Shear stress-induced Ca2+ elevation is mediated by autocrine-acting glutamate in osteoblastic MC3T3-E1 cells

Abstract

Mechanical loading is an important regulatory factor in bone homeostasis. Neurotransmitters, such as glutamate and ATP, are known to be released from osteoblasts, but their roles have been less studied. In this study, we investigated the role of transmitter release in mechanotransduction. To identify from where transmitters were released, focal fluid flow was applied to a single cell of MC3T3-E1, mouse calvaria-derived osteoblastic cell line, by using a glass micropipette. Intracellular Ca2+ elevation induced by the focal shear stress was eliminated by either GdCl3, a mechanosensing channel inhibitor, or removal of extracellular Ca2+. On the other hand, the focal shear stress-induced Ca2+ elevation was also significantly suppressed by inositol triphosphate receptor antagonist or vesicular release inhibitors. These results suggest that not only mechanosensitive channel-mediated Ca2+ influx but also some autocrine transmitters are involved in mechanotransduction. Additionally, glutamate receptor antagonists, but not ATP receptor antagonist, suppressed most of the focal shear stress-induced Ca2+ elevation. Therefore, it is suggested that glutamate is released from osteoblasts following the activation of mechanosensitive Ca2+ channels and acts in an autocrine manner. The glutamate release may have a significant role in the initial event of mechanotransduction in bone tissue.