Parathyroid hormone enhances fluid shear-induced [Ca2+]i signaling in osteoblastic cells through activation of mechanosensitive and voltage-sensitive Ca2+ channels

Osteoblasts respond to both fluid shear and parathyroid hormone (PTH) with a rapid increase in intracellular calcium concentration ([Ca2+](i)). Because both stimuli modulate the kinetics of the mechanosensitive cation channel (MSCC), we postulated PTH would enhance the [Ca2+](i) response to fluid shear by increasing the sensitivity of MSCCs. After a 3-minute preflow at 1 dyne/cm(2), MC3T3-E1 cells were subjected to various levels of shear and changes in [Ca2+](i) were assessed using Fura-2. Pretreatment with 50 nM bovine PTH(1-34) [bPTH(1-34)] significantly enhanced the shear magnitude-dependent increase in [Ca2+](i). Gadolinium (Gd3+), an MSCC blocker, significantly inhibited the mean peak [Ca2+](i) response to shear and sheer + bPTH(1-34). Nifedipine (Nif), an L-type voltage-sensitive Ca2+ channel (VSCC) blocker, also significantly reduced the [Ca2+](i) response to shear + bPTH(1-34), but not to shear alone, suggesting VSCC activation plays an interactive role in the action of these stimuli together. Activation of either the protein kinase C (PKC) or protein kinase A. (PKA) pathways with specific agonists indicated that PKC activation did not alter the Ca2+ response to shear, whereas PKA. activation significantly increased the [Ca2+](i), response to lower magnitudes of shear, bPTH(1-34), which activates both pathways, induced the greatest [Ca2+](i) response at each level of shear, suggesting an interaction of these pathways in this response. These data Indicate that PTH significantly enhances the [Ca2+](i) response to shear primarily via PKA modulation of the MSCC and VSCC.