Regulatory effects of Ca-binding proteins on the Ca-pump of rat intestinal basolateral membranes.

1. CaBP-9k affected Ca uptake by actions on both the apparent affinity of the pump for Ca and on the maximum Ca transport rate giving stimulation or inhibition at different Ca concentrations. 2. Ca uptake by vesicles prepared from a number of different sites was stimulated similarly. 3. Batches of CaBP that had been run on HPLC did not give the same results. 4. CaBP-28k increased Ca uptake by an effect on the ATP-independent binding of Ca. 5. The complex effects of CaBP-9k on the intestinal Ca-pump may be a method by which 1,25-dihydroxycholecalciferol controls the exit of Ca from the enterocyte during Ca absorption.