| pubmed-article:2557419 | pubmed:abstractText | We investigated the mechanism of guanyl-5'-yl-imidodiphosphate (GppNHp) regulation of peptidoleukotrienes (LTs) and LT-antagonists binding to LTD4 receptors on guinea pig lung membranes (GPLMs). In saturation experiments, [3H]LTD4 saturable (maximum binding = 943 +/- 39 fmol/mg of protein) binding to GPLM was significantly (P less than .01) inhibited by GppNHp (60 nM, maximum binding = 446 +/- 113 fmol/mg of protein) in a concentration-dependent manner. No significant change in the affinity (Kd = 0.29 +/- 0.02 nM vs. 0.43 +/- 0.12 nM for control and treated GPLM, respectively) for [3H]LTD4 was observed. The binding affinity for the selective LTD4 antagonist ICI 198,615 (Ki = 0.13 +/- 0.04 nM) as determined by competition against [3H]LTD4, was not changed by GppNHp. Saturation analysis of [3H]ICI 198,615 binding confirmed that GppNHp did not change the apparent affinity or site-density for this ligand. In competition experiments against [3H]-ICI 198,615, GppNHp (1 microM) caused a significant (P less than .01) rightward shift of the inhibition by agonists (94-, 50- and 8-fold shifts for LTD4, LTE4 and YM-17690, respectively). In contrast, inhibition of [3H]ICI 198,615 by four LTD4 antagonists (ICI 198,615, 4-[5-cyclopentylcarbonylamino-1-[3-cyanobenzyl] indol-3-yl-methyl]3-methoxybenzoic acid, 4-[5-cyclopentylcarbonylamino-3-chloroindol-1-y-methyl]3-met hoxybenzoic acid and FPL55712) was not affected by GppNHp. Taken together the data suggest that LTD4 receptors are coupled to a G-protein that modulates the affinity of agonists but not antagonists binding. | lld:pubmed |
