Vol. 285, Issue 1, 170-177, April 1998

Characterization of I₂ Imidazoline and  Binding Sites in the Rat and Human Stomach

Gerhard J. Molderings, Kurt Donecker, Maria Burian, W. Alexander Simon, Detlev W. Schröder and Manfred Göthert

Institute of Pharmacology and Toxicology (G.J.M., K.D., M.B., M.G.), University of Bonn, Bonn, Germany, Byk Gulden Lomberg GmbH (W.A.S.), Konstanz, Germany and Evangelisches Krankenhaus (D.W.S.), Bonn, Germany

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 (K_d = 11.1 nM and 12.4 nM, respectively) and [³H]DTG (K_d = 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 -site ligands, respectively, at different rank orders of affinity, which suggests the existence of I₂-imidazoline binding sites as well as ₂-sites. In two functional models, the direct effects of I₂-site ligands and ₂-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 µM did not induce acid secretion from rabbit isolated gastric glands. Our data provide evidence that the rat stomach is endowed with ₂ 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  binding sites in the human stomach. Neither the I₂ and [³H]clonidine binding sites nor the  sites in rat stomach are directly related to a postsynaptic effect on gastric smooth muscle or to acid release from isolated gastric glands.