TRPC channels play significant roles in the regulation of neuronal plasticity and development. The mechanism by which these nonselective cation channels exert their trophic actions appears to involve entry of Ca(2+) into the cells. Using a neuronal cell model (differentiated human IMR32 neuroblastoma cells), we demonstrate a central role for sodium entry via TRPC3/6 channels in receptor-mediated increases in intracellular calcium. These Na(+)-dependent Ca(2+) influxes, which were observed in a subpopulation of cells, were efficiently blocked by protein kinase C activation, by the Na(+)/Ca(2+) exchanger inhibitors, and by molecular disruption of TRPC3/6 channel function. On the other hand, another subpopulation of cells showed a Na(+)-independent Ca(2+) entry upon stimulation of the same receptors, orexin/hypocretin and bradykinin receptors. This second type of response was not affected by the above mentioned treatments, but it was sensitive to polyvalent cations, such as ruthenium red, spermine and Gd(3+). The data suggest that a NCX-TRPC channel interaction constitutes an important functional unit in receptor-mediated Ca(2+) influx in neuronal cells.
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