Effects of endothelin on the mechanical activity and cytosolic calcium levels of various types of smooth muscle

Abstract

Effects of porcine/human endothelin (endothelin‐1), a novel vasoconstrictor peptide, on various smooth muscles were examined. In rat aorta, endothelin (1 pM‐30 nm) induced contraction in a concentration‐dependent manner. Removal of endothelium shifted the concentration‐response curve to the left. When added during the sustained contraction induced by 0.1 μm noradrenaline, endothelin (1 nm) induced a relaxation that was inhibited by removing endothelium or by methylene blue. In rat aorta without endothelium, endothelin (1–30 nm) increased cytosolic Ca²⁺ level ([Ca²⁺]_cyt) followed by contraction. Endothelin induced less contraction than high K⁺ at a given [Ca²⁺]_cyt when the concentration of endothelin was lower (1–3 nm) and/or during the early phase of the contraction (< 10 min). In contrast, endothelin induced a greater contraction than KCl after prolonged exposure to high concentrations (> 10 nm). The increase in [Ca²⁺]_cyt due to endothelin was strongly inhibited by 10 μm verapamil or 0.3 μm nicardipine although muscle contraction was only partially inhibited. In Ca²⁺‐free solution, endothelin (30 nm) induced a transient increase in [Ca²⁺]_cyt and a slow increase in muscle tension. After a prolonged incubation in Ca²⁺‐free solution, endothelin (30 nm) still induced a slow increase in tension without changing [Ca²⁺]_cyt. This contraction was inhibited by 1 μm sodium nitroprusside or 10 μm forskolin. In canine trachea and guinea‐pig uterus, endothelin (30 nm) induced sustained contraction with an increase in [Ca²⁺]_cyt. In the absence of external Ca²⁺, endothelin (30 nm) induced a sustained contraction in canine trachea without changing [Ca²⁺]_cyt. In guinea‐pig vas deferens, taenia caeci and ileal longitudinal muscle, endothelin induced small increases in [Ca²⁺]_cyt and tension. In permeabilized smooth muscles, endothelin (30 nm) did not change the muscle tone. These results suggest that endothelin acts on the endothelium and increases the synthesis or release of endothelium‐derived relaxing factor (EDRF). These results also suggest that endothelin acts directly on smooth muscle and increases [Ca²⁺]_cyt by releasing Ca²⁺ from storage sites and increasing Ca²⁺ influx through the verapamil‐ and 1,4‐dihydropyridine‐sensitive pathway. Endothelin seems to decrease Ca²⁺‐sensitivity of contractile elements at lower concentrations and/or during the early phase of the contraction, whereas it increases Ca²⁺‐sensitivity at higher concentrations during the sustained phase of the contraction. Furthermore, endothelin induces a contraction that is not dependent on [Ca²⁺]_cyt.