Imidazoline recognition sites and stomach function.

Radioligand binding experiments carried out in cell membranes from rat and human stomach revealed the existence of non-adrenoceptor [3H]clonidine and [3H]idazoxan binding sites and of [3H]DTG (1,2-di-(2-tolyl)guanidine) binding sites. In rat stomach, specific binding was inhibited by imidazolines and guanidines and by non-imidazoline sigma-site ligands, respectively, at different rank orders of affinity, suggesting the existence of non-I1/non-I2 [3H]clonidine binding sites, I2-imidazoline binding sites as well as sigma 2-like-sites. These sites are not directly related to a postsynaptic contractile effect on rat gastric smooth muscle or to acid release from isolated gastric glands. Finally, we demonstrated that the gastric pathogen Helicobacter pylori is able to form and to release the endogenous imidazoline receptor ligand agmatine and that considerable amounts of agmatine are present in human gastric juice. The quantities of agmatine were higher in gastric juice from H. pylori-positive than H. pylori-negative patients.

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.

Characterization of non-adrenergic [3H]clonidine binding sites in rat stomach: high affinity of imidazolines, guanidines and sigma ligands.

We have identified and characterized non-adrenergic [3H]clonidine binding sites in rat stomach. The binding of [3H]clonidine was rapid, reversible, partly specific (as defined by cirazoline 0.1 nmol/l), saturable and of high affinity. The specific binding of [3H]clonidine to rat stomach membranes was concentration-dependently inhibited by various imidazolines and guanidines including the sigma site ligand 1,2-di-(2-tolyl)guanidine (DTG), by the butyrophenone derivative (+)-3-PPP[(R)-3-(3-hydroxyphenyl)-N-propylpiperidine]; the latter two compounds are also known to exhibit affinity for sigma sites. In contrast, rauwolscine, histamine, ranitidine and the non-hydrolysable GTP-analogue Gpp(NH)p (5' guanylylimidodiphosphate) did not, or with negligible affinity, inhibit [3H]clonidine binding. In most cases, the competition curves were best fitted to a two-site model. The rank order of affinity for the high affinity site (in a few cases for a single detectable site) was as follows: cirazoline > idazoxan > or = DTG > (+)-3-PPP > chlonidine > guanabenz > haloperidol. This rank order is not compatible with the pharmacological properties of either I1- or I2-imidazoline binding sites. However, the ability of haloperidol, (+)-3-PPP and DTG to displace [3H]clonidine (the latter two with high affinity) suggests that the [3H] clonidine binding sites in rat stomach may be related to sigma-like sites.