The aim of this study was to investigate the possibility that an abnormality related to parathyroid hormone (PTH) action is involved in the increased bone turnover observed in otosclerosis. To do so, expression and function of the PTH-PTH-related peptide (PTHrP) receptor were studied in the involved tissue (stapes) and compared with that in control bone sample obtained from the external auditory canal (EAC) in the same patient in 10 cases of otosclerosis and in 1 case of osteogenesis imperfecta. PTH-PTHrP receptor expression was studied by RT-PCR of RNA prepared from cultured cells in three patients and RNA directly extracted from bone samples in four patients. PTH-PTHrP receptor function was assessed by measuring the stimulation of cAMP production by 0.8, 8, and 80 nM PTH in bone cell cultures in seven cases. Results showed that PTH-PTHrP receptor mRNA expression in the otosclerotic stapes was lower than that in EAC samples (P < 0.05), whereas it was higher in stapes than that in EAC in the case of osteogenesis imperfecta. cAMP production after PTH stimulation was lower in bone cells cultured from otosclerotic stapes compared with that in cells cultured from EAC (range of increase in stimulation: 0.8-4.5 and 1.5-7 in stapes and EAC bone cells, respectively, P < 0.05). In contrast, the stimulation of cAMP production by forskolin was not significantly different in otosclerotic stapes and EAC bone cells (range of increase in stimulation: 20.7-83.1 and 4.9-99.8 in stapes and EAC, respectively, P > 0.05). These results show a lower stimulation of cAMP production in response to PTH associated with a lower PTH-PTHrP receptor mRNA expression in pathological stapes from patients with otosclerosis compared with that in control EAC samples. This difference supports the hypothesis that an abnormal cellular response to PTH contributes to the abnormal bone turnover in otosclerosis.
