Extracellular calcium and potassium changes in hippocampal slices

Ca2+ and K+ ion sensitive microelectrodes were used to measure changes in ionic activities in the CA1 region of hippocampal slices during orthodromic (stratum radiatum) stimulation. Baseline levels of [K+]o and [Ca2+]o were those of the bathing medium which contained 5 mM K+ and 2.0 mM Ca2+. During stimulation [K+]o rose to maximal levels of 12 mM while [Ca2+]o decreased to as low as 1.4 mM. Systematic alterations in extracellular field potentials in stratum pyramidale accompanied the ionic shifts. Following stimulation K+ undershoots occurred. An active K+ uptake mechanism was demonstrated using iontophoretic K+ pulses. [K+]o and [Ca2+]o changes occurred in parallel and in a laminar distribution with maximal changes recorded in stratum pyramidale. Maximal [K+]o changes occurred from baselines of 5 mM and declined progressively at higher baseline levels. During epileptiform activity produced by exposure of slices to penicillin, larger ionic shifts with a more rapid onset occurred. The alterations in [K+]o and [Ca2+]o in the hippocampal slice are similar in some respects to those obtained by stimulation in vivo, making this preparation a potentially useful one for determination of mechanisms and effects of alterations in the ionic microenvironment.