We studied the effect of PTH (10-100 nM) on transductive mechanisms (adenylate cyclase activity, Ca2+ metabolism, IP3 levels) in cell cultures derived from normal and otosclerotic human bone fragments. The cultured cells were osteoblast-like but with calcitonin-receptors still present and with PTH receptors coupled with the adenylate cyclase system. The results showed that PTH activated adenylate cyclase and increased the intracellular Ca2+ levels with qualitative and quantitative differences between the two cellular populations. In particular, otosclerotic cells responded less to hormone stimulation, which is in accord with the current hypothesis of a desensitization of the receptor/enzyme complex associated with the pathological status.

The authors first reviewed the main theories concerning the pathogenesis of otoselerosis and studied the morphologic and functional characteristics of cell cultures derived from normal and otosclerotic bones. Light transmission and scanning electron microscopy did not permit definite identification of the cultured cells as predominantly osteoblasts, nor did these techniques show significant differences between cultured ceils derived from normal and pathologic bone. Functional tests of the cell cultures proved more interesting. First, the bony nature of the cultured cells was demonstrated by studying the intracellular 45Ca++ uptake after stimulation with calcitonin and dybutryl-cAMP. Second, cell cultures derived from otosclerotic bone behaved differently from those derived from normal bone. Their peak uptake of calcium appeared later, and post-stimulatory values were higher, suggesting that cells derived from otosclerotic bone store a greater quantity of 4SCa ++. Furthermore, after stimulation with calcitonin and propranolol, we observed an inhibition of the calcium uptake and decreased intracellular cAMP levels in normat bone cell cultures. In contrast, the cell cultures derived from otosclerotic bone exhibited an initial inhibition of calcium absorption followed by massive calcium penetration.The response of adenylate cyclase to the action of Mg + § Ca + +, and F-ions was evaluated in cultures derived from normal bone, otosclerotic bone, and normal skin fibroblasts. The resulting data show that activation due to Mg + § is much lower in cultured cells derived from otosclerotic bone than in those from either normal bone or skin fibroblasts. No significant differences were found after .Ca ++ inhibition in any of the cell cultures. Moreover, in cell cultures derived from normal bone, F-ions induced a strong activation that was lower than the levels observed in cultures of otosclerotic bone or in normal fibroblasts. We hypothesize that an alteration at the calcitonin receptor site is responsible for the difference in calcium uptake and cAMP levels observed in the cells derived from otosclerotic bone as compared to those cultured from normal cells.

An investigation on cell cultures obtained from temporal human bone fragments showed that they provide a suitable model for studying the mechanism involved in calcitonin action on bone cells. Furthermore they demonstrated: a transitory increase in 45Ca uptake that returned to control values ten minutes after the hormone was added; a relation between 45Ca uptake and increased cAMP concentrations when these were measured at the same time intervals; a reproduction of the salmon calcitonin (sCT) effect after incubation of the cultures with either db-cAMP or db-cGMP and inhibition of 45Ca uptake and parallel decrease in cAMP levels with propanol. These results suggest that in human bone cell cultures, sCT acts as a temporary promoter of 45Ca uptake, probably by activating an adenylate-cyclase system through a beta-receptor.