This paper documents for the first time a volumesensitive Ca 2؉ influx pathway in osteocytes, which transmits loading-induced signals into bone formation. Stretch loading by swelling rat and chicken osteocytes in hypo-osmotic solution induced a rapid and progressive increase of cytosolic calcium concentration, [Ca 2؉ ] i . The influx of extracellular Ca 2؉ explains the increased [Ca 2؉ ] i that paralleled the increase in the mean cell volume. Gadolinium chloride (Gd 3؉ ), an inhibitor of

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... -activated cation channels, blocked the [Ca 2؉ ] i increase caused by hypotonic solutions. Also, the expression of ␣1C subunit of voltage-operated L-type Ca 2؉ channels (␣1C) is required for the hypotonicity-induced [Ca 2؉ ] i increase judging from the effect of ␣1C antisense oligodeoxynucleotides. Parathyroid hormone (PTH) specifically potentiated the hypotonicity-induced [Ca 2؉ ] 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.