Slow diffusion of Ca2+ in the rat's hippocampus

Ca2+-sensitive microelectrodes, attached to CaCl2-containing micropipettes, were inserted into the dorsal hippocampus of rats under urethane. When Ca2+ was released iontophoretically, the amplitude and time course of the resultant increase in extracellular Ca2+ concentration could be fitted to a simple diffusion model, but the apparent diffusion coefficient of Ca2+ was only about 1/100 of its value in water, possibly because of reversible Ca2+ binding to hippocampal tissue. A further anomaly was a very low transport number (<0.01) for the release of Ca2+ from microelectrodes in vivo.