Table 2. Downstream phototransduction cascade parameters.

Symbol	Description	Default value a	Model 1 value b	Units
βDark	Dark rate constant of cGMP hydrolysis	3.2	4	s−1
αmax	Maximal rate of cGMP synthesis by GC	120	150	µM/s
fCa	Fraction of CNGC current carried by Ca2+	0.12
KGCAP	Ca2+ concentration parameter of GCAP	80		nM
mGCAP	Ca2+ cooperativity of GCAP	1.5
JcG, max	Maximal CNGC current for the OS	2000		pA
KcG	cG concentration parameter of CNGC	20		µM
Jex, max	Maximal exchange current for the OS	4.6		pA
Kex	Ca2+ concentration parameter of exchanger	1100		nM

	Calculated resting dark state
cGDark	Dark cGMP concentration	4.12		µM
CaDark	Dark Ca2+ concentration	322		nM
JDark	Dark current	18.4		pA

	Other kinetic parameters
νRE	Rate of G*-E* activation by an R*	300 c		s−1
kE	Rate of G*-E* shut-off	5 c		s−1
βsub	Rate constant of cGMP hydrolysis by an E*	0.024 d	0.063 d	s−1
BCa	Buffering power of cytoplasm for Ca2+	50
Vcyto	Available cytoplasmic volume of OS	0.02		pL

	Longitudinal diffusion parameters
DcG	Longitudinal diffusion coefficient for cG	40		µm2/s
DCa	Longitudinal diffusion coefficient for Ca2+	2		µm2/s
L	Length of outer segment	22		µm

a. All values apply to the entire outer segment; however, suction pipette experiments record only a fraction of the total current. b. The default values applied in all cases, except for the three values listed under Model 1. c. The formation (ν_RE) and shut-off (k_E) of E* are modeled as either deterministic or stochastic. d. For comparison with the values used in Gross et al. [11], note that they employed a different parameter for hydrolysis by E*, that they referred to as β_idv, which is shown in Appendix 1 to be related to β_sub by β_idv = N_disks β_sub, where N_disks = 710 is the number of disks in the outer segment.