Influence of lighting conditions on daily rhythm of bone metabolism in rats and possible involvement of melatonin and other hormones in this process.

OBJECTIVE

To evaluate the influence of lighting conditions on daily fluctuations of biochemical markers of bone and mineral metabolism such as the activity of serum alkaline phosphatase (ALP), the concentration of carboxyterminal propeptide of type I procollagen (PICP), cross-linked carboxyterminal telopeptide of type I collagen (ICTP) and inorganic phosphorus (iP) as well as urinary excretion of hydroxyproline (HYP) and calcium (Ca). In addition, possible involvement of daily secretion of melatonin (MEL), GH/IGF-I axis activity and parathyroid, thyroid, adrenocortical and gonadal function in this process.

METHODS

Three series of 48 adult male Wistar rats weighing 180 +/- 10 g were conditioned to different lighting conditions for 4 weeks: 1. control group (LD 12:12 h, light on from 7.00 to 19.00); 2. short-day group (LD 0.5:23.5 h, light on from 7.00 to 7.30); 3. long-day group (LD 23.5:0.5 h, light on from 19.30 to 19.00). The blood was obtained by decapitation. Material for studies was collected every 3 hours during a day. The concentration of hormones, PICP and ICTP was determined with the use of RIA method, whereas ALP, iP, HYP and Ca levels were estimated spectrophotometrically.

RESULTS

A significant influence of different lighting conditions on the daily profile of investigated markers of bone metabolism in rats was confirmed. Thus a short day was found to have an inducing effect upon the level of these markers, while a long day showed a suppressing effect. Furthermore, substantial changes in the values of amplitude and phase of their daily rhythms were found. Also some distinct anomalies in daily oscillations of ALP, PICP, ICTP, HYP and Ca were found which were dependent on the time of the day and showed a negative correlation with the changes in endogenous MEL concentrations. Moreover, ICTP, HYP and Ca concentrations correlated positively with daily fluctuations of IGF-I and triiodothyronine.

CONCLUSIONS

Lighting conditions can influence daily fluctuations of studied bone metabolism markers in rats, and in the mechanism of this dependence the changes in endogenous MEL concentrations seem to play an important role. Secondary changes in daily IGF-I and triiodothyronine oscillations, caused by short- and long-day conditions, result in altered daily bone resorption rhythmicity.