A pharmacological model for studying the role of Na+ gradients in the modulation of synaptosomal free [Ca2+]i levels and energy metabolism.

Lactate production (Jlac), oxygen consumption rate (QO2), plasma membrane potentials (Em) and cytosolic free calcium levels [Ca2+]i were studied on synaptosomes isolated from rat brains, incubated in presence of high doses of nicardipine (90 microM), diltiazem (0.5 mM) and verapamil (0.25 mM), and submitted to depolarizing stimulation or inhibition of mitochondrial respiration. Nicardipine was able to completely prevent the veratridine-induced stimulation of Jlac, QO2 and Em depolarization, whereas diltiazem and verapamil were less effective, although the concentrations used were 5 and 3 times higher, respectively, than nicardipine. Diltiazem, verapamil and nicardipine (9 microM) also prevented the veratridine-induced increase in [Ca2+]i, this effect being much less pronounced if the drugs were added after veratridine. Monensin (20 microM) was also able to increase [Ca2+]i but this effect was not affected by verapamil. Synaptosomes were also submitted to an inhibition of respiration of intrasynaptic mitochondria by incubation with rotenone (5 microM); in this condition of mimicked hypoxia Em was more positive of about 11 mV; none of the drugs utilized modified this situation. The rotenone-induced 3-fold increase in Jlac was barely modified by diltiazem and verapamil but it was completely abolished by nicardipine. The possible mechanism of the counteracting action of the drugs towards veratridine stimulation and rotenone inhibition and the involvement of Na+/Ca2+ exchanger in affecting [Ca2+]i are discussed.