Treatment with N‐ethylmaleimide selectively reduces adenosine receptor‐mediated decreases in cyclic AMP accumulation in rat hippocampal slices

Abstract

1 N-ethylmaleimide (NEM) has been reported to interact with the GTP-binding N_i-protein; we have examined its effect on adenosine receptor binding in feline cortical membranes and on adenosine-receptor mediated effects on cyclic AMP accumulation in rat hippocampal slices. 2 Treatment of cortical membranes with NEM (100 μM for 5 min) altered the binding of [³H]-phenylisopropyladenosine (PIA) from being almost exclusively to a single class of high affinity sites (K_D = 1.65 nM) to binding at two classes of sites (K_DH = 2.1 nM, K_DL = 102 nM). The total number of binding sites was similar (825–845 fmol mg⁻¹ in control membranes, 944–1428 fmol mg⁻¹ in NEM-treated membranes). 3 In rat hippocampal slices treated with forskolin (0.3 μM) L-PIA produced a biphasic effect on cyclic AMP accumulation: an inhibition at 0.03 to 1 μM and at higher concentrations, a stimulation. Treatment with 50 μM NEM selectively inhibited the inhibitory phase, causing stimulation at lower concentrations of L-PIA. At 50 μM, NEM did not alter basal or forskolin-stimulated cyclic AMP accumulation but at higher concentrations inhibition was observed. 4 It is concluded that NEM can, in certain doses, selectively block adenosine A₁-receptor-mediated effects without affecting A₂-receptor-mediated actions in the same tissue. It is suggested that this is due to NEM affecting the N_i guanine nucleotide binding protein.