Table 3. Inhibition of fluorowillardiine binding to rat brain membranes by other nucleotides.

	IC50 (μM)	N
GDP	228 ± 18	4
2′-deoxy-GDP	209 ± 15	2
2′,3′-dideoxy-GTP	231 ± 9	3
GDP-mannose	389 ± 5	3
GDP-fucose	394 ± 13	4
Cyclic GMP	612 ± 14	2
IMP	2758 ± 195	3

Inhibition constants were determined in rat brain membranes by measuring binding of [³H]fluorowillardiine (20 nM) at 0°C in the presence of 3–5 concentrations of the nucleotide and fitting a sigmoidal inhibition curve (with n_Hill = 1) to the data points. No Cheng-Prusoff correction was applied because brain receptors exhibit two or more affinity components.

In additional experiments (not shown), no detectable inhibition was observed with 1 or 2 mM ATP, UDP-glucuronic acid or UDP-N-acetyl-galactosamine. Inhibition by GDP was not significantly influenced by inclusion of 100 μM Ca²⁺, 2 mM Mg²⁺, 300 μM Zn²⁺, 3.2 mM CX546 (positive AMPA receptor modulator; Kessler and Arai, 2005), or 200 μM GYKI 52466 (negative modulator; Donevan and Rogawski, 1993). Also, 5–10 mM ribose-5′-phosphate did not significantly inhibit [³H]fluorowillardiine binding (<3%) and it did not antagonize the inhibition by GDP.