Role of Intracellular Na(+) in the Regulation of [Ca(2+)](i) in the Rat Suprachiasmatic Nucleus Neurons

Transmembrane Ca(2+) influx is essential to the proper functioning of the central clock in the suprachiasmatic nucleus (SCN). In the rat SCN neurons, the clearance of somatic Ca(2+) following depolarization-induced Ca(2+) transients involves Ca(2+) extrusion via Na(+)/Ca(2+) exchanger (NCX) and mitochondrial Ca(2+) buffering. Here we show an important role of intracellular Na(+) in the regulation of [Ca(2+)](i) in these neurons. The effect of Na(+) loading on [Ca(2+)](i) was determined with the Na(+) ionophore monensin and the cardiac glycoside ouabain to block Na(+)/K(+)-ATPase (NKA). Ratiometric Na(+) and Ca(2+) imaging was used to measure the change in [Na(+)](i) and [Ca(2+)](i), and cell-attached recordings to investigate the effects of monensin and ouabain on spontaneous firing. Our results show that in spite of opposite effects on spontaneous firing and basal [Ca(2+)], both monensin and ouabain induced Na(+) loading, and increased the peak amplitude, slowed the fast decay rate, and enhanced the slow decay phase of 20 mM K(+)-evoked Ca(2+) transients. Furthermore, both ouabain and monensin preferentially enhanced nimodipine-insensitive Ca(2+) transients. Together, our results indicate that in the SCN neurons the NKA plays an important role in regulating [Ca(2+)](i), in particular, associated with nimodipine-insensitive Ca(2+) channels.