Some Endogenous Parathyroid Hormone Effects Manifested by Bone in vitro. ∗

Summary

A system of bone incubation is described in which pooled rat serum was used for the incubation of femoral fragments. The following results were observed: 1. In vitro calcium and Ca⁴⁵ equilibration studies on metaphyseal and diaphyseal femoral rat bone indicate that metaphyseal bone equilibrates at a higher level than does diaphyseal bone. 2. Incubated bone taken from parathyroid intact animals maintains higher calcium levels in vitro than bone from parathyroidectomized rats; this is also the case for Ca⁴⁵ injected longer than 24 hours before sacrifice and subsequent incubation. If the Ca⁴⁵ is injected less than 24 hours before sacrifice, the same amount of radioactivity is released into the medium for both intact and parathyroidectomized groups. Dead bone, however, always releases less radioactivity than live bone. 3. These studies indicate that endogenous parathyroid hormonal effects are detectable when bone is removed from parathyroid intact rats and incubated. These effects are exerted through an influence on cellular mediated processes; however, bone from parathyroidectomized rats also maintains media calcium levels partly through energy requiring processes. 4. Evidence presented illustrates 2 distinct processes of bone salt removal exist, one of which is influenced by endogenous parathyroid hormone. Data also imply that diaphyseal (compact) bone may be the primary site of parathyroid hormone influences on calcium homeostasis.