Uptake of calcium and magnesium by rat duodenal mucosa analysed by means of competing metals.

1. The short-term uptake of Ca and Mg by rat duodenal mucosa consisted of two parts, one apparently complete within 1 min (uptake I) and a second which increased linearly with time from 1 to 12 min (uptake II).2. Uptake II for both Ca and Mg showed a curvilinear dependence on concentration. Uptake I was related linearly to concentration up to the highest concentration tested.3. Mg caused a significant inhibition of Ca uptake II. Ca was without effect on Mg uptake.4. Mn reduced significantly both uptakes I and II for Ca; Ba and Sr were without effect on Ca uptake.5. Two lanthanides, La and Pr, both inhibited uptakes I and II for Ca. Inhibition by Pr could be reversed by increasing the concentration of Ca. Some uptake II for Ca still persisted when uptake I was completely abolished by 3 mM-La.6. It is suggested that a small part of Ca uptake I may be associated with the subsequent mediated entry into the mucosa, since inhibition of Ca uptake consistently involves both forms of uptake. However, since some Ca still enters the mucosa when uptake I is completely inhibited, it appears that some entry of Ca can occur by diffusion.