Synaptosomal calcium influx is activated by sodium fluoride

Neuronal calcium channels can be modulated by changes in membrane potential or by activation of channel-associated receptors. The latter may be modulated by guanine nucleotide binding proteins. NaF, which activates guanine nucleotide binding proteins, caused a large stimulation of ⁴⁵Ca²⁺ uptake by synaptosomes prepared from rat brain. Stimulation of ⁴⁵Ca²⁺ influx by NaF (i) was apparent in media containing either 5 mM-K⁺ or 50 mM-K⁺, (ii) was slower than the fast-phase of voltage-dependent ⁴⁵Ca²⁺ influx but continued for a longer period of time than did depolarization-induced ⁴⁵Ca²⁺ influx, and (iii) was not mimicked or modified by a number of drugs, including ouabain, dinitrophenol, sodium azide or sodium vanadate. These results are consistent with the hypothesis that NaF activates a guanine nucleotide binding protein associated with receptor-coupled calcium channels, resulting in stimulation of calcium influx.