Low Mobility of the Ca2+ Buffers in Axons of Cultured Aplysia Neurons

Cellular Ca2+ buffers determine amplitude and diffusional spread of neuronal Ca2+ signals. Fixed Ca2+ buffers tend to retard the signal and to lower the apparent diffusion coefficient (D(app)) of Ca2+, whereas mobile buffers contribute to Ca2+ redistribution. To estimate the impact of the expression of specific Ca2+-binding proteins or the errors in Ca2+ measurement introduced by indicator dyes, the diffusion coefficient De and the Ca2+-binding ratio kappa(e) of endogenous Ca2+ buffers must be known. In this study, we obtain upper bounds to these quantities (De < 16 microm2/s; kappa(e) < 60) for axoplasm of metacerebral cells of Aplysia california. Due to these very low values, even minute concentrations of indicator dyes will interfere with the spatiotemporal pattern of Ca2+ signals and will conceal changes in the expression of specific Ca2+-binding proteins, which in the native neuron are expected to have significant effects on Ca2+ signals.