Specific binding of a stable prostacyclin analogue (Iloprost) to rat oxyntic mucosa

It has been reported that prostacyclin (PGI₂) is the predominant species of prostanoid in rat oxyntic mucosa. However since PGI₂ is inactivated under physiological conditions it has not been possible to demonstrate specific PGI₂ binding to the rat stomach. Therefore a stable PGI₂ analogue, Iloprost, was chosen as ligand in this study. Binding of labelled Iloprost to the 20,000 xg homogenate fraction of rat oxyntic mucosa was specific, dissociable, saturable and dependent upon the temperature and time of incubation. Neither tritiated PGE₂ nor 6 keto PGF_1α displayed any significant specific binding to rat stomach. A Scatchard plot of the equilibrium binding data for Iloprost was curvilinear and could be resolved into at least two binding sites. The average parameters determined from Scatchard analysis were: dissociation constants of 1.8 × 10⁻¹¹ M and 7.1 × 10⁻⁸ M and corresponding binding site concentrations of 12.0 pmole/mg and 4800 pmoles/mg protein respectively. PGI₂ was less potent than unlabelled Iloprost in displacing ³H-Iloprost from its binding site. The addition of PGE₂ to the incubation medium resulted in an increase in ³H-Iloprost binding. It is concluded that rat oxyntic mucosa has specific binding sites for PGI₂-like agents but not for either PGE₂ or 6 keto PGF_1α.