Abstract

The effects of leukotriene D₄ (LTD₄) on the concentration of intracellular cytosolic free calcium ([Ca⁺⁺]_i) and on phosphoinositide hydrolysis were studied in cultured guinea pig tracheal smooth muscle cells. In Fura-2-loaded cells, LTD₄(10⁻⁹–10⁻⁶ M) induced concentration-dependent changes in [Ca⁺⁺]_iconsisting of a slow, transient increase followed by a sustained phase. Preincubation of cells with LTD₄ receptor antagonist MK-571 (10⁻⁶ M) blocked the increase in [Ca⁺⁺]_i. Similarly, LTD₄-induced inositol phosphate ([³H]InsP_s) synthesis was transient, concentration-dependent and inhibited by the LTD₄ antagonist. In the absence of extracellular Ca⁺⁺, LTD₄ failed to induce [Ca⁺⁺]_i increases and [³H]InsP_s formation. Accordingly, NiCl₂ completely inhibited the LTD₄-stimulated [³H]InsP_s synthesis. Nifedipine (10⁻⁵ M) had a slight inhibitory effect on [Ca⁺⁺]_i increase but significantly reduced (40–50%) the [³H]InsP_s accumulation. These findings indicate that LTD₄-stimulated inositol phosphate synthesis and [Ca⁺⁺]_i increases in tracheal smooth muscle cells are receptor-mediated events and are dependent on the availability of extracellular Ca⁺⁺. It is suggested that Ca⁺⁺ influx plays a major role in the LTD₄signal transduction mechanism.