Ngoài ra việc con thoi canxi qua các tế bào, protein ràng buộc giữ nồng độ canxi cytosolic thấp và do đó duy trì một gradient thuận lợi cho dòng canxi trong khi nơi có bảo vệ khỏi tác hại của nồng độ cao của canxi miễn phí. | The Vitamin D Endocrine System 283 3Na Ca2 Ca2 2K 3Na 3Na mRNAs 1 25 OH 2D3 PO4 VDR Ca2 CaT1 Ca2 Ca2 Ca2 Ca2 ECaC duodenal epithelial cell Figure 15 Actions of 1 25 OH 2D3 on intestinal transport of calcium. VDR Vitamin D receptor CaT1 calcium transporter 1 ECaC epithelial calcium channel transporter CaB calbindin. calcium pump and sodium calcium antiporters. In addition to shuttling calcium across cells binding proteins keep the cytosolic calcium concentration low and thus maintain a gradient favorable for calcium influx while affording protection from deleterious effects of high concentrations of free calcium. It appears that the abundance of ECaC and CaT1 in the luminal membrane and at least one of the calbindins in the cytosol depend on 1 25 OH 2D3 through regulation of gene transcription. Similarly 1 25 OH 2D3 is thought to regulate expression of sodium phosphate transporters in the luminal membrane. Some evidence obtained in experimental animals and in cultured cells suggests that 1 25 OH 2D3 may also produce some rapid actions that are not mediated by altered genomic expression. Among these are rapid transport of 284 Chapter 8. Hormonal Regulation of Calcium Metabolism calcium across the intestinal epithelium by a process that may involve both the IP3 DAG and the cyclic AMP second messenger systems see Chapter 1 and activation of membrane calcium channels. The physiological importance of these rapid actions of 1 25 OH 2D3 and the nature of the receptor that signals them are not known. Actions on Bone Although the most obvious consequence of vitamin D deficiency is decreased mineralization of bone 1 25 OH 2D3 apparently does not directly increase bone formation or calcium phosphate deposition in osteoid. Rather mineralization of osteoid occurs spontaneously when adequate amounts of these ions are available. Ultimately increased bone mineralization is made possible by increased intestinal absorption of calcium and phosphate. Paradoxically perhaps 1 25 OH 2D3 .
