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) a ctivity in bone cells was examined. Bone cells were isolated from the calvaria of 18 to 21-day-old fetal rats and cultured in BGJ(b) or alph a-minimum essential medium (alpha-MEM) with calcium concentrations of 1.87 mM (control group), 1.20 mM (middle group) or 0.34 mM (low-Ca gro up). 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 cell s with 0.34, 1.20 and 1.87 mM calcium was 106.3 +/- 11.9, 73.3 +/- 10. 4, and 38.8 +/- 7.3 unit per mg protein (n = 9), respectively. When th e 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 conflue nce, cell growth in the low-Ca group was poorer than that in the contr ol group with time. The ALP activity markedly increased and the minera lized nodular structures were observed in the control group of alpha-M EM, but not in the low-Ca group. These findings suggest that stimulati on 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-C a environment may be different from that on mineralization.