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 no-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+4 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+4 in cases of hyperactivity, this in analogy with what has been suggested for K+.