| [ Summary ] |
Transcellular Ca2+ in the intestines and kidneies plays an essential role in calcium homeostasis. It is well known that active vitamin D3 plays a major role in this process, but its precise molecular mechanism remains unclear. Recently, new calcium channels, ECaC1 and ECaC2 have been cloned from the epithelia of the intestine and kidney, and their molecular structures and gene structures have been identified. Both the proteins function in the rate -limiting steps of Ca transport and their activities are believed to be regulated by hormones. It has been reported that the expression of ECaC2 is regulated by 1,25-dihydroxyvitamin D3 and the factor(s) regulating the expression of ECaC1 remain unclear. Transcellular Ca transport is realized in three steps, consisting of Ca2+ entry across the apical plasma membrane, followed by cytosolic diffusion of Ca2+ bound to calbindin-D and then extrusion across the basolateral membrane by a plasma membrane Ca2+-ATPase (PMCA) and/or Na+/Ca2+-exchanger into the blood. |