Contraction of rat thoracic aorta strips by endothelin‐1 in the absence of extracellular Ca2+

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Abstract
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. 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 × 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. 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 × 10−8 m)‐induced contraction. 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.

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