Studies on the mode of action of vitamin D XXXVII 1α-hydroxyvitamin D3: A long-acting 1,25-dihydroxyvitamin D3 analog

Summary This study was undertaken to determine whether 1α-hydroxyvitamin D₃ [1α(OH)D₃] administration to chicks in vivo results in 1,25-dihydroxyvitamin D₃ [1,25(OH)₂D₃] intestinal receptor occupancy and to compare the temporal characteristics of the physiological effects of 1α(OH)D₃ and 1,25(OH)₂D₃ for several days after a single dose of either steroid. Occupied 1,25(OH)₂D₃ receptors of the chick duodenal mucosa were measured by the recently developed exchange assay procedure [J Biol Chem (1980) 255:9534–9537]. Within 2 h after 1α(OH)D₃ injection in rachitic chicks, there was a significant elevation of 1,25(OH)₂D₃ receptor occupancy in the intestinal mucosa. This observation represents the first direct confirmation that this synthetic analog exerts biological effects through occupancy of 1,25(OH)₂D₃ receptors. Serum 1,25(OH)₂D₃ levels reached a 3-fold higher peak after 1,25(OH)₂D₃ injection (3.25 nmol) than after 1α(OH)D₃ injection (6.5 nmol); further, after 1α(OH)D₃ injection the peak was delayed by 2–4 h. However, serum 1,25(OH)₂D₃ levels remained elevated for only 3–6 h after 1,25(OH)₂D₃, compared to 48 h after 1α(OH)D₃ injection. Occupied 1,25(OH)₂D₃ receptor levels paralleled serum 1,25(OH)₂D₃ levels at all times after administration of either steroid. At 24 h, duodenal vitamin D-dependent calcium binding protein (CaBP) levels were similarly elevated in both treatment groups, but by 48 and 72 h after 1α(OH)D₃ administration CaBP and serum Ca²⁺, respectively, were more significantly elevated. These data confirm that 1α(OH)D₃ induces its major biological effects via intracellular 1,25(OH)₂D₃ receptors and reinforce the concept that 25-hydroxylation is a prerequisite for these effects. These results also suggest that 1α(OH)D₃ may become useful in the therapy for sustained treatment of vitamin D deficiency diseases.