Bar, A., Institute of Animal Science, Agricultural Research Organization, The Volcani Center, Bet Dagan 50250, Israel
Hurwitz, S., Institute of Animal Science, Agricultural Research Organization, The Volcani Center, Bet Dagan 50250, Israel

The binding of 1,25-dihydroxycholecalciferol (1,25-(OH)2D3) to its intestinal receptor was studied in chicks fed a phosphorus (P)-deficient diet. The equilibrium association constant (K(a)) was determined by Scatchard analysis. Association (K(ass)) and dissociation (K(dis) rate constants were determined in experiments on hormone uptake and release respectively. The K(a), determined at 4,12 and 19° C, decreased progressively during P deficiency, due to the decrease in K(ass), but K(dis) was not affected. During prolonged P deficiency the concentration of binding sites (N(max)) also decreased. Duodenal calcium-binding protein (CaBP) increased during 10 days of P deficiency and then decreased. The long-term decrease in receptor affinity and N(max) may account for the observed reduction in receptor occupancy and the decrease in the high level of intestinal CaBP stimulated during early P deficiency. The resulting decrease in Ca absorption may minimize the hypercalcaemia induced by the deficiency.