TABLE IV.

Kinetic Parameters for ACh Activation of Human Adult nAChRs

	−70 mV	−110 mV	−150 mV
k+1	211 ± 11	140 ± 9	138 ± 15
k−1	4,126 ± 260	2,654 ± 248	1,845 ± 228
K1	19.6	19	13.4
k+2	165 ± 4	148 ± 5	139 ± 5
k−2	20,261 ± 262	13,410 ± 231	10,345 ± 226
K2	123	90.6	74.4
β	53,211 ± 1,163	54,585 ± 1,254	57,352 ± 1591
α	2,290 ± 49	1,770 ± 46	1,373 ± 48
Θ	23.2	30.8	41.8
k+b	38 ± 4	77 ± 3	90 ± 3.2
k−b	149,000 ± 4,122	97,761 ± 2,004	105,963 ± 1,671
KB	3,921	1,270	1,177

Parameters were derived from maximum likelihood fitting of Scheme A (Fig. 3) to single-channel data obtained at ACh concentrations from 6–300 mM and are expressed ± SE. The rate constants are defined in the legend to Fig. 3. Association rate constants are given in units of μM⁻¹ s⁻¹, and all others in units of s⁻¹. K₁, K₂ and K_B (mM) are dissociation constants governing the binding of ACh to the high- and low-affinity agonist binding sites and the channel block site, respectively. Θ is the equilibrium constant governing channel opening. K₁, K₂, K_B, and Θ were calculated from their constituent rate constants.