Intracellular diffusion, binding, and compartmentalization of the fluorescent calcium indicators indo-1 and fura-2

We studied intracellular binding and possible compartmentalization of the fluorescent Ca2+ indicators, indo-1 and fura-2, in single mammalian cardiac ventricular cells that had been loaded with indo-1 and fura-2 by exposure to the acetoxymethylester form of the indicators (indo-1/AM and fura-2/AM). Techniques similar to those used in experiments on fluorescence recovery after photobleaching (FRAP) were used. It was assumed that reversible binding in myoplasm would be evident as slowed recovery of fluorescence after photobleaching, and that irreversible binding of the indicators to immobile myoplasmic sites (or "compartmentalization" in organelles) would be evident as incomplete recovery. Through the use of a mask, one half of a cell was exposed to high-intensity ultraviolet (UV) light to bleach the indo-1 or fura-2 in only that part of the cell. Upon removal of the mask and termination of the high-intensity UV illumination, fluorescence recovered in the bleached half of the cell, indicating diffusion of indo-1 and fura-2. Mathematical modeling of the diffusional redistribution of the indicators indicated that in these cells the apparent diffusion coefficient for indo-1 is 1.57 x 10(-7) cm2 s-1 (SD 0.48 x 10(-7) cm2 s-1; n = 5 cells, 21 degrees C), and for fura-2 is 3.19 x 10(-7) cm2 s-1 (SD 1.85 x 10(-7) cm2 s-1; n = 6 cells, 21 degrees C). These values are approximately 6 and 3, respectively, times smaller than those expected for free diffusion in the myoplasm. In the bleached half of the cell the recovered level of fluorescence never reached the final level in the half not exposed to UV light. The extent of incomplete recovery was variable amongst the cells. Our analysis indicated that, under the conditions we used, approximately one-third of the intracellular dye is not diffusible in the myoplasm.