Apparent Increase in Carrier Affinity for Intestinal Calcium Transport following Dietary Calcium Restriction

Rats fed a diet lacking calcium adapt to this deprivation by increased active transport of this ion across the intestine. When the characteristics of the transport system were analyzed in vitro by use of saturation kinetics, increased net absorptive calcium transport appeared to be the result of an increased affinity (decrease in K t from 1.12 to 0.59 m m ) rather than an increase in the capacity or V max of the transport process. Adaptation through substrate induction in yeast results in an analogous increase in carrier affinity for sugar transport by a linkage to metabolic energy. Such a mechanism could account for the lowered dietary calcium effect, however the authors suggest that rather than an actual change in carrier affinity a second molecule (such as intestinal calcium-binding protein) could also alter carrier-substrate equilibria and produce a decrease in the observed K t if access to the carrier were rate limiting at low substrate concentrations and this second molecule increased access of substrate. This model is consistent with the changes in calcium transport system characteristics and calcium-binding protein levels under conditions of restricted dietary calcium levels.