Simultaneous determination of 45calcium and 65zinc uptake by caco-2 cells

A simple method for simultaneously determining cell-associated Ca and Zn in Caco-2 cells is described. Calcium and zinc uptake was measured via radioisotopes Ca-45 and Zn-65. Preliminary studies revealed that Zn-65, a positron (beta(+)) and gamma emitter, contributed to Ca-45 counts in a liquid scintillation counter (LSC). However, Ca-45, being a true beta emitter, did not contribute to the counts in a gamma counter (gammaC). To differentiate the counts of Ca-45 from those of Zn-65, first a Zn-65-labeled cell suspension was read in a gammaC and an LSC, thus obtaining the relationship between the radioactive counts obtained from the gammaC and LSC. This information defined the linear relationship between gammaC Zn-65 counts per minute (CPM) and LSC Zn-65 CPM. Because the Ca-45 and Zn-65 counts obtained in the LSC are additive, giving total LSC CPM, the value of LSC Ca-45 CPM was obtained by subtracting LSC Zn-65 CPM from total LSC CPM for the dual-labeled cell sample, obtaining then LSC Ca-45 CPM. To determine the absolute activity or disintegrations per minute (DPM) of each isotope in the dual-labeled sample, the linear relationship between DPM and CPM was determined for each isotope. The method is simple and straightforward for the determination of Ca-45 counts from a sample also containing Zn-65, using gamma and liquid scintillation counters.