CONTRACTION OF RAT THORACIC AORTA STRIPS BY ENDOTHELIN-1 IN THE ABSENCE OF EXTRACELLULAR CA2+

1 Endothelin-1 (ET-1) caused a concentration-dependent contraction of helical strips from rat thoracic aorta in the absence of extracellular Ca2+. The Ca2+ -depleted muscle strips, prepared by three repeated applications of 10(-2) M caffeine or 10(-6) M noradrenaline in Ca2+ -free buffer, were contracted by 10(-8) M ET-1 in the same manner as non-treated strips. 2 In the absence of extracellular Ca2+, 10(-7) M phorbol 12-myristate 13-acetate (PMA), an activator of protein kinase C, induced a small but sustained contraction of the rat thoracic aorta strips within 60 min. Preincubation of the strips with 10(-7) M PMA for 60 min in Ca2+-free buffer, did not affect the 10(-8)M ET-1-induced contraction, but decreased the 5 x 10(-8)M phorbol 12,13-dibutyrate (PDB)-, or the 10(-7)M PMA-induced contraction, and potentiated the contraction induced by 10(-8)M urotensin II. Preincubation with 10(-8)M ET-1 (which induced maximum contraction) for 25 min in Ca2+-free buffer did not change the subsequent contraction induced by PMA (10(-7)M) or urotensin II (10(-8)M) but gave a somewhat lower maximum tension than in non-treated strips. 3 Calyculin-A, a potent inhibitor of phosphatase, also induced a contraction of the Ca2+-depleted muscle strips in Ca2+-free buffer. Preincubation of the strips with ET-1 (10(-8)M) or PMA (10(-7)M) decreased the calyculin-A (3 x 10(-8)M)-induced contraction. 4 These results suggest that ET-1 may induce phosphorylation of an unknown protein either without an increase in myoplasmic Ca2+ concentration or, alternatively, with mobilization of intracellular Ca2+ from noradrenaline- and caffeine-insensitive Ca2+ sources, through a mechanism different from that of phorbol ester.