Comparison of properties of α4β2 AChR subtypes
Subtype | (α4β2)2β2 | (α4β2)2α4 | (α4β2)2α5 | (α4β2)2β3 |
|---|---|---|---|---|
| Accessory subunit | β2 | α4 | α5 | β3 |
| Expression level in cell line (pmol/mg protein)a | 1.25 | 1.25 | 1.75 | 30 |
| ACh sensitivity for activation | ≡1 | 0.004 | 0.19 | 0.0033 |
| EC50 | 0.23 | 57 | 1.2 | 48 |
| Nicotine sensitivity for activation (μM) | ≡1 | 0.044 | 0.32 | 0.067 |
| EC50 | 0.12 | 2.7 | 0.37 | 1.78 |
| Nicotine sensitivity for long-term desensitization (μM) | ≡1 | ≈1 | 0.68 | 0.23 |
| IC50 | 0.006 | 0.006 | 0.0089 | 0.027 |
| Nicotine sensitivity for up-regulation (μM)b | ≡1 | ≈0 | 1 | ≈0 |
| EC50 | 0.35 | 0.035 | ||
| Ca2+ permeabilityc | ≡1 | 2.4 | 5.8 | 2.6 |
↵ a In the α4β2 cell line, the (α4β2)2β2 and (α4β2)2α4 stoichiometries are present in the ratio of 1:9 based on normalized peak currents in response to ACh (Nelson et al., 2003) The number of [3H]epibatidine sites per milligram of protein reported is the total for both.
↵ b The lack of nicotine-induced upregulation in the α4β2β3 cell line does not necessarily mean that β3 prevents upregulation. This line has excess β3 and expresses 25-fold more AChR than the parent line, probably depleting pools of α4 or β2.
↵ c The Ca2+ permeability values reflect the relative Ca2+ permeabilities of these AChR subtypes expressed in X. laevis oocytes as reported in Tapia et al. (2007), whereas all other values were measured in cell lines.