Table 1.
Description of model variables and parameters
| Symbol | Units | Description |
|---|---|---|
| Variables | ||
| R* (R) | # | Number of photoactivated (inactive) rhodopsin molecules at time t |
| E* | # | Number of activated PDE catalytic subunits per rod at time t |
| g | μM | Concentration of free outer segment (OS) cGMP |
| c | μM | Concentration of intracellular free OS Ca2+ at time t |
| cb | μM | Concentration of Ca2+ bound to dynamic Ca2+ buffer at time t |
| F | # | Normalized circulating current at time t |
| Parameters | ||
| τR | s | Time constant for first-order inactivation of R* |
| τE | s | Time constant for first-order inactivation of E* (E* = G*·PDE*) |
| νrp | s−1 | Rate of production of E* per R* |
| cdark | μM | Dark resting concentration of free intracellular Ca2+ |
| fse | # | Suction-electrode collecting efficiency |
| Jex | pA | Na+/Ca2+-K+ exchange current |
| Jex,dark | pA | Na+/Ca2+-K+ exchange current in the dark |
| Jex,sat | pA | Saturated magnitude of Na+/Ca2+-K+ exchange current |
| Kex | μM | Cai2+ giving rise to half-maximal exchange current |
| c0 | μM | Minimum value of free OS Ca2+ (Ca2+ floor) |
| γCa | s−1 | Rate constant of Ca2+ extrusion by exchanger |
| Vcyto | pL | Effective volume of rod OS |
| Fca | # | Fraction of inward circulating current carried by Ca2+ |
| b | μMs−1 pA−1 | Converts Ca2+ influx to current. Note: For Vcyto = 1 pL, Fca = 0.193 b |
| BCa | # | Instantaneous Ca2+-buffering power of the rod OS |
| k1 | μM−1 s−1 | On-rate constant for binding of Ca2+ to dynamic Ca2+ buffer |
| k2 | s−1 | Off-rate constant for unbinding of Ca2+ from dynamic Ca2+ buffer |
| et | μM | Total concentration of dynamic Ca2+ buffer |
| gdark | μM | Resting cytoplasmic concentration of free cGMP in the dark |
| Amas | μMs−1 | Maximum activity of guanylate cyclase |
| βdark | s− | Rate constant of cGMP hydrolysis (by E*) in the dark |
| βsub | s−1 | Rate constant of a catalytic PDE subunit in a well-stirred volume |
| Kc | μM | Concentration of Ca2+ at which cyclase activity is half-maximal |
| nca | # | Hill coefficient for Ca2+ modulation of cGMP synthesis via cyclase |
| ncg | # | Hill coefficient for opening of cGMP-gated channels by cGMP |
| Jdark | pA | Dark circulating current |
| Fdark | pA | Normalized circulating current in the dark |