Table 1.

Parameter values used for the different involved buffer species and calcium

KDm)kon(m−1 sec−1)Diffusion coefficient (μm2/sec)
EGTA0.182.5  × 106220
BAPTA0.224.0  × 108220
ATP2300.05.0  × 108220
Endogenous mobile buffer50.01.0  × 10815
  • The values for ATP are chosen to correct for the presence of 3 mm total Mg2+ in a typical internal solution, for instance 2 mm MgATP and 1 mmMgCl, giving rise to 0.17 mm free ATP. Hence, the literature values for KD and kon are scaled by the factor 2 mm/0.17 mm ≈ 11.8 (Klingauf and Neher, 1997). The kinetic parameters for EGTA and BAPTA are according to M. Naraghi and E. Neher (unpublished results). The diffusion coefficient of the mobile endogenous buffer is chosen as in Zhou and Neher (1993), its on-rate and KD according to Xu et al. (1997), and its total concentration such that the binding ratio is 10, as estimated in Zhou and Neher (1993). The diffusion coefficient of free calcium corresponds to the number measured in oocyte cytoplasm (Allbritton et al., 1992); for the exogenous buffers, the value expected in water divided by 2 (to correct for viscosity) is taken. The resting calcium concentration [Ca2+] is always assumed to be 0.1 μm.