Binding of [3H]ICIA 5165, an H2-receptor antagonist to guinea pig gastric mucosa

ICIA 5165, 2-guanidino-4-4-(2-cyano-3-methylguanidino)butyl] thiazole, a selective histamine H₂-receptor antagonist was radiolabelled with tritium to a specific activity of 50.8 Ci/mmoll for use in binding studies. Radiolabelling did not impair bioactivity.Binding characteristics of ³H]ICIA 5165 to guinea pig gastric mucosa were determined. Ligand binding was rapid, reaching equilibrium within five minutes at 0°C, reversible and saturable. Specific ³H]ICIA 5165 binding had an equilibrium dissociation constant of 1.29×10^–8 M, determined by Scatchard plot analysis, and of 1.02×10^–8 M, calculated from the ratio of the dissociation to association rate constants. A Hill number, n_H, of 1.02 was determined for the specific binding component. Specific binding of ³H]ICIA 5165 to gastric mucosal supernatant was not inhibited by methapyrilene, diphenhydramine, mepyramine, d-chlorpheniramine or I-chlorpheniramine (all at 10^–7 M), or by atropine or propranolol (both at 10^–6 M). Specific ³H]ICIA 5165 binding was inhibited in a concentration dependent manner by non-radioactive ICIA 5165 and tiotidine, as well as by a variety of other agents, with H₂ agonist or H₂ antagonist properties. In competition experiments, however, difficulties encountered in accurately defining the degree of specific binding indicate some reservation should be observed in interpreting these results.