Regulatory effect of calcium-regulating hormones on the synthesis and/or secretion of bone gamma-carboxyglutamic acid-containing protein in chick embryonic calvaria in vitro

Regulatory effects of 1,25-dihydroxyvitamin D3 (1,25-(OH)2-D3), 24,25-dihydroxyvitamin D3 (24,25-(OH)2-D3), parathyroid hormone (PTH) and calcitonin (CT) on BGP synthesis and/or secretion were studied using chick embryonic calvaria in vitro. BGP contents in calvaria and culture medium were determined by radioimmunoassay using antiserum to purified chick BGP. After 72 h culture, 1,25-(OH)2-D3 (10(-10)-10(-7) M) increased the BGP content in culture medium significantly, and the effect was maximum at 10(-8)M, while the BGP content in calvaria was not changed by 1,25-(OH)2-D3. 24,25-(OH)2-D3 increased the BGP content both in medium at 10(-6) M-10(-5) M after 24 h culture and in calvaria at 5 X 10(-7)-10(-5) M after 48 h culture. PTH increased BGP contents both in calvaria after 72 h culture and medium at 1 U/ml after 24 h culture, while it decreased them at 5-10 U/ml after 24 h culture. CT (0.5-10 U/ml) had no effect on BGP contents in calvaria and medium. 1,25-(OH)2-D3-induced increase of BGP in culture medium was observed from 24 to 72 h of culture and then reached a plateau at 120 h of culture. 1,25-(OH)2-D3-induced increase in the BGP level in medium after 72 h culture was 3-4 times the control value. On the other hand, the BGP content in calvaria was not affected by 1,25-(OH)2-D3 until 72 h of culture and then significantly increased at 120 h. The addition of 1,25-(OH)2-D3 (10(-8) M), 24,25-(OH)2-D3 (10(-6) M), and 1 U PTH (and 1 U CT added to those hormones) into culture medium of chick embryonic calvaria significantly increased BGP contents in calvaria and medium at 72 h after culture. The effect of three hormones was not synergistic, but had a tendency to be greater than that of a single addition of 1,25-(OH)2-D3, 24,25-(OH)2-D3 or PTH. Further addition of 1 U CT into culture medium did not affect the combined effects of 1,25-(OH)2-D3, 24,25-(OH)2-D3 and PTH except for the calcium content in calvaria. The addition of individual calcium-regulating hormones into culture medium of chick embryonic calvaria did not affect the calcium and phosphorus contents in calvaria, but the calcium content in calvaria was significantly increased by the addition of 1,25-(OH)2-D3, 24,25-(OH)2-D3 and PTH to the culture medium.