Action of vitamin D on intestinal calcium transport

The means by which 1,25(OH)2D3 regulates calcium transport across the intestine still remains an enigma. The movement of calcium across the brush border membrane into the cell occurs down a steep concentration gradient. Stimulation of this process by 1,25(OH)2D3 occurs within 2h and does not require new protein synthesis. Although the mitochondria may be involved in moving the calcium across the cell under rather high concentrations of cytosol calcium (10(-5) M Ca), a different organelle (lysosome-like vesicle) may assume primary responsibility for intracellular transport at lower calcium concentrations. This change from mitochondrial to vesicular calcium movement requires time (within 18 h) and may involve new protein synthesis. In the absence of this changeover, calcium transport can still occur but only at the cost of higher intracellular calcium concentrations. Movement of calcium out of the cell across the basolateral membrane occurrs against a steep concentration gradient. The difficulty in preparing pure basolateral membranes with homogeneous orientation of the inner and outer surfaces has limited progress in our understanding of calcium movement across this membrane. However, the need for an energy-driven calcium pump at this membrane that is responsive to micromolar calcium concentrations is apparent. Whether 1,25(OH)2D3 induces the synthesis of such a pump or leads to its activation by factors such as CaBP, cAMP, and calmodulin remains to be determined.