Parathyroid Hormone Enhances Fluid Shear-Induced [Ca super(2+)] sub(i) Signaling in Osteoblastic Cells Through Activation of Mechanosensitive and Voltage-Sensitive Ca super(2+) Channels

Osteoblasts respond to both fluid shear and parathyroid hormone (PTH) with a rapid increase in intracellular calcium concentration ([Ca super(2+)] sub(i)). Because both stimuli modulate the kinetics of the mechanosensitive cation channel (MSCC), we postulated PTH would enhance the [Ca super(2+)] sub(i) response to fluid shear by increasing the sensitivity of MSCCs. After a 3-minute preflow at 1 dyne/cm super(2), MC3T3-E1 cells were subjected to various levels of shear and changes in [Ca super(2+)] sub(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 [Ca super(2+)] sub(i). Gadolinium (Gd super(3+)), an MSCC blocker, significantly inhibited the mean peak [Ca super(2+)] sub(i) response to shear and shear + bPTH(1-34). Nifedipine (Nif), an L-type voltage-sensitive Ca super(2+) channel (VSCC) blocker, also significantly reduced the [Ca super(2+)] sub(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 Ca super(2+) response to shear, whereas PKA activation significantly increased the [Ca super(2+)] sub(i) response to lower magnitudes of shear. bPTH(1-34), which activates both pathways, induced the greatest [Ca super(2+)] sub(i) response at each level of shear, suggesting an interaction of these pathways in this response. These data indicate that PTH significantly enhances the [Ca super(2+)] sub(i) response to shear primarily via PKA modulation of the MSCC and VSCC.