Serotonin-Induced Calcium Signaling via 5-HT1A Receptors in Human Leukemia (K 562) Cells

In this study, serotonin (5-HT) was found to diminish the intracellular calcium, [Ca2+]i, concentrations in a dose-dependent manner in Fura-2-loaded human leukemia (K 562) cells. Prior addition of verapamil (a calcium channel blocker) to the cells abolished the serotonin-induced response, suggesting that the diminution of free [Ca2+]i contents was due to the opening of calcium channels. Thapsigargin (an agent which increases [Ca2+]i via its action on endoplasmic reticulum) augmented the [Ca2+]i contents. Addition of serotonin before or after thapsigargin (THAP) curtailed the THAP-stimulated increases in [Ca2+]i, supporting the notion that 5-HT acts on the calcium channels and that it empties, in part, the THAP-stimulated calcium contents. Spiperone and NAN-190, antagonists to 5-HT1A receptor subtype, abolished the serotonin effects on calcium signaling, whereas an agonist to 5-HT1A receptor, 8OHDPAT, mimicked the serotonin-like action on the diminution of the intracellular calcium contents. These results indicate that the 5-HT response is mediated via 5-HT1A serotonergic receptors in these cells. Furthermore, we characterized the 5-HT receptors in K 562 cells. The specific binding of serotonin to these cells was saturable and hyperbolic. The Kd and the Bmax of serotonin binding were 100 nM and 1690 fmol/106 cells, respectively. 8-[3H]OH-DPAT also labeled these cells with a Bmax of 2.8 fmol/106 cells and a Kd of 20 nM. The specific binding of 8-[3H] OH-DPAT to K 562 cells was displaced by serotonin, 8OH-DPAT, and NAN-190. These results suggest that K 562 cells possess functional 5-HT1A receptors coupled with calcium signaling.