Parathyroid hormone, prostaglandin E2, and 1,25-dihydroxyvitamin D3 decrease the level of Na+-Ca2+ exchange protein in osteoblastic cells.

We previously described Na+-Ca2+ exchange in osteoblastic rat osteosarcoma cells (UMR-106) and demonstrated that Na+-dependent Ca2+ transport was inhibited by 24-hour treatment of cells with parathyroid hormone (PTH), prostaglandin E2 (PGE2), or 1,25(OH)2D3. To determine whether this inhibition of Na+-Ca2+ exchange is at the level of exchanger protein synthesis we have examined exchanger protein levels using immunoblot analysis. UMR-106 cells were treated for 24 hours with or without PTH, PGE2, or 1,25(OH)2D3. Plasma membrane fractions (7500 g) were obtained and proteins were separated by SDS-PAGE, transferred to nylon membranes, and immunoblotted with a polyclonal antibody to the canine cardiac Na+-Ca2+ exchanger. In rat cardiac membranes, we detected 125 and 75 kD bands, similar to findings for the canine exchanger. In the osteoblastic UMR cell membranes, a specific band was detected at 90 kD that decreased 65% after treatment of cells with PTH. Inhibition by PTH was dose dependent, was maximal with 10(-7) M PTH, and required 16-24 hour treatment time. Similar inhibition was observed after a 24 hour treatment with 10(-6) M PGE2 or 10(-8) M 1,25(OH)2D3. These results demonstrate the presence of a specific protein in UMR cells that cross-reacts with antibody directed against the cardiac Na+-Ca2+ exchanger. Thus, the previously reported inhibition of Na+-Ca2+ exchange activity by calcemic agents in osteoblasts appears to be due to regulation of exchanger protein levels in these osteoblastic cells.