Role of intracellular Na⁺ in the regulation of [Ca²⁺]i in the rat suprachiasmatic nucleus neurons

Transmembrane Ca²⁺ 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 Ca²⁺ following depolarization-induced Ca²⁺ transients involves Ca²⁺ extrusion via Na⁺/Ca²⁺ exchanger (NCX) and mitochondrial Ca²⁺ buffering. Here we show an important role of intracellular Na⁺ in the regulation of [Ca²⁺]i in these neurons. The effect of Na⁺ loading on [Ca²⁺]i was determined with the Na⁺ ionophore monensin and the cardiac glycoside ouabain to block Na⁺/K⁺-ATPase (NKA). Ratiometric Na⁺ and Ca²⁺ imaging was used to measure the change in [Na+]i and [Ca²⁺]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 [Ca²⁺], 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 Ca²⁺ transients. Furthermore, both ouabain and monensin preferentially enhanced nimodipine-insensitive Ca²⁺ transients. Together, our results indicate that in the SCN neurons the NKA plays an important role in regulating [Ca²⁺]i, in particular, associated with nimodipine-insensitive Ca²⁺ channels.