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 PGF1 alpha 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 X 10(-11) M and 7.1 X 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 PGF1 alpha.