Table III

Parameters Involving Calcium and Affecting Recovery

SymbolValue/UnitInterpretation/comment
Vcyto1.0 plVolume of the outer segment cytoplasm
Jdark/fse70 pADark/circulating current measured by suction electrode (s.e.) divided by s.e. collecting efficiency *‡§
fCa0.1Fraction of inward circulating current carried by Ca2+‡
Cadark385 nMDark/resting concentration of Ca2+i in outer segment; see ii‡
Jex,sat12.5 pASaturated magnitude of Na+/Ca2+-K+ exchange current‡
Kex1600 nMCa2+i giving rise to half-maximal exchange current‡
KCa100 nMCa2+i at which cyclase activity is half-maximal§
nCa2.0Cooperativity coefficient for Ca2+ dependence of cyclase activity§‖
βdark0.8–1.2 s−1Rate constant of cGMP hydrolysis in dark‖¶
α′max10–15 s−1Maximum rate of cGMP synthesis divided by cGdark
γ5.6 s−1Factor for converting Ca2+ currents into concentration changes; Eq. 15
η0.8Fraction of unused exchange current capacity at rest; Eq. 15
μ2.7 s−1Real component of the oscillatory term governing cyclase feedback near Cadark; Eqs. 15, 20, and A6.8
ν4.5 s−1Imaginary component of the oscillatory feedback term; Eqs. 20 and A6.8
θ3.8 radiansPhase term of the calcium feedback; Eqs. 20 and A6.8
  • Parameters in rows 1–8 were fixed at the values listed, based on the references. βdark was varied within the range given to optimize fitting; the most commonly used value was 1.0 s−1; in the perturbation analysis α′max is not free, but takes a value consequent to initial conditions based on equations 6, 11, and 13 (see Appendix II). The last five parameters in the table arise in the perturbation analysis of the dim-flash response in Ringer's (Theorem 6); the values listed are typical ones.  

  • *  Lyubarsky et al., 1996;  

  •  Lagnado et al., 1992;  

  • §  Koutalos et al., 1995;  

  •  Pugh et al., 1997;  

  •  Hodgkin and Nunn, 1988.