Table 2. Values of model parameters that... | Rockefeller University Press

Table 2.

Values of model parameters that best simulated dark current in bass single cones

	Parameters^a	Category	Units		N
PDE	${}^{dark}V_{PDE}$	Statistical	µM/s	6.5 ± 1.48^b	6
	K_m	Invariant	µM	26^c
	$β_{dark}$	Statistical	µM/s	12.64 ± 0.89^d	18
GC	${}^{dark}V_{GC}$	Statistical	µM/s	6.5 ± 1.48^b	6
	Vmax	Invariant	µM/s	110^e
	${}^{GC}K_{Ca}$	Invariant	µM	0.1^e
	$n_{G C}$	Invariant		2^e
CNG ion channel	$I_{dark}$	Statistical	pA	27.3 ± 10.5	18
	$n_{C N G}$	Invariant		2.5^f
	${}^{dark}K_{cGMP}$	Invariant	µM	172.3^f
Dark Ca²⁺ influx	${}^{in}J_{Ca}^{dark}$	Statistical	µM/s	14.2 ± 5.4^g	18
	P_f	Invariant		0.34^h
	$C_{H A}$	Adjustable	µM	20.9 ± 12.9ⁱ	18
	$K_{H A}$	Adjustable	µM	0.052 ± 0.024	18
	B	Adjustable		11.5 ± 3.9ⁱ	18
Dark Ca²⁺ efflux	${}^{out}J_{Ca}^{dark}$	Statistical	µM/s	14.2 ± 5.4^g	18
	$J_{C a}^{\max}$	Statistical	pA	4.87 ± 1.88^j	18
	$K_{C a}^{e x c}$	Adjustable	µM	0.019 ± 0.009^j	18

	Parameters^a	Category	Units		N
PDE	${}^{dark}V_{PDE}$	Statistical	µM/s	6.5 ± 1.48^b	6
	K_m	Invariant	µM	26^c
	$β_{dark}$	Statistical	µM/s	12.64 ± 0.89^d	18
GC	${}^{dark}V_{GC}$	Statistical	µM/s	6.5 ± 1.48^b	6
	Vmax	Invariant	µM/s	110^e
	${}^{GC}K_{Ca}$	Invariant	µM	0.1^e
	$n_{G C}$	Invariant		2^e
CNG ion channel	$I_{dark}$	Statistical	pA	27.3 ± 10.5	18
	$n_{C N G}$	Invariant		2.5^f
	${}^{dark}K_{cGMP}$	Invariant	µM	172.3^f
Dark Ca²⁺ influx	${}^{in}J_{Ca}^{dark}$	Statistical	µM/s	14.2 ± 5.4^g	18
	P_f	Invariant		0.34^h
	$C_{H A}$	Adjustable	µM	20.9 ± 12.9ⁱ	18
	$K_{H A}$	Adjustable	µM	0.052 ± 0.024	18
	B	Adjustable		11.5 ± 3.9ⁱ	18
Dark Ca²⁺ efflux	${}^{out}J_{Ca}^{dark}$	Statistical	µM/s	14.2 ± 5.4^g	18
	$J_{C a}^{\max}$	Statistical	pA	4.87 ± 1.88^j	18
	$K_{C a}^{e x c}$	Adjustable	µM	0.019 ± 0.009^j	18

Invariant parameters are taken from the literature, and their values are held constant in all cells and all simulations of dark and photocurrents. “Statistical” are parameters measured experimentally in each and every cone analyzed. “Adjustable” parameters are arrived at by optimized fit of simulated to experimental data and are constrained, when possible, by data in the literature. The values in this table are the mean ± SD.

Experimental data (Holcman and Korenbrot, 2005).

Experimental data (Huang et al., 2004).

Experimental data calculated in each cell from its measured dark current (and hence dark cGMP concentration), PDE K_m, and dark PDE activity. See Eqs. 2.2 and 2.3.

Experimental data in carp cones (Takemoto et al., 2009), but calcium dependence as measured in rods (Koch and Stryer, 1988).

Experimental data (Picones and Korenbrot, 1992; Rebrik et al., 2000).

Computed from Eq. 2.8 since dark current is measured. Vcos is 0.19 pL, about one half of the geometrical outer segment volume. The outer segment is a truncated cone of approximate dimensions: base diameter, 7.5 µm; tip diameter, 3.5 µm; length, 17 µm (Miller and Korenbrot, 1993). Ca²⁺ influx and efflux are the same in the dark.

Experimental data (Ohyama et al., 2000).

Experimental values in aequorin-loaded rods are estimated to be C_HA = 24 µM and B =10 (Lagnado et al., 1992).

The time constant of Ca²⁺ clearance from the bass cone outer segment determined from simulations with these values is 40.0 ± 15.4 msec, comparable to the experimental value of 43 ± 9.8 msec measured in the tiger salamander cone (Sampath et al., 1999). Time constant is the time it takes to reduce cytoplasmic free Ca²⁺ from 0.4 to 0.147 µM (1/e).

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