Effect of Phototherapy on Bone Metabolism in Newborn Rats

Kim, Sang Hee; Park, Jae Hong

Abstract

PURPOSE: Neonatal hyperbilirubinema, the single most common abnormal physical finding in the first week of life, is seen in 60-80% of newborn infants. It is commonly managed by phototherapy, which has been associated with an increased incidence of hypocalcemia. As one of the mechanisms of the phototherapy-induced hypocalcemia, it was hypothesized that phototherapy dicreases the secretion of melatonin and the low serum concentration of melatonin permits the unopposed uptake of calcium by bone. The aim of the bony uptake of calcium by assessing the effect of phototherapy on the bone mineral density.
METHODS
Newborn male rats were exposed to blue fluorescent light (420-470 nm wave length) intermittently at 12-hours intervals for 4 weeks from the second day of life. The newborn rats were sacrified and the blood was obtained by a direct puncture of heart at first , second, third, and fourth weeks, respectively. Total bone mineral contents and bone mineral concentration were measured by QDR 4500 FAN BEAM X-Ray densitometer, and serum intact parathyroid hormone levels by a radioimmunoassay.
RESULTS
1) There were no significant differences in total bone mineral contents between the photo-treated groups and control groups, except those between a 1-week photo-treated and its groups. 2) Bone mineral concentrations in photo-treated grups were similar to those of control groups.3) Although the levels of intact parathyroid hormone tend to be lower in all of the photo-treated groups than in control groups. it was statistically significant only in the 1-week photo-treated group (P<0.05).
CONCLUSION
These results are inconsistent with previous hypothesis that phototherapy-induced hypocalcemia results from the unopposed uptake of calcium by bone due to decreased secretion of melatonin and suggest that decreased secretion of parathyroid hormone may be one of the causes for phototherapy-induced hypocalcemia.