Leukotriene C4 receptors on guinea pig tracheocytes

Leukotriene (LT) C4 receptors have been characterized on freshly isolated guinea pig tracheal epithelial cells (tracheocytes). The [3H]LTC4 receptor affinity was enhanced by increasing the sodium (60-160 mM) and the magnesium (0-10 mM) concentrations. Low concentrations of calcium (0-3 mM) increased [3H]LTC4 binding, but high concentrations (3-10 mM) decreased it. The pH (6.5-8.0) had no effect on [3H]LTC4 binding to tracheocytes. Under our experimental conditions, binding equilibrium was reached after 20 min. The association and the dissociation rate constants were estimated to be 2.75 +/- 0.25 x 10(6) M-1.min-1 and 0.093 +/- 0.008 min-1, respectively. The Kd (35.4 +/- 8.6 nM) and the Bmax values (2.4 +/- 0.6 x 10(5) receptors/cell) were determined by Scatchard analysis. LTB4, LTD4 and LTE4 did not inhibit [3H]LTC4 binding to the receptors. However, the compound FPL 55712 inhibited the binding of [3H]LTC4 with an IC50 value of 9.0 +/- 1.0 microM. [3H]LTC4 was not metabolized during the binding assays, as confirmed by reverse-phase high-performance liquid chromatography. The lack of [3H]LTC4 binding to glutathione-S-transferase was demonstrated in the presence of an excess of reduced glutathione. LTC4 produced a concentration-dependent increase of free Ca++ in tracheocytes. Our results suggest that guinea pig tracheocytes possess a specific LTC4 receptor coupled to a Ca++ signaling pathway. This LTC4 receptor may play a key role in the epithelium-dependent responses of airway smooth muscle.