Parathyroid hormone induces a rapid increase in the number of active osteoclasts by releasing histamine from mast cells.

Intraperitoneal injection (i.p.) of parathyroid hormone (PTH, 50 micrograms/kg) into young rats (7-day postnatal) induced, within one hour, an increase in the number of osteoclasts which showed well-developed clear zone. Histological observations showed that degranulation of mast cells adjacent to bone surface occurred within 15 min after the injection of PTH. Injection of histamine into rats pretreated with cimetidine, an H2 antagonist of histamine, also induced an increase in the number of active osteoclasts within one hour after the injection of histamine. Furthermore, pretreatment of mice with an H1 antagonist, pyrilamine, completely inhibited the rapid increase in the number of active osteoclasts by PTH. These results suggest that PTH may stimulate osteoclasts to the active form by releasing histamine from mast cells and by stimulating H1 receptors for histamine.