Facilitated diffusion of calcium across the rat intestinal epithelial cell

Similarities and differences between the models for facilitated diffusion of substrates across cytoplasm and across cell membranes are discussed. It is valuable to consider that calbindin acts to increase the partitioning of calcium within the cytoplasm. Calculations on the basis of a simple model for this facilitated diffusion show that the rate of transcellular calcium movement depends in a complex way on KCAB, the dissociation constant for calcium and calbindin. At each value of the transcellular concentration gradient of free calcium, there is an optimal value of KCAB that gives the maximal augmentation of calcium flow. For low values of the ratio of free calcium concentrations at the opposite poles of the cell, the optimal value of KCAB is close to the prevailing concentration of free calcium. An analysis of possible effects of pH on binding of calcium to calbindin shows that it may be useful to consider cotransport of calcium and protons within the cell. Small values of a transcellular pH gradient can have large effects on enhancing transcellular calcium flow, provided the hydrogen ion concentration affects the binding of calcium to calbindin or the rate of diffusion of the calcium/calbindin complex.