On the Ca²⁺‐permeability of neurons and glia

Fractions enriched in pinched‐off nerve terminals and astrocytic glial cells were used to analyse the permeability of Ca⁺⁺of the two membranes. Three experimental models were used to illustrate a principle difference of the excitatory versus the non‐excitatory cell membrane, with respect to Ca‐permeability. The effect of an increased [Ca⁺⁺] on ⁸⁶Rb⁺and ³H‐GABA transport was measured in medium containing low and high [K⁺]. The possibility of GABA stimulation of release of preaccumulated ⁴⁵Ca⁺⁺with low and high [K⁺] was compared in neurons and glia. Finally, the effect of depolarizing [K⁺] on the transmembranal Ca⁺⁺‐gradient was measured by comparing the efficacy of the Ca‐ionophore A23187 to depolarize or to inhibit the ³H‐GABA uptake at these different [K⁺]. The data essentially confirms the picture of a potential dependent Ca⁺⁺‐permeability in the neuron, while the permeability of the glial cells seems a “true” constant. It might be relevant to suggest the astrocyte a role as a “Ca‐buffer” with possibilities to control extracellular Ca⁺⁺in cases of hyperactivity, this in analogy with what has been suggested for K⁺.