Effect of Vitamin D Status on the Equilibrium between Occupied and Unoccupied 1,25-Dihydroxyvitamin D Intestinal Receptors in the Chick

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Abstract

The dynamic equilibrium between in vivo occupied and unoccupied 1,25-dihydroxyvitamin D3[1,25(OH)2D3] receptors of the chick intestinal mucosa was investigated by the exchange assay previously reported [(1980). J. Biol. Chem.255: 9534-9537]. These parameters and their correlation to biological response, i.e., the levels of intestinal vitamin D-dependent calcium binding protein (CaBP), were assessed under different physiological conditions. After a single 1,25(OH)2D3 injection (3.25 nmol), occupied receptor levels increased sharply to a maximum between 1 and 2 h, followed by a rapid decline. A single dose of 1α-hydroxy-vitamin D3 [1α(OH)D3], an analog that requires 25-hydroxylation for biological activity, resulted in a protracted, albeit lower, response with maximal receptor occupancy at 6 h and half maximal levels 24 h after injection. The intestinal receptor occupancy patterns mirrored the serum 1,25(OH)2D3 levels after either 1,25(OH)2D3 or 1α(OH)D3 treatment. Additionally, time-course (half-life) of blood disappearance of 1,25(OH)2D3 and occupied receptor levels were similar (1.9 and 2.3 h, respectively), suggesting that the amount of occupied 1,25(OH)2D3 receptor is determined by a simple equilibrium between serum 1,25(OH)2D3 and unoccupied receptors. A dose-response study after intramuscular 1,25(OH)2D3 injection yielded a hyperbolic curve with an apparent plateau at 70% receptor occupancy, corresponding to 5 nmol 1,25(OH)2D3 injected. Half-maximal occupancy was reached after a dose of 1 nmol 1,25(OH)2D3, corresponding to 1.5 ng 1,25(OH)2D3/ml serum. From this value the apparent Kd in vivo is 3.7 nM, which is similar to that determined in vitro. A 10-fold increase in the 1α(OH)D3 dose resulted in less than a doubling of the levels of serum 1,25(OH)2D3, occupied 1,25(OH)2D3 receptors, or CaBP. Under all experimental conditions, there was a positive correlation between occupied receptor and CaBP levels; however, the slope of the lines depended on the times chosen for the assays due in part to the lag period for CaBP induction and its accumulation within the cell. Conversely, the correlation between serum 1,25-(OH)2D3 levels and occupied receptor levels yielded a single regression line independent of the observation time. Short and long-term treatment with different vitamin D metabolites, estrogen, progesterone, or cortisol did not affect the levels of total intestinal 1,25(OH)2D3 receptor. Under normal physiological conditions, only 10-15% of the total 1,25(OH)2D3 receptor population was occupied by ligand. These studies provide a basis for further investigations of physiological and biochemical parameters of the vitamin D endocrine system and their clinical applications.

Authors

Willi Hunziker, Marian R. Walters, June E. Bishop, Anthony W. Norman