Table 1.
Subunit-specific properties of nAChRs
Subtype | EC501-a (μm) | IC501-b (μm) ±Ca | Desensitization at 500 nm (%) ±Ca | Desensitization time course1-c (τd; min) | Desensitization recovery1-d (τ; min) ±Ca | ||
---|---|---|---|---|---|---|---|
+Ca | −Ca | +Ca | −Ca | ||||
α3β4 | 62 | 65 | 1.15 | 39 | 32.5 ± 2.5 (3) | 36.8 ± 4.8 (7) | 42.6 ± 5.5 (4) |
α4β4 | 9 | 11 | 0.042 | 87 | 35.5* ± 3.2 (5) | 16.2 ± 2.8 (3) | 413.81-f ± 305.5 (2) |
α3β2 | 123 | 75 | 0.33 | 47 | ≈5 (3)1-e | 3.9 ± 2.1 (4) | 11.7 ± 4.1 (2) |
α4β2 | 15 | 12 | 0.061 | 71 | 3.7 ± 0.9 (4) | 2.7 ± 0.3 (2) | 86.9 ± 64.7 (4) |
α7 | 90* | 234 | 6.2 (−Ca) | 1 (−Ca) | 0.5 ± 0.03 (3) | 1.9 ± 0.5 (4) | 1.9 ± 0.5 (5) |
1.3* (+Ca) | 12 (+Ca) |
EC50 calculated from the activation concentration–response curve in calcium-containing (+Ca) and calcium-free (−Ca) media.
IC50 calculated from the concentration–response curves constructed from the mean desensitization produced in both +Ca and −Ca media. These combined data are indicated by ±Ca.
The time constant (τd) from a single exponential fit to the time course of desensitization at near IC50 concentrations of nicotine. These concentrations were 1 μm (α3β4), 100 nm (α4β4), 300 nm (α3β2), 300 nm (α4β2), 10 μm (−Ca, α7), and 3 μm (+Ca, α7). The number of measurements is indicated in parentheses.
The time constant (τ) from a single exponential fit to the time course of recovery from desensitization at 1 μm nicotine for all subtypes except α7 (10 μm). The number of measurements is indicated in parentheses. In the majority of experiments, the exponential was fit by assuming full recovery to control values.
The reported values are estimates. It was difficult to obtain an accurate exponential fit for some of the α3β2 data sets, because the interpulse interval for the test pulses was large (5–10 min) and desensitization mainly was complete by the time of the first test pulse.
Two additional oocytes expressing α4β4 receptors showed no recovery after 20–30 min wash.
Indicates significance (p < 0.05), as compared with data obtained in calcium-free media.