Involvement of small-conductance Ca2+-activated K+ (SKCa2) channels in spontaneous Ca2+ oscillations in rat pinealocytes.

Melatonin secretion from the pineal glands regulates circadian rhythms in mammals. Melatonin production is decreased by an increase in cytosolic Ca2+ concentration following the activation of nicotinic acetylcholine receptors in parasympathetic systems. We previously reported that pineal Ca2+ oscillations were regulated by voltage-dependent Ca2+ channels and large-conductance Ca2+-activated K+ (BKCa) channels, which inhibited melatonin production. In the present study, the contribution of small- and intermediate-conductance Ca2+-activated K+ (SKCa and IKCa) channels to the regulation of spontaneous Ca2+ oscillations was examined in rat pinealocytes. The amplitude and frequency of spontaneous Ca2+ oscillations were increased by a SKCa channel blocker (100 nM apamin), but not by an IKCa channel blocker (1 μM TRAM-34). On the other hand, they were decreased by a SKCa channel opener (100 μM DCEBIO), but not by an IKCa channel opener (1 μM DCEBIO). Expression analyses using quantitative real-time PCR, immunocytochemical staining, and Western blotting revealed that the SKCa2 channel subtype was abundantly expressed in rat pinealocytes. Moreover, the enhanced amplitude of Ca2+ oscillations in the presence of apamin was further increased by a BKCa channel blocker (1 μM paxilline). These results suggest that the activity of SKCa2 channels regulates cytosolic Ca2+ signaling and melatonin production during parasympathetic activation in pineal glands.