Table 2.

Activation rate constants of wild-type, hybrid and mutant αY93F and αW149F receptors in the presence of ACh

Receptor	k+1 (μm−1 s−1)	k−1 (s−1)	Kd1 or Kd (μm)	k+2(μm−1 s−1)	k−2 (s−1)	Kd2 (μm)	β (s−1)	α (s−1)	Θ
Wild type	—	18000 ± 422	164	110 ± 20	—	—	(60 000)	1321 ± 15	45
αY93F hybrid	(110)	(18 000)	164	27 ± 3	10 363 ± 2151	384	5657	2099 ± 35	2.7
αY93F mutant	—	5153 ± 763	628	8.2 ± 1.2	—	—	1200	2919 ± 65	0.42
αW149F hybrid	(110)	(18 000)	164	16.3 ± 0.2	(27 320)	1676	3337	1909 ± 17	1.75
αW149F mutant	—	27320 ± 15517	1910	14.3 ± 8.0	—	—	648	1651 ± 22	0.39

The rate constants and error limits were determined from single-channel kinetic analysis using Model 1 (wild-type and mutant receptors) or Model 2 (hybrid receptors). The opening rate constants for hybrid and mutant receptors were constrained to values obtained from fitting the β′ curve (Table 1). The rate constants for the unmutated site in the hybrid receptor were constrained to values for the wild-type receptor binding site. K_D was calculated as k₋/k₊; Θ is β/α. The values for the wild-type receptor are from Akk & Auerbach (1996). The opening rate constant for the wild-type receptor is from Maconochie & Steinbach (1998). For the hybrid aY93F receptor, the rate constants were estimated from simultaneous fitting of data from two patches recorded with 200 μm (4366 events) or 500 μm ACh (4388 events). The dead time was 45 μs. For the hybrid αW149F receptor, the rate constants were estimated from simultaneous fitting of data from three patches recorded with 200 μm (7431 events), 500 μm (10 346 events) or 1000 μm ACh (7943 events). The dead time was 60 μs. For the mutant receptor, the rate constants were estimated from simultaneous fitting of data from three patches recorded at 1000 μm (3213 events), 2000 μm (7585 events) or 5000 μm ACh (1324 events). The dead time was 60 μs.