High affinity block by nimodipine of the internal calcium elevation in chronically depolarized rat cerebellar granule neurons

Chronic depolarization enhances survival of cultured rat cerebellar granule neurons by elevating the internal calcium concentration ([Ca²+](i)). In Fura2-loaded cells maintained in 25 mM KCl, [Ca²+](i) was close to 150 nNI and decreased to approximate to 50 nM when KCl was lowered to 5.4 mM. The effect of nimodipine (IC50 = 0.45 nM) was similar to depolarization removal, while agatoxin IVA (up to 500 nM) was ineffective. In whole-cell-clamp experiments, the IC50 for current inhibition was 57 nM, while with transient, KCl-induced depolarizations, the dose dependence of nimodipine inhibition had a 'double-affinity' shape, with IC50(1) = 0.30 nM and IC50(2) = 71 nM. We concluded that L-type calcium channels are the main responsible of the elevated internal calcium level necessary for survival in these neurons. These channels do not inactivate and bind nimodipine with different affinity, depending on their state.