Effect of Ca intake on saturable and nonsaturable components of duodenal Ca transport.

Calcium absorption was studied by an in situ ligated-loop procedure in 9-wk-old male Wistar rats that had been placed from weaning on one of three semisynthetic regimens, 0.17% Ca, 0.44% Ca, or 0.44% Ca plus lactose. Lactose was added because it is known to increase intestinal calcium retention. When the amount of calcium absorbed was expressed as a function of calcium instilled in the loop, it became possible to describe absorption as the sum of a hyperbolic and a linear function, equivalent to a saturable and a nonsaturable process, respectively. The slope of the nonsaturable component was independent of prior calcium intake, while the maximum saturable flux (Jmax) decreased as calcium intake increased. Analysis of the duodenal content of the vitamin D-dependent calcium-binding protein (CaBP, Mr congruent to 10(4)) revealed a positive relation between Jmax and CaBP. Thus, vitamin D appears to be implicated in the saturable, but not in the nonsaturable, component of calcium absorption.