Table 1.
Description of Model Variables & 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 Ca++ at time t |
| cb | μM | Concentration of Ca++ bound to dynamic Ca++ -buffer at time t |
| F | # | Normalized circulated current at time t |
| Rec* | μM | Concentration of recoverin bound to Ca++ |
| RK* | μM | Concentration of activated (disinhibited) rhodopsin kinase |
| Parameters | ||
| Front-End Feedback | ||
| Rectot | μM | Total concentration of recoverin |
| RKtot | μM | Total concentration of rhodopsiin kinase |
| Krec,Ca | μM | Concentration of Ca++ at which 1/2Rectot is bound to Ca++ |
| w | # | Hill coefficient for Ca++ binding to recoverin |
| q3 | sec−1 μM−1 | rate constant for binding of Rec* to RK* |
| q4 | sec−1 | rate constant for unbinding of Rec* to RK* |
| q5 | sec−1 μM−1 | rate constant for interaction between R* and RK* |
| Front-End | ||
| τR | s | Time constant for 1st -order inactivation of R* |
| τE | s | Time constant for 1st -order inactivation of E* (E*=G*PDE*) |
| nrp | s−1 | Rate of production of E* per R* |
| τRi | sec | Time constant for depletion of inactivated rhodopsin |
| τb | sec | Time constant for back reaction from R to R* |
| Ca++, Ca++ -Buffering | ||
| cdark | μM | Dark resting concentration of free intracellular Ca++ |
| c0 | μM | Minimum value of free OS Ca++ (Ca++ floor) |
| γCa | s−1 | Rate constant of Ca++ extrusion by exchanger |
| Vcyto | pL | Effective volume of rod OS |
| FCa | # | Fraction of inward circulating current carried by Ca++ |
| k1 | μM−1 s−1 | On-rate constant for binding of Ca++ to dynamic Ca++ -buffer |
| k2 | s−1 | Off-rate constant for unbinding of Ca++ to dynamic Ca++ -buffer |
| eT | μM | Total Concentration of dynamic Ca++ buffer |
| cGMP, Cyclase | ||
| gdark | μM | Resting cytoplasmic concentration of cGMP in the dark |
| Amax | μM s−1 | Maximum activity of guanylate cyclase |
| βdark | s−1 | Rate constant of cGMP hydrolysis in the dark |
| βsub | s−1 | Rate constant of a catalytic PDE subunit in a well-stirred volume |
| Kc | μM | Concentration of Ca++ at which cyclase activity is half-maximal |
| nca | # | Hill coefficient for Ca++ -modulation of cGMP synthesis via cyclase |
| cGMP-Gated Channel, Photocurrent | ||
| neg | # | Hill coefficient for opening of cGMP-gated channels by cGMP |
| Jdark | pA | Dark circulating current |
The symbold “#” means that the quantity is a unitless number.