Maintenance by cortisone of the calcemic response to parathyroid extract of rats on a diet without vitamin D and low in calcium.

Weanling rats raised for 21 days on a vitamin D deprived, low Ca diet (0.02%), and given chronic cortisone administration (5 mg/kg/d), maintain responsiveness to the hypercalcemic effect of endogenous and exogenous parathyroid extract (PTE). The PTE action is a bone effect that does not require the presence of the kidneys, and is not related to a higher concentration of calcium or cortisone. Maintained sensitivity of D- Ca- Cort+ rats to PTE does not appear to be the consequence of a lesser degree of D-deficiency: the whole body vitamin D pool and its chloroform-soluble fraction in these animals are not different from those of their D- Ca- PTE- unresponsive controls. Repeated PTE injections for 4 days exhaust the sensitivity to the hypercalcemic action of PTE of D- Ca- Cort+ rats. The present data seem to indicate that cortisone-treated D- Ca- rats, responsive to the bone action of PTE, are characterized by a near normal bone calcium content and Ca/P ratio, and a significant increase in the number of osteoclasts.