R-Type Ca2+-channel Activity Is Associated with Chromogranin A Secretion in Human Neuroendocrine Tumor BON Cells

This electrophysiological study was undertaken to investigate the role of voltage-operated Ca2+ channels (VOCCs) in cultivated human neuroendocrine tumor (NET) cells. Patch-clamp techniques, measurements of intracellular Ca2+ ([Ca2+]i), and secretion analysis were performed using cultured human NET BON cells. Ba2+ inward currents through R-type channels (CaV2.3) were measured and identified by SNX-482 (10 nM), a novel voltage-sensitive R-type Ca2+ channel antagonist. In the presence of nifedipine (5 µM), ω-Conotoxin GVIA (100 nM) and ω-Agatoxin IVA (20 nM), R-type channel currents were also detectable. Release of Ca2+ from intracellular Ca2+ stores by intracellular application of inositol-1,4,5-trisphosphate (InsP3; 10 µM) via the patch pipette during whole-cell configuration as well as induction of capacitative Ca2+ entry (CCE), a passive maneuver to release Ca2+ from intracellular Ca2+ stores, led to an increase in [Ca2+]i. This effect could be reduced by SNX-482 (20 nM). In addition, SNX-482 (25 nM) also decreased chromogranin A (CgA) secretion, whereas ω-Conotoxin GVIA (500 nM) and nifedipine (5 µM) failed to reduce CgA secretion. We conclude that these data reveal neuronal R-type channel activity (CaV2.3), for the first time associated with CgA secretion in BON cells. Influx of Ca2+ by activation of R-type channels may lead to an increase of intracellular Ca2+, which stimulates CgA secretion. Thus, R-type channels could play an important role in certain clinical characteristics of NETs, such as the hypersecretion syndrome.