Bar, A., Department of Physical Biology, New York State Veterinary College, Cornell University Ithaca, NY 14850, United States
Wasserman, R.H., Department of Physical Biology, New York State Veterinary College, Cornell University Ithaca, NY 14850, United States

A current hypothesis suggests that the degree of Ca absorption is hormonally controlled via the feed-back regulation of 1,25-dihydroxycholecalciferol (1,25-(OH)2D3) production from 25-hydroxycholecalciferol (25-OHD3) by kidney 1-hydroxylase. To test this hypothesis, dihydrotachysterol3 (DHT3), a steroid not requiring 1-hydroxylation for biological activity, was given to chicks as the only source of vitamin D-activity. As expected, DHT3-treated chicks did not adapt to a calcium-deficient diet. However, both the efficiency of Ca absorption and net synthesis of CaBP were stimulated in DHT3-treated chicks by a low phosphorus intake, providing evidence for an alternate pathway of control. © 1973.