Prior to the experiments, HEPES-buffered Ringer’s solution (in some cases EBM-2 medium) was added to empty wells of a 12-well plate (Costar). Filter inserts, containing confluent monolayers of CD34+-ECs, were subsequently placed in the 12-well plate, filled with compound solution containing the fluorescent integrity marker Lucifer Yellow (20 µM; Life Technologies), and then placed on an orbital shaker. After 1 h, filter inserts were withdrawn from the receiver compartment. Aliquots from the donor solution were taken at the beginning and at the end of the experiments and the fluorescence was quantified. At least three inserts with cells and three without cells were tested in each permeability measurement. Fluorescence detection was carried out on a Synergy H1 multiplates reader (Biotek) using the following excitation/emission wavelength (nm) settings: 432/538; 490/516; 542/570 for Lucifer yellow, Fluorescein Na and Cy3-Human Serum Albumin and Human IgG respectively.
To obtain a concentration-independent transport parameter, the clearance principle was used. The increment in cleared volume was calculated by dividing the amount of compound in the receiver compartment by the drug concentration in the donor compartment [7] (link). The volume cleared was plotted versus time and the slope estimated by linear regression analysis. The slope of the clearance curve with inserts alone and inserts with cells is equal to PSf and PSt, respectively, where PS (microliters/minute) is the permeability surface area (square centimeter) product. The PS-value for endothelial monolayer (PSe) was calculated. To generate the endothelial permeability coefficient, Pe (cm/min), the PSe value was divided by the surface area of the filter (A in cm2) insert using the following equation: Pe = [1/PSt−1/PSf]−1/A. To assess possible adsorption to plastics and non-specific binding to cells, the mass balance (%) was calculated from the amount of compound recovered in both compartments at the end of the experiment divided by the total amount added in the donor compartment at time zero. For Pe determination, mass balance value should be between 80% and 120%.
To obtain a concentration-independent transport parameter, the clearance principle was used. The increment in cleared volume was calculated by dividing the amount of compound in the receiver compartment by the drug concentration in the donor compartment [7] (link). The volume cleared was plotted versus time and the slope estimated by linear regression analysis. The slope of the clearance curve with inserts alone and inserts with cells is equal to PSf and PSt, respectively, where PS (microliters/minute) is the permeability surface area (square centimeter) product. The PS-value for endothelial monolayer (PSe) was calculated. To generate the endothelial permeability coefficient, Pe (cm/min), the PSe value was divided by the surface area of the filter (A in cm2) insert using the following equation: Pe = [1/PSt−1/PSf]−1/A. To assess possible adsorption to plastics and non-specific binding to cells, the mass balance (%) was calculated from the amount of compound recovered in both compartments at the end of the experiment divided by the total amount added in the donor compartment at time zero. For Pe determination, mass balance value should be between 80% and 120%.
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