenotype, we developed a system in which embryonic smooth muscle cells exhibit phasic and tonic contractile properties i n vitro. Single-cell force measurements showed that cultured embryonic gizzard (phasic) cells developed force more rapidly (8+/-2 seconds) a nd achieved greater force (3.0+/-0.7 mu N) than did cultured embryonic aortic (tonic) cells (20+/-0.7 seconds, 0.76+/-0.01 mu N; P<.05) in r esponse to depolarization. Chronic exposure of the phasic (gizzard) ce lls to endothelin-1 prolonged the time to peak force (24+/-3 seconds) and reduced the peak force (1.0+/-0.1 mu N), SO that the contraction r esembled the tonic type. This effect, mediated by the endothelin-a rec eptor, was associated with a shift in MLC17 splicing to the tonic patt ern. These results demonstrate that endothelin-1 is highly enriched in developing aortic compared with intestinal tissues and can convert ph asic smooth muscle cells to the tonic type in vitro, suggesting a role for this growth factor during development in determining the contract ile phenotype of smooth muscle cells.