BONE TURNOVER IN RATS TREATED WITH 1,25-DIHYDROXYVITAMIN-D(3), 25-HYDROXYVITAMIN-D(3) OR 24,25-DIHYDROXYVITAMIN-D(3)

Female rats were given 1,25-dihydroxyvitamin D3 (1,25(OH)2D3), 0-25 mu g per 100 g body weight (bw), 25-hydroxyvitamin D3 (25(OH)D3), 1.7 mug /100 g bw or 24,25-dihydroxyvitamin D, (24,25(OH)2D3) 1.7 mug/100 g bw , subcutaneously three times a week for 12 weeks. Traditional variable s pertaining to calcium homeostasis and growth, i.e. blood and urine c alcium (Ca) and phosphate (P), serum levels of vitamin D, metabolites parathyroid hormone, (PTH), calcitonin (CT), prolactin (PRL) and growt h hormone (GH) were measured every four weeks. This data pool was corr elated with bone matrix turnover parameters, i.e. serum levels of alka line phosphatase (ALP) and urinary hydroxyproline (u-HYP) excretion. A fter 12 weeks of treatment, 1,25(OH)2D3 significantly enhanced serum t otal and ionized Ca, urine Ca and urine P, and also diminished urine c AMP due to reduced renal function (creatinine clearance). However, 25( OH)D3 administration had no such impact. 24,25(OH)2D3 opposed the effe ct of 1,25(OH)2D3 after 12 weeks by significantly augmenting serum P a nd diminishing serum levels of total Ca and ionized Ca. Cross sectiona l group analyses showed that circulating levels of ALP were directly r elated with serum 1,25(OH)2D3 and inversely related to serum 24,25(OH) 2D, and CT. Total u-HYP and per cent non-dialysable HYP (ndHYP) were r eciprocally and positively correlated with serum PRL, respectively. Ho wever, no such relations were observed with serum GH. It appears that rats with elevated circulating levels of 1,25(OH)2D3 exhibit increased bone resorption, while augmented 24,25(OH)2D3 is associated with the opposite. Apparently, high bone turnover i.e. reduced total urinary HY P and enhanced ndHYP) is associated with high serum PRL.