On Histamine - Receptor Interaction

The effects of external divalent cations on drug-receptor interaction were examined using guinea pig taenia. The effects of histamine on membrane potential of taenia in various solutions were also examined using the sucrose gap method. Ca2+ and Me2+ played an important role in the combination of histamine with its receptor. Sr2+, Mn2+ and Co2+ do not substitute for. Ca2+ and Me2+ in this interaction. None of divalent cations used were involved in the competitive antagonist-receptor interaction or in the irreversible antagonist-receptor interaction. Histamine which induced membrane depolarization and increased tension in normal Locke Ringer solution had no effect on the membrane potential and tension in Mg2+free Se2+-solution. The mechanism for this phenomenon has been discussed herein. In a previous paper (1) it was reported that Ca2+ or Mg2+ is involved in the interactions of the agonists with their receptors and that neither ion is involved in the combinations of the competitive antagonists and ƒÀ-chloroethylamines with their receptors. In this paper the effects of Sr2+, Mn2+ and Co2+ on the histamine-receptor interaction were studied. As Hotta and Tsukui (2) and Bando et al. (3) reported that actions of acetylcholine and histamine were greatly reduced, when Ca2+ in Locke solution was substituted for Sr2+. The mechanism for this phenomenon was also investigated herein. METHODS A. A piece of taenia isolated from the caecum of the guinea pig (250 to 350 g) was suspended in a 10 ml organ bath filled with Locke Ringer solution, kept at 32•Ž and bubbled with air. Responses of the taenia were recorded through an isotonic lever. B. In order to study effects of divalent cations on histamine-receptor interaction, "drug -receptor protection" technique originally described by Furchgott (4) was used as accordingly in a previous paper of Takagi, Takayanagi and Liao (1). A cumulative concentration action curve of histamine was obtained with taenia suspended in the normal Locke Ringer solution (5). The taenia was incubated with modified Locke Ringer solution for 20 min. A protecting agent and dibenamine, an irreversible antagonist, were applied to the taenia suspended in the modified Locke Ringer solution. Thus the taenia was incubated with the protecting agents and dibenamine in the modified Locke Ringer 78 C.S. LIAO, I. TAKAYANAGI & K. TAKAGI solution for 10 min. After that, the taenia was washed with the modified Locke Ringer solution for 20 min. In addition, the taenia was washed with the normal Locke Ringer solution for 2 hr and the cumulative concentration action curve of histamine was again obtained. C. To examine effects of divalent cations on change of the membrane potential by histamine, experiments were also made on strips of the taenia from the guinea pig caecum. A 10 mm in situ length of the taenia was dissected free from the underlying tissue and mounted in the sucrose gap apparatus originally described by Biilbring and Burnstock (6). Bath fluid was gassed with a mixture of 95% O2 and 5% CO2 and kept at 32•Ž. The membrane potential was amplified by a high gain d.c. amplifier and recorded by a pen oscillograph. Tension change was simultaneously recorded by using a mechanoelectric transducer (RCA 5734). All the results were presented as the means of at least 8 experiments. D. The following solutions were used as bath fluids. Locke Ringer solution : NaC1, 154 mM; KCL, 5.6 mM ; CaCl2, 2.2 mM; MgCl2, 2.1 mM; NaHCO3, 5.9 mM and glucose, 2.8 mM. "Ca2+-free solution" : CaCl2 was removed from Locke Ringer solution and 0.1 mM GEDTA (glycoletherdiamine tetraacetic acid) was added. "Mg2+-free solution": MgCl2 was removed from Locke Ringer solution. "Ca2+and Mg2+-free solution": CaCl2 and MgCl2 were removed and 0.1 mM GEDTA was added. "Sr2+-solution": Ca2+ in Locke Ringer solution was substituted by Sr2+. "Mn2+ (or Co2+)-solution": Ca2+ in Locke Ringer solution was substituted by Mn2+ (or Co2+). "Mg2+-free Sr2+-solution": Ca2+ in Locke Ringer solution was substituted by Sr2+ and MgCl2 was removed. "Mg2+free Mn2+ (or Co2+)-solution": Ca2+ in Locke Ringer solution was substituted by Mn2+ (or Co2+) and MgCl2 was removed. Drugs used. An agonist : histamine hydrochloride. A competitive antagonist : di-