Potency of μ-opioid agonists to stimulate binding of [35S]GTPγS to different PTX-insensitive Gα proteins C6μ cells transiently transfected with C351(2)I-PTXi Gα subunits were treated overnight with 100 ng/ml PTX. Agonist stimulation of [35S]GTPγS (100 pM) binding in membranes from these cells was measured with at least five concentrations of each agonist in the presence of 30 μM GDP as described under Materials and Methods. Shown are the mean EC50 ± S.E.M. from at least three separate experiments, each performed in duplicate, and the relative potency compared with DAMGO (in parentheses).
Ligand | EC50 (Potency Relative to DAMGO)a | ||||||||
|---|---|---|---|---|---|---|---|---|---|
| Gαi3 | GαoA | Gαi1 | Gαi2 | Gαi2/Gαi3 | |||||
| nM | |||||||||
| DAMGO | 28 ± 9 (1.0 ± 0.3) | 48 ± 9 (1.0 ± 0.2) | 207 ± 73 (1.0 ± 0.4) | 145 ± 36 (1.0 ± 0.2) | 5.2 | ||||
| Etorphine | 0.30 ± 0.02 (0.014 ± 0.001) | 0.45 ± 0.07 (0.010 ± 0.002) | 0.94 ± 0.08 (0.005 ± 0.0004) | 1.3 ± 0.1 (0.009 ± 0.0002) | 4.3 | ||||
| Fentanyl | 33 ± 12 (1.2 ± 0.4) | 59 ± 18 (1.2 ± 0.4) | 151 ± 50 (0.7 ± 0.2) | 170 ± 30 (1.2 ± 0.2) | 5.2 | ||||
| Endomorphin-1 | 28 ± 2 (1.0 ± 0.1) | 71 ± 4 (1.5 ± 0.1) | 112 ± 3 (0.54 ± 0.02) | 134 ± 11 (0.92 ± 0.10) | 4.9 | ||||
| Endomorphin-2 | 45 ± 5 (1.6 ± 0.2) | 132 ± 28 (2.8 ± 0.6) | 206 ± 57 (1.0 ± 0.3) | 219 ± 33 (1.5 ± 0.2) | 4.9 | ||||
| Morphine | 31 ± 6 (1.1 ± 0.2) | 57 ± 17 (1.2 ± 0.4) | 168 ± 109 (0.82 ± 0.53) | 162 ± 61 (1.1 ± 0.4) | 5.2 | ||||
| Meperidine | 3550 ± 960 (130 ± 30) | 5020 ± 1340 (110 ± 30) | 11,200 ± 1680 (54 ± 8) | 8180 ± 740 (56 ± 5) | 2.3 | ||||
↵ a Differences in the potency of ligands across the Gα proteins expressed were significant for DAMGO [F(3,8) = 4.2, P < 0.05], etorphine [F(3,8) = 38.8, P < 0.001] (Gαi3 = Gαo > Gai1, P < 0.01; Gαi3 = Gαo > Gai2, P < 0.001), fentanyl [F(3,8) = 4.7, P < 0.05], endomorphin-1 [F(3,8) = 58, P < 0.0001] (Gαi3 > Gαo, P < 0.01; Gαi3 = Gαi1, P < 0.001; Gαi3 > Gαi2, P < 0.001), endomorphin-2 [F(3,8) = 5.0, P < 0.05], and meperidine [F(3,8) = 7.6, P < 0.01]