Uptake of 45-Calcium by Spirostomum ambiguum

SYNOPSIS. Spirostomum ambiguum equilibrated with 45Ca in the external medium after about 14 days under normal conditions. As the external Ca concentration was increased, uptake was increased but only until the internal concentration was ∼ 60 mM. Changing the Ca/K ratio or increasing the K or Mg concentrations had no effect on uptake of the isotope; this was taken as an indication that most of the uptake reflected Ca binding as apatite. Increasing and decreasing the temperature increased and decreased Ca uptake, but Ca bound at higher temperatures remained in the animal at lower temperatures. Feeding caused a marked increase in uptake of isotope, probably due to the availability of a large number of binding sites on the products of digestion perhaps accompanied by increased permeability of the cell membrane. It is argued that most of the 45Ca taken up is incorporated into Ca phosphate deposits, although it is likely that there will be a certain amount of free or weakly bound Ca.