The circadian rhythm of serum osteocalcin concentrations: Effects of 1,25 dihydroxyvitamin D administration

The effects of 1,25 dihydroxyvitamin D₃ (1,25(OH)₂D₃) administration on serum osteocalcin (Oc) concentrations were determined. 2.0 μg doses of 1,25(OH)₂D₃ were administered orally and intravenously to four healthy adult males. Blood was sampled hourly for 24 hours on four occasions: once prior to the two treatment days (i.v. and p.o.), on each of the treatment days, and during a second nontreatment day 2 years later. Mean circadian Oc rythms of the four subjects on each study day were compared with each other and with a previously derived mathematical representation of the normative Oc rhythm, the circadian Oc rhythm model. We found overall conservation of the mean Oc pattern across time and 1,25(OH)₂D₃ treatment. However, 1,25(OH)₂D₃ administration resulted in a rapid rise (within 6 hours) in Oc concentrations that blunted or eliminated the morning fall in Oc levels. The increased Oc levels were sustained for the remainder of the 24 hour period though pattern shapes converged with those of the nontreatment days and the model. We conclude that serum Oc levels are rapidly responsive to near physiological doses of 1,25(OH)₂D₃ in healthy adult males and that the effects are maintained for at least 24 hours.