Store-operated calcium entry induced by activation of Gq-coupled alpha1B adrenergic receptor in human osteoblast

Recent studies have revealed that the sympathetic nervous system is involved in bone metabolism. We previously reported that noradrenaline (NA) suppressed K+ currents via Gi/o protein-coupled alpha1B-adrenergic receptor (α1B-AR) in human osteoblast SaM-1 cells. Additionally, it has been demonstrated that the intracellular Ca2+ level ([Ca2+]i) was increased by NA via α1B-AR. In this study, we investigated the signal pathway of NA-induced [Ca2+]i elevation by using Ca2+ fluorescence imaging in SaM-1 cells. NA-induced [Ca2+]i elevation was suppressed by pretreatment with a PLC inhibitor, U73122. This suggested that the [Ca2+]i elevation was mediated by Gq protein-coupled α1B-AR. On the other hand, NA-induced [Ca2+]i elevation was completely abolished in Ca2+-free solution, which suggested that Ca2+ influx is the predominant pathway of NA-induced [Ca2+]i elevation. Although the inhibition of K+ channel by NA caused membrane depolarization, the [Ca2+]i elevation was not affected by voltage-dependent Ca2+ channel blockers, nifedipine and mibefradil. Meanwhile, NA-induced [Ca2+]i elevation was abolished following activation of store-operated Ca2+ channel by thapsigargin. Additionally, the [Ca2+]i elevation was suppressed by store-operated channel inhibitors, 2-APB, flufenamate, GdCl3 and LaCl3. These results suggest that Ca2+ influx through store-operated Ca2+ channels plays a critical role in the signal transduction pathway of Gq protein-coupled α1B-AR in human osteoblasts.