Characterization of I2 imidazoline and sigma binding sites in the rat and human stomach

Radioligand binding experiments were carried out to identify and characterize nonadrenoceptor [3H]idazoxan binding sites and [3H](1, 2-di-(2-tolyl)guanidine) binding sites in the rat and human stomach. Furthermore, we examined two selected aspects of their potential functional significance. Binding of [3H]idazoxan (Kd = 11.1 nM and 12.4 nM, respectively) and [3H]DTG (Kd = 932 nM and 242 nM, respectively) to cell membranes from rat and human stomach was rapid, reversible, specific and saturable. In rat stomach, binding of the radioligands was inhibited by imidazolines and by nonimidazoline sigma-site ligands, respectively, at different rank orders of affinity, which suggests the existence of I2-imidazoline binding sites as well as sigma2-sites. In two functional models, the direct effects of I2-site ligands and sigma2-site ligands on gastric smooth muscle and glands were investigated. (1) Cirazoline, clonidine and 4-chloro-2-(2-imidazolin-2-ylamino)-isoindoline (BDF 6143) failed to contract the longitudinal muscle of the rat stomach fundus; BDF 6143 also failed to induce relaxation of this preparation when it was precontracted with 30 mM KCl. (2) Clonidine, idazoxan, BDF 6143, 1, 2-di-(2-tolyl)guanidine, agmatine and (R)-3-(3-hydroxyphenyl)-N-propylpiperidine up to 100 microM did not induce acid secretion from rabbit isolated gastric glands. Our data provide evidence that the rat stomach is endowed with sigma2 sites and I2 binding sites in addition to the previously identified non-I1/non-I2 [3H]clonidine binding sites. Our experiments also offer basic evidence of the existence of I2 and sigma binding sites in the human stomach. Neither the I2 and [3H]clonidine binding sites nor the sigma sites in rat stomach are directly related to a postsynaptic effect on gastric smooth muscle or to acid release from isolated gastric glands.