2.6. Calcium transport studies
The Caco-2 cell monolayers with TEERs exceeding 500 Ω·cm−2 were used for the calcium transport experiments. The monolayers were set in a 12-well plate and washed twice with warm (37 °C) balanced salt solution without calcium and magnesium (HBS) (125 mM NaCl, 4 mM KCl, 4 mM l-Glu, 10 mM Glucose, 30 mM HEPES, adjusted to pH 7.4) and incubated at 37 °C for 20 min. The wells were then transferred to a new cluster plate containing 1.95 ml HBS buffer. Subsequently, 5 mmol/L dietary factors (phosphate, oxalate, zinc ions) and DPs-calcium chelate and DPs-calcium mixtures were added to the apical side of the insert and the calcium transport rates across Caco-2 cell monolayers were measured after 2 h. The TEERs were monitored before and after the addition of the calcium and DPs samples. Finally, the calcium contents were measured with an atomic absorption spectrophotometer (AA6300C, Shimadzu, Kyoto, Japan).