Characterization and localization of leukotriene C4 receptors in bullfrog lung

Specific binding sites for [3H]leukotriene C4, ([3H]LTC4) were characterized in lung membrane preparations and localized to the septa of lung tissue from the American bullfrog (Rana catesbeiana). Binding assays were carried out at 23 degrees C for 30 min in the presence of serine (5 mM)-borate (10 mM) to prevent metabolism of LTC4 to LTD4. Specific binding reached steady state within 10 min and was completely reversible with the addition of 1000-fold excess unlabeled LTC4. Scatchard analysis predicted a single class of binding sites with a Kd of 55.7 nM and a Bmax of 61.9 pmol/mg protein. The Hill coefficient was 0.958. LTC4 was the most potent inhibitor of [3H]LTC4 binding, with a Ki of 33 nM. LTD4 and LTE4 are less potent inhibitors, with Ki values of 3350 and 5979 nM, respectively. Mammalian LTD4 antagonists LY171883, L-649,923 and ICI 198,615 only displaced [3H]LTC4 specific binding at concentrations in excess of 1 x 10(-4) M. Guanosine 5' triphosphate and its analog guanyl-5'-yl-imidodiphosphate did not affect specific binding of [3H]LTC4, suggesting that a Gi protein is not involved in the LTC4 transduction mechanism. Glutathione, S-decyl-glutathione and hematin had Ki values of 35,000, 280 and 29,000 nM, respectively, suggesting the binding site is not the enzyme LTC4-synthase. LTC4 contracted lung strips, with S-decyl-glutathione a partial agonist. Computer-enhanced autoradiographs localized the binding sites to the smooth muscle regions of the septa in lung tissue. These data suggest that LTC4 binds to distinct receptors in bullfrog lung.