Chlortetracycline, a fluorescent probe for pH of calcium stores in cells

Endoplasmic reticulum is the major storage site for Ca 2 + that is mobilized during activation of cell functions. Chlortetracycline (CTC) loaded into calcium stores of fresh, isolated human blood lymphocytes was found to be sensitive to changes in H + concentration, in addition to its established sensitivity to Ca 2 + . Determination of pH of calcium stores was based on dissociation of fluorescent CTC.Ca 2 + with decrease in pH. Concentrations of H + at the calcium stores were calibrated to different levels by equilibrating it with different extracellular pH buffers using the H + ionophore, nigericin. It was possible to determine the pH of two pools of calcium stores; they were 7.2 and 6.2.