The "domain" calcium (Ca2+) concentration near an open Ca2+ channel can be mod- eled as buffered diffusion from a point source. The concentration profiles can be well approximated by hemispherically symmetric steady-state solutions to a system of reaction-diffusion equations. After nondimensionalizing these equations and scaling space so that both reaction terms and the source amplitude are 0(1), we identify two dimensionless parameters, Cc and Eb, that correspond to the diffusion coefficients of dimensionless Ca2+ and buffer, respectively. Using perturbation methods, we derive approximations for the Ca2+ and buffer profiles in three asymptotic limits: (1) an "excess buffer approximation" (EBA), where the mobility of buffer exceeds that of Ca2+ (Eb > Ec) and the fast diffusion of buffer toward the Ca2+ channel prevents buffer saturation (cf. Neher [Calcium Electrogenesis and Neuronal Functioning, Exp. Brain Res. 14, Springer-Verlag, Berlin, 1986, pp. 80-96]); (2) a "rapid buffer approximation" (RBA), where the diffusive time-scale for Ca2+ and buffer are comparable, but slow compared to reaction (ec << 1, Eb reaction (ec << 1, Eb reaction (ec << 1, Eb
SIAM Journal on Applied Mathematics
Smith, Gregory D.; Dai, Longxiang; Miura, Robert M.; and Sherman, Arthur, Asymptotic analysis of equations for the buffered diffusion of intracellular Ca2+. (2001). SIAM Journal on Applied Mathematics, 61(5).