Table 1

Effect of Ca2+ on the affinities of several GABABreceptor ligands on native GABAB receptors

Native receptorsGABABR1a
Ca2+EGTAEGTA/Ca2+Ca2+EGTAEGTA/Ca2+
AgonistsIC50 (μM)IC50(μM)IC50/IC50 IC50(μM)IC50 (μM)IC50/IC50
GABA0.27  ± 0.026.8  ± 0.3625.220.9  ± 0.8780  ± 14337.3
APPA0.036  ± 0.0040.47  ± 0.1013.02.8  ± 0.6149  ± 2053.2
Baclofen0.45  ± 0.090.45  ± 0.021.030.2  ± 1.734.2  ± 7.21.13
AntagonistsIC50(nM)IC50 (nM)IC50/IC50 IC50(nM)IC50 (nM)IC50/IC50
CGP642131.17  ± 0.022.00  ± 0.201.71.53  ± 0.252.74  ± 0.961.79
CGP54626A2.8  ± 0.18.6  ± 1.33.11.90  ± 0.225.4  ± 0.82.84
CGP56999A0.6  ± 0.10.84  ± 0.021.40.38  ± 0.060.26  ± 0.060.68

[125I]CGP64213 binding on rat brain membranes (native receptors) or on membranes prepared from HEK 293 cells expressing GABABR1a receptors (GABABR1a) was measured in the absence and presence of increasing concentrations of the indicated compounds. IC50 values (micromolar for the agonists GABA, APPA and baclofen, and nanomolar for the antagonists CGP64213, CGP54626A and CGP56999A) were determined as described under Experimental Procedures. Experiments were conducted in the presence of 1.8 mM Ca2+ (Ca2+), or in the presence of 1 mM EGTA with no added Ca2+ (EGTA). Values are means ± S.E.M. of at least three independent experiments performed in triplicate. The ratio of the mean IC50 value measured in the absence of Ca2+ over that measured in the presence of Ca2+ is also indicated for each compound (EGTA/Ca2+) for both native and recombinant GABABR1a receptors.