To investigate whether the secretion of phosphatidylcholine (PC) in intestinal mucus occurs by apical secretion or via basolateral excretion and to determine its subsequent passage across the tight junctions to the apical mucus. We addressed this question using the polarized intestinally differentiated tumor cell line CaCo-2 grown on filters to confluence in Transwell culture chambers. The released PC and sphingomyelin (Sph) from apical and basolateral media were analyzed by mass spectrometry. The secreted PC species were identical in both compartments indicating the same intracellular origin of PC. However, PC secretion into the basolateral compartment was more effective, and the PC:Sph ratio in the basolateral compartment was significantly higher than that in the apical compartment (8.18 +/- 1.84 vs 4.31 +/- 1.22, P = 0.01). Both pathways were temperature sensitive and were unaltered in the presence of cyclosporine. The data demonstrate the PC secretion capacity of CaCo-2 cells and indicate two separated apical and basolateral release mechanisms.
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