Table 4. Values of model parameters that... | Rockefeller University Press

Table 4.

Values of model parameters that best simulate photocurrents in dark-adapted bass single cones: Response to 2-s steps of light

	Parameter	Category^a	Units	Cone 1			Cone 2			Cone 3
VP*	Intensity	Statistical	VP*/s	61	1,177	25,872	171	773	35,236	50	1,975	41,466
	$γ_{m a x}$	Adjustable	µM/s	100^b	100	90	100	100	63	100	100	55
T*	$Ψ_{0}$	Adjustable	1/s	250^c	250^c	250^c	250	250	250	220	220	220
PDE*	$β_{s u b}$	Adjustable	µM/s per molecule	0.439^d	0.439^d	0.439^d	0.347	0.347	0.347	0.317	0.317	0.317
	${}^{init}α_{PDE}$	Adjustable	1/s	5^e	6.5	2	9	10	—	3	5	—
	${}^{final}α_{PDE}$		1/s	—	9	97	—	—	55	—	8.4	42
	$k_{α_{PDE}}$		1/s	—	9	2	—	—	—	—	11	—
Ca²⁺ influx	$K_{H A}$	Adjustable	µM	0.073	0.073	0.073	0.052	0.052	0.052	0.098	0.098	0.098
	$C_{H A}$	Adjustable	µM	21.1	21.1	21.1	28.2	28.2	28.2	30.8	30.8	30.8
	B	Adjustable		11.3	11.3	11.3	10	10	10	15	15	15
Ca²⁺ efflux	$J_{C a}^{\max}$	Statistical	pA	4.59	4.59	4.59	5	5	5	3.56	3.56	3.56
	$K_{C a}^{e x c}$	Adjustable	µM	0.025	0.025	0.025	0.025	0.025	0.025	0.025	0.025	0.025

	Parameter	Category^a	Units	Cone 1			Cone 2			Cone 3
VP*	Intensity	Statistical	VP*/s	61	1,177	25,872	171	773	35,236	50	1,975	41,466
	$γ_{m a x}$	Adjustable	µM/s	100^b	100	90	100	100	63	100	100	55
T*	$Ψ_{0}$	Adjustable	1/s	250^c	250^c	250^c	250	250	250	220	220	220
PDE*	$β_{s u b}$	Adjustable	µM/s per molecule	0.439^d	0.439^d	0.439^d	0.347	0.347	0.347	0.317	0.317	0.317
	${}^{init}α_{PDE}$	Adjustable	1/s	5^e	6.5	2	9	10	—	3	5	—
	${}^{final}α_{PDE}$		1/s	—	9	97	—	—	55	—	8.4	42
	$k_{α_{PDE}}$		1/s	—	9	2	—	—	—	—	11	—
Ca²⁺ influx	$K_{H A}$	Adjustable	µM	0.073	0.073	0.073	0.052	0.052	0.052	0.098	0.098	0.098
	$C_{H A}$	Adjustable	µM	21.1	21.1	21.1	28.2	28.2	28.2	30.8	30.8	30.8
	B	Adjustable		11.3	11.3	11.3	10	10	10	15	15	15
Ca²⁺ efflux	$J_{C a}^{\max}$	Statistical	pA	4.59	4.59	4.59	5	5	5	3.56	3.56	3.56
	$K_{C a}^{e x c}$	Adjustable	µM	0.025	0.025	0.025	0.025	0.025	0.025	0.025	0.025	0.025

Values of invariant parameters first used to compute dark and flash photocurrents are not repeated here. They have the same values listed in Table 2.

$γ_{m a x}$ experimental value is 100–150 s⁻¹ (Tachibanaki et al., 2005).

This value was initially constrained to 150 s⁻¹ from experimental data (Leskov et al., 2000).

Experimental values of PDE* $k_{c a t}$ range from 2.2 to 8 × 10³ s⁻¹ (Dumke et al., 1994; D’Amours and Cote, 1999; Muradov et al., 2009). From the definition of $β_{s u b}$ (Eq. 3.11) and the highest $k_{c a t}$ value, the mean value $β_{s u b}$ 0.334 (Table 5) indicates that the effective intracellular volume for cGMP is, on average, 0.08 pL, rather than the 0.19 pL used throughout these calculations. This factor of approximately two can probably be accounted for by the fact that the volume, in fact, changes some from cell to cell and is not the rigid assigned value of 0.19 pL. There is experimental uncertainty because the size of each and every cone recorded from was not measured. Also, the actual concentration of cGMP may be a little smaller than the computed value because there is intracellular cGMP binding (Olson and Pugh, 1993), a fact that would be revealed as a smaller apparent volume.

This value was initially constrained to 10 s⁻¹ at dim lights from the analysis of dark current noise in bass single cones (Holcman and Korenbrot, 2005).

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