An analysis of intestinal calcium transport across the rat intestine.

Kinetic analysis of transmural calcium transport, as evaluated by in situ intestinal loops, has confirmed the existence of two transport processes, a saturable, transcellular one that is regulated by vitamin D and predominates in the proximal intestine and a nonsaturable process similar in intensity throughout the intestine. Transport data obtained from everted sac experiments are kinetically consistent with events in the in situ loop. Analysis of the three component steps making up the saturable process, i.e., entry across the brush-border membrane, intracellular diffusion, and extrusion across the basolateral membrane, indicates that intracellular diffusion is likely to be the limiting step. Active calcium transport varies directly and proportionately with the content of calcium-binding protein (CaBP), a specific molecular expression of the action of vitamin D. Since CaBP is a cytosolic protein, it may act to facilitate calcium diffusion, a proposition advanced by Kretsinger, Mann, and Simmons and supported here quantitatively. We calculate that the rate of intracellular calcium diffusion in the absence of CaBP is only approximately 1/70 of what is found in the vitamin D-replete cell. Similar considerations have led to the proposal that calcium moved by the nonsaturable process travels largely via the paracellular route. The kinetic parameters derived here, i.e., Vm = 22 mumol X h-1 X g (wt wt-1, Km = 3.9 mM, and a nonsaturable rate of 0.16/h, can be used to predict calcium absorption data as determined in previously published balance experiments.