Abstract
We analyzed preparative and analytical aspects of the dynamic localization of Ca2+ during cell stimulation, using a combination of quenched flow and energy-dispersive X-ray microanalysis (EDX). Calcium (or Sr, as a substitute) was retained as fluorides during freeze-substitution, followed by epoxide embedding. The quenched-flow used allowed analyses, during stimulation, in the subsecond time range. Sections of 500 nm were analyzed and no artificial Ca or Sr leakage was recognizable. We calculated a primary beam spread from 63 to 72 nm that roughly indicated the resolution of EDX/structure correlation. These values are quite compatible with the size of potential structures of interest, e.g., Ca stores (∼100-nm thickness) or cilia (∼250-nm diameter). We used widely different standards to calibrate the ratio of CaKα net counts in relation to actual [Ca]. Calibration curves showed a linear relationship and a detection limit of [Ca] = 2 mM, while [Ca] in cytosol was 3 mM and in stores was 43 mM, both in nonactivated cells. Eventually Sr2+ can rapidly be substituted for Ca2+ in the medium before and during stimulation, thus allowing one to determine Me2+ fluxes. With our “model” cell, Paramecium, we showed that, upon stimulation (causing rapid Ca2+ mobilization from subplasmalemmal stores), Ca was immediately exchanged for Sr in stores.
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