Bicarbonate‐dependence of responses to ethylenediamine in the guinea‐pig isolated ileum: involvement of ethylenediamine‐monocarbamate

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Abstract
γ‐Aminobutyric‐acid (GABA)‐mimetic responses were induced by ethylenediamine (EDA) in the isolated ileum of the guinea‐pig maintained in bicarbonate buffered Krebs‐Henseleit (KBC) solution, pH 7.4, 37°C, the responses consisting of a contraction followed by a relaxation. There were no such responses to EDA in bicarbonate‐free phosphate buffered (KPO) or HEPES buffered (KHO) Krebs solution, gassed with 100% O2, pH 7.4, 37°C, yet the ileum responded to GABA in bicarbonate‐free Krebs solution. Similar GABA‐mimetic responses were induced by EDA in the isolated ileum maintained in bicarbonate‐free KPO or KHO modified Krebs solution, gassed with O2, if HCO3− (5 mm) was first added immediately before the test dose of EDA (0.1–1 mm), the threshold [HCO3−] being 2 mm for EDA‐induced responses in these preparations. However, ileal GABA‐mimetic responses were induced in bicarbonate‐free KPO or KHO solutions by EDA that had been pretreated with carbon dioxide, where the final [HCO3−] in the bath did not exceed 25 μm. Ethylenediamine monocarbamate (synthetic EDAC) released [3H]‐GABA from preloaded segments of ileum maintained in bicarbonate‐free KPO or KHO solution containing amino‐oxyacetic acid and β‐alanine, the release being sensitive to 3‐mercaptopropionic acid which prevents GABA release. EDA itself did not evoke any such release in the absence of bicarbonate, but released [3H]‐GABA from segments maintained in KBC solution. GABA‐mimetic responses were induced by EDAC in the isolated ileum maintained in bicarbonate‐free KPO solution, as was a δ‐aminovalerate‐sensitive depression of ileal twitch responses elicited by transmural stimulation, all of which were also sensitive to 3‐mercaptopropionic acid. It is concluded that GABA‐mimetic responses to EDA in the isolated ileum of the guinea‐pig, maintained in normal Krebs bicarbonate medium, result from the release of endogenous GABA by ethylenediamine monocarbamate formed through the rapid reaction of EDA with the carbon dioxide of bicarbonate buffered Krebs solution. Furthermore, in the ileum, HCO3− ions per se are not necessary for this GABA‐releasing property of EDA if the latter is first converted to the monocarbamate, since synthetic ethylenediamine monocarbamate elicits ileal GABA‐mimetic responses in the total absence of bicarbonate.