Assessment and validation of a microinjection method for kinetic analysis of [Ca²⁺](i) in individual cells undergoing apoptosis

Normal and malignant epithelial cells are induced to undergo apoptosis by a large variety of mechanistically diverse agents. Regardless of inducing agent, apoptosis characteristically occurs asynchronously within a population of epithelial cells over a period of 12-96 h and is associated with permeability and enzymatic perturbations. Pre-loading of cells with acetoxymethyl esters (AM) derivatives of fluorescent Ca²⁺ indicators (i.e. fura-2, indo-1, fluo-3) by passive diffusion allows longitudinal kinetic analysis of acute [Ca²⁺](i) changes subsequent to exposure to apoptosis inducing agents. Using prostate cancer cell lines, however, it is demonstrated that dye leakage and compartmentalization into organelles limit such passive loading to longitudinal [Ca²⁺](i) measurements of < 2 h. Post-loading of cells exposed to the apoptosis inducing agent for several hours is also inaccurate owing to decreased loading efficiency and de-esterification of the probes resulting in increased production of fluorescent Ca²⁺-insensitive dye species. To accurately measure kinetics of [Ca²⁺](i) changes longitudinally in individual cells undergoing apoptosis, cells were microinjected with fura dextran and maintained in a physiologic environment. [Ca²⁺](i) and morphological changes characteristic of apoptosis were then followed simultaneously in individual cells over several days following exposure to the apoptosis inducing agent.