Effect of ATP-induced permeabilization on loading of the Na+ probe SBFI into endothelial cells

The fluoroprobe sodium-binding benzofuran isophthalate (SBFI) is used to measure intracellular cytosolic sodium concentration ([Na](i)). A problem with the use of this probe is the difficulty in loading it into cells. ATP reversibly increases membrane permeability of some cells via activation of receptors of the tetrabasic form of ATP (ATP(4-)). We investigated the effect of ATP-induced membrane permeabilization on loading of the acetoxymethyl ester (AM) form of SBFI (SBFI-AM) into bovine pulmonary arterial endothelial cells. Monolayers were incubated in a series of solutions that reversibly opened pores, loaded the fluoroprobe, and finally sealed the pores. ATP (1-5 mM) or 3'-O-(4-benzoyl)benzoyl-ATP (0.1-1 mM), an analogue 30-100x more specific for ATP(4-) receptors, was utilized to permeabilize the cell membrane. The signal-to-background ratio of the intracellular SBFI fluorescent signal was used as an indicator of the effectiveness of dye loading. ATP and 3'-O-(4-benzoyl)benzoyl-ATP significantly increased the signal-to-background ratio compared with the values obtained with the standard dye-loading procedure without ATP, indicating that permeabilization increased SBFI-AM entry into the cells. The permeabilization procedure produced a small decrease in cell viability, as determined with a fluorescent viability assay (ethidium dimer uptake), compared with the standard method of loading SBFI-AM. We used the procedure to measure baseline [Na](i) and changes in [Na]i after the administration of ouabain (10(-4) M) and monensin (10(-5) M). Baseline [Na](i) with this procedure (19.7 +/- 2.7 mM; n = 15 monolayers) was similar to measurements made in other cell types with the standard method of loading the probe. We conclude that i) the ATP-induced permeabilization technique is an improved dye-loading method for SBFI-AM in endothelial cell monolayers that facilitates measurement of [Na](i) and 2) these data suggest the presence of an ATP(4-) pore-forming mechanism in this cell type.