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

Abstract

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 × 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 × 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.