Native receptors | GABABR1b + GABABR2 | |||||
---|---|---|---|---|---|---|
Ca2+ | EGTA | EGTA/Ca2+ | Ca2+ | EGTA | EGTA/Ca2+ | |
Agonists | EC50 (μM) | EC50(μM) | EC50/EC50 | EC50(μM) | EC50 (μM) | EC50/EC50 |
GABA | 2.4 ± 0.1 | 200 ± 20 | 83.3 | 4.0 ± 0.6 | 91 ± 9 | 22.7 |
APPA | 0.45 ± 0.01 | 36 ± 9 | 80.0 | 0.45 ± 0.08 | 13 ± 2 | 28.9 |
Baclofen | 3.2 ± 0.2 | 17 ± 2 | 5.3 | 7.0 ± 0.6 | 6.6 ± 1.1 | 0.94 |
[35S]GTPγS binding on rat brain membranes (native receptors) or on membranes prepared from Chinese hamster ovary K2 cells stably expressing the human GABABR1b and the rat GABABR2 was measured in the absence and presence of increasing concentrations of the indicated compounds. EC50 values (micromolar) were determined as described under Experimental Procedures. Experiments were conducted in the presence of 1.8 mM Ca2+, or in the presence of 0.2 mM EGTA with no added Ca2+. Values are means ± S.E.M. of three to seven independent experiments performed in triplicate. The ratio of the mean EC50 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 GABAB receptors.