Effect of a low-calcium environment on alkaline phosphatase activity in embryonic rat calvarial bone cells in culture

The effect of a low Ca environment on the alkaline phosphatase (ALP) activity in bone cells was examined. Bone cells were isolated from the calvaria of 18 to 21-day-old fetal rats and cultured in BGJb or α-minimum essential medium (α-MEM) with calcium concentrations of 1.87 mM (control group), 1.20 mM (middle group) or 0.34 mM (low-Ca group). The control and low-Ca groups grew and reached a confluent state at 10 days of culture and there were no significant differences in the number of cells, total protein, or DNA content between the two groups. The specific ALP activity measured at 10 days of culture of the cells with 0.34, 1.20 and 1.87 mM calcium was 106.3 ± 11.9, 73.3 ± 10.4,and38.8 ± 7.3 unit per mg protein (n = 9), respectively. When the low-Ca medium was replaced with the control medium during 10 days of culture, the activity decreased to the level of the control group. On the other hand, when control medium was replaced with low-Ca medium, the activity increased to the level of the low-Ca group. After confluence, cell growth in the low-Ca group was poorer than that in the control group with time. The ALP activity markedly increased and the mineralized nodular structures were observed in the control group of α-MEM, but not in the low-Ca group. These findings suggest that stimulation of ALP activity is a compensation mechanism to maintain normal cell functions in a low-Ca condition and that the role of ALP in the low-Ca environment may be different from that on mineralization.