Table 3. Potency of NE, DA, clonidine, 7-OH-PIPAT and quinpirole obtained from G protein activation experiments mediated by α_2A and α_2C adrenoceptors coupled to the different Gαi/o subtypes.

Gα subunit	Receptor	NE	DA	Clonidine	7-OH-PIPAT	Quinpirole	DA/NE
Gαil	α2A	11±2**	170 ± 40	3±1	80 ± 20 *	700 ±250	15
	α2C	90 ±30	150 ± 40	6±2	(11 ±4)	ND	1.7
Gαi2	α2A	1.3 ±0.3*	30 ±3	2.0 ±0.8	120 ± 6 *	(1000 ±300)	23
	α2C	0.4 ±0.2	5±3	3±1	10 ±2	(200 ±100)	12.5
Gαi3	α2A	0.6 ±0.2	15 ±5	1.0 ±0.2	60 ± 20 *	(700 ± 400)	25
	α2C	0.4 ±0.2	30 ±20	4±1	5±3	400 ±100	75
Gαol	α2A	3.0 ±0.5*	80 ±10	2.0 ± 0.4*	100 ± 20	1300 ± 200**	27
	α2C	19 ±6	126 ±8	(12 ±5)	20 ±3	230 ±50)	7
Gαo2	α2A	6±1*	100 ± 20	4.0 ± 0.2**	66 ±7*	820 ± 80 ***	17
	α2C	50 ±10	140 ± 20	11 ± 2	14 ±7	(100 ±10)	2.8

Potency (EC₅₀ values, in nM) of NE, DA, clonidine and D₂-like receptor ligands obtained from G protein activation experiments mediated by α_2A and α_2C coupled to the different Gαi/o subtypes (Figs. 3 and 4). EC₅₀ values were obtained from a sigmoidal concentration-response function adjusted by nonlinear regression analysis and are expressed as means ± S.E.M. of 3 to 11 experiments performed in triplicate. In parenthesis, values corresponding to experiments showing low efficacy, E_max lower than 50% (Table 4). DA/NE: ratio of EC₅₀ values of DA and NE for each receptor and Gαi/o protein subtype. Statistical differences between α_2A and α_2C adrenoceptors were calculated by non-paired, two-tailed Student's t test;

^*p<0.05,

^**p<0.01 and

^***p<0.001.