The influence of bicarbonate ions on the GABA-mimetic activity of ethylenediamine

Abstract

A study was performed to investigate the GABA-mimetic activity of ethylenediamine (EDA) and piperazine at mammalian gamma-aminobutyric acid (GABA) receptors using radioligand binding assays and in vitro isolated tissues. The potency of ethylenediamine and piperazine as inhibitors of the binding of GABA receptors to synaptic membranes from rat brain was measured in Tris-buffers and Krebs-Henseleit solution (KHS). The potency of ethylenediamine and piperazine at GABAA and ethylenediamine at GABAB receptors was raised if Krebs-Henseleit solution was used for the assay. Piperazine was inactive at GABAB receptors. The potency of the antagonist of GABAA receptors bicuculline methobromide, was also increased in Krebs-Henseleit when compared with Tris-citrate buffer. Of the ions present in Krebs-Henseleit, bicarbonate ions were responsible for the increase in the GABA-mimetic potency of ethylenediamine and piperazine. Addition of either NaHCO3 or KHCO3 (25 mM) to Tris-HCl buffer (for GABAA binding) or Tris-HCl plus 2.5 mM CaCl2 (for GABAB binding) yielded IC50 values similar to those measured in Krebs-Henseleit solution. Bicarbonate ions also enhanced the ability of ethylenediamine to potentiate the binding of [3H]diazepam to membranes from rat brain (raising both the potency of ethylenediamine and its maximum effect) in this system. In the absence of HCO-3 ions, ethylenediamine potentiated the binding of [3H]diazepam by raising the maximum binding capacity (Bmax) without changing the affinity (Kd) of the receptors. Potassium bicarbonate (25 mM) caused ethylenediamine to further potentiate the binding of [3H]diazepam by changing both Bmax and Kd.(ABSTRACT TRUNCATED AT 250 WORDS)