Effect of a prolonged glutamate challenge on plasmalemmal calcium permeability in mammalian central neurones. Mn2+ as a tool to study calcium influx pathways

The rate of Mn(2+)-induced fluorescence quenching (RFQ) was used as a relative measure of plasma membrane Ca2+ permeability (PCa) in fura-2-loaded cultured hippocampal neurons and cerebellar granule cells during and after protracted (15-30 min) glutamate (GLU) treatment. Some limitations of this method were evaluated using a kinetic model of a competitive binding of Mn2+ and Ca2+ to fura-2 in the cell. In parallel experiment a contribution of Ca2+ influx to the cytoplasmic Ca2+ ([Ca2+]i) was repeatedly examined during and following a prolonged GLU challenge by short-duration "low-Ca2+ trials" (50 microM EGTA) and by measurements of 45Ca2+ uptake. Experiments failed to reveal a putative persistent increase in PCa that earlier was thought to underlie Ca2+ overload of the neuron caused by its toxic GLU treatment. By contrast, a sustained increase of [Ca2+]i was found to be associated with a progressive decrease in PCa and Ca2+ influx both in the period of GLU application and after its termination. These findings give new evidence in favour of the hypothesis that the GLU-induced Ca2+ overload of the neuron mainly from an impairment of its Ca2+ extrusion systems.

Bepridil exacerbates glutamate-induced deterioration of calcium homeostasis and cultured nerve cell injury

Application of 50 microM bepridil (BPD) to cultured nerve cells did not greatly affect the resting cytoplasmic Ca2+ concentration ([Ca2+]i) but caused its pronounced increase both during prolonged glutamate (GLU, 100 microM) treatment and, especially, in the postglutamate period in case of partial [Ca2+]i recovery. In contrast, in cells exhibiting a high [Ca2+]i plateau in the postglutamate period, BPD application either did not cause any additional elevation of [Ca2+]i or caused a very small increase. Under identical conditions replacement of external Na+ by Li+ or N-methyl-D-glucamine (NMDG) either did not change [Ca2+]i or produced a very small increase, strongly indicating that the BPD-evoked Ca2+ responses could not be explained solely by Na+/Ca2+ exchange inhibition but resulted from some other BPD effects. Indeed, in experiments with Rhodamine 123-loaded neurons it has been shown that 50 microM BPD induced prominent mitochondrial depolarization which is known to abolish the mitochondrial Ca2+ uptake. Finally it was revealed that BPD application to the cell culture either in the period of a prolonged (15 min) GLU action or, especially, in the postglutamate period greatly exacerbated delayed neuronal death, apparently due to a complex inhibitory action of the drug on both Ca2+ buffering and Ca2+ extrusion systems.