LOW MOBILITY OF THE CA2+ BUFFERS IN AXONS OF CULTURED APLYSIA NEURONS

Cellular Ca2+ buffers determine amplitude and diffusional spread of ne uronal Ca2+ signals. Fixed Ca2+ buffers tend to retard the signal and to lower the apparent diffusion coefficient (D-app) of Ca2+, whereas m obile buffers contribute to Ca2+ redistribution. To estimate the impac t of the expression of specific Ca2+-binding proteins or the errors in Ca2+ measurement introduced by indicator dyes, the diffusion coeffici ent D-e and the Ca2+-binding ratio kappa(e) of endogenous Ca2+ buffers must be known. In this study, we obtain upper bounds to these quantit ies (D-e < 16 mu m(2)/s; kappa(e) < 60) for axoplasm of metacerebral c ells of Aplysia california. Due to these very low values, even minute concentrations of indicator dyes will interfere with the spatiotempora l pattern of Ca2+ signals and will conceal changes in the expression o f specific Ca2+-binding proteins, which in the native neuron are expec ted to have significant effects on Ca2+ signals.