Interrelationships of vitamin D, bone metabolism and blood calcium concentration in the chick

It is not known if the effects of vitamin D deficiency on chick bone are due to direct actions of the vitamin or if they are secondary to other changes, such as hypocalcemia. Day-old cockerels were fed either a rachitogenic diet containing no Ca (-D-Ca), 1.4% Ca (-D), or 3% Ca (-DHiCa) and given corn oil (-D groups) or vitamin D3 in corn oil (+D and +D-Ca) p.o. for up to 21 days. Radii were harvested and incubated for 6-8 h in a defined medium. Medium samples were taken every 2 h and analysed for Ca, Pi and lactate. Some bones were incubated in a respirometer to measure O2 consumption. Compared to +D, -D birds showed evidence of D deficiency by decreased plasma Ca concentration (35%), bone and body weight (43%) and Ca release from bone (70%) and by histological changes in bone characteristic of rickets. Increases were seen in total and bone alkaline phosphatase activity in plasma (270 and 706%, respectively), Pi release (23%), O2 consumption (23%) and lactate production (52%) by the -D radii. The marked hypocalcemia seen in the -D chicks did not occur in -DHiCa birds. Nevertheless, bone and body weights were decreased in this group and bone lactate production, O2 consumption and total and bone alkaline phosphatase in plasma were increased. Rachitic bone lesions were only partially corrected by the high-Ca diet. Release of Ca and Pi from the -DHiCa bone was not different than from +D radii. Comparing +D-Ca and -D-Ca groups with +D chicks, both were hypocalcemic with decreased bone weight, body weight and bone Ca release, while showing elevated lactate production and Pi release. The only difference between the +D-Ca and -D-Ca groups was a 50% decrease in Ca release by -D-Ca bone. The results suggest that in chicks: (1) some, but not all, of the effects of vitamin D deficiency on bone can be corrected by normalizing plasma Ca and (2) evaluation of the effects of vitamin D deficiency on bone may require hypocalcemia, since some responses are masked by normocalcemia.