] i increase in a dose-dependent manner through the activation of adenyl cyclase. The increases induced by both PTH and hypotonicity were observed primarily in the processes of the osteocytes. In cyclically stretched osteocytes on flexible-bottomed plates, PTH also synergistically elevated the insulin-like growth factor-1 mRNA level. Furthermore, Gd 3؉ and ␣1C antisense significantly inhibited the stretch-induced insulin-like growth factor-1 mRNA elevation. The volume-sensitive calcium influx pathways of osteocytes represent a mechanism by which PTH potentiates mechanical responsiveness, an important aspect of bone formation.Mechanical loading applied to the skeleton has been shown to cause osteotropic influences. On the other hand, prolonged immobilization is one of the important reasons of bone loss. However, the intracellular mechanisms by which bone cells sense mechanical strain are not well known (1).Although osteocytes are the most abundant cells in bone, the role of osteocytes in bone remodeling was not clear until recently. Direct inhibition of osteoclastic activity by osteocytederived protein through osteoclast-osteocyte attachment was demonstrated (2, 3). Furthermore, intermittent mechanical loading within the physiological range enhances IGF-1 1 mRNA expression in osteocytes (4, 5), suggesting that these cells transduce signals induced by mechanical stress to osteoblasts. Although various other mediators such as prostaglandins (6 -8), cyclooxygenase-2 (encoding prostaglandin G/H synthase), (8, 9), endothelial, constitutive nitric oxide synthase (10), or c-fos (8, 11-13) have been suggested, the exact mechanosensing mechanisms in these cells are not clear. Although the involvement of stretch-activated cation channels (SA-Cat) in the reception of mechanical stress has been reported in many other cell types, none has been functionally demonstrated in osteocytes so far.The localization of PTH receptors by in situ hybridization (14), as well as the synergistic effects of mechanical stress and PTH (13, 15), on the other hand, indicated the important role of PTH in regulating the signal transduction of mechanical stress in osteocytes. Stretch-activated cation channels and their activation by PTH were demonstrated in UMR 106 osteoblast-like cells by who suggested either the expression of isoforms of the ␣1C subunit of the voltage-operated Ca 24 channel (VOCC) is required for the activity or that the channel may have homology to the ␣1C subunit of VOCC. Osteocytes differentiate from osteoblasts along with a dramatic elongation of cytoplasmic processes. This morphological change suggests that their levels of sensitivity to mechanical loading and the mechanisms of response could be distinct.In this study, we have identified a stretch-activated Ca 2ϩ entry pathway for the first time in both rat and chicken osteocytes by swelling the cells with hypo-osmotic stress. Also, by using single cell [Ca 2ϩ ] i video-image analysis, we demonstrated that osteocyte processes are furnished with volumesensitive, stretch-act...

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The effects of the vacuolar-type H(+)-ATPase inhibitor bafilomycin A1 (baf.A1) and the carbonic anhydrase II inhibitor acetazolamide (AZ) on bone resorption and procathepsin L secretion of rat osteoclasts were investigated using the bone slice assay method, pit formation test. Baf.A1 completely suppressed osteoclastic bone resorption stimulated by parathyroid hormone (PTH), but did not affect procathepsin L secretion, while AZ suppressed both bone resorption and procathepsin L secretion. These findings suggest that bone resorption by procathepsin L secretion and its processing are regulated by proton production and proton secretion.