Rates of activation calculated for cells expressing RGS-insensitive Gα proteins
Kact was calculated from the EC50 for inhibition of forskolin-stimulated cAMP accumulation (equation 11). Ki values measured in the presence of NaCl and GDP were used for Kd (DAMGO Ki = 279 nM; morphine Ki = 132 nM; from Emmerson et al., 1996) and khydrol = 0.1 s−1 for absence of RGS proteins (from Shea et al., 2000).
| Cells | μ-Opioid Receptors | [35S]GTPγS Maximal DAMGO Stimulation | kact | |
|---|---|---|---|---|
| DAMGO | Morphine | |||
| pmol/mg protein | % | s−1 | ||
| Gαi2CIGS/A | 6.6 ± 0.4 | 380 ± 80 | 13 | 1.8 |
| Gαi2CIGS/B | 4.8 ± 1.1 | 210 ± 30 | 8.9 | 1.2 |
| GαoCIGS/B | 6.4 ± 0.4 | 65 ± 20 | 25 | 1.5 |
| GαoCIGSA | 3.6 ± 0.6 | 190 ± 10 | 16 | 2.3 |
| Gαi3CIGS/A | 4.5 ± 1.4 | 32 ± 6 | 7.2 | 0.45 |
| Gαi3CIGS/B | 0.08 ± 0.07 | 6 ± 3 | 0.8 | N.A. |
N.A., not available.