Role of P-glycoprotein in restricting propranolol transport in cultured rabbit conjunctival epithelial cell layers

Purpose. To determine the role of P-glycoprotein (P-gp) in propranolol tran sport in cultured rabbit conjunctival epithelial cell layers (RCEC). Methods. The localization of P-gy in the cultured RCEC as well as in the ex cised conjunctiva was determined by immunofluorescence technique. The role of P-gp in transepithelial transport and uptake of propranolol in conjuncti val epithelial cells cultured on Transwell filters was evaluated in the pre sence and absence of P-gp competing substrates, an anti-P-gp monoclonal ant ibody (4E3 mAb), or a metabolic inhibitor, 2, 4-dinitrophenol (2,4-DNP). Results. Immunofluorescence studies revealed positive staining in the apica l membrane of cultured RCEC and in the apical surface of the superficial ce ll layers in the excised conjunctiva, but not the basolateral membrane of c ultured RCEC. Transport of propranolol showed preference in the basolateral -to-apical direction. The net secretory flux was saturable with a K-m of 71 .5 +/- 24.0 nM and a J(max) of 1.45 +/- 0.17 pmol/cm(2)/hr. Cyclosporin A, progesterone, rhodamine 123, verapamil, 4E3 mAb and 2,4-DNP all increased a pical 50 nM propranolol uptake by 43% to 66%. On the other hand, neither be ta-blockers (atenolol, metoprolol, and alprenolol) nor organic cation trans porter substrates (tetraethylammonium (TEA) and guanidine), affected apical 50 nM propranolol uptake. Conclusions. The energy-dependent efflux pump P-gp appears to be predominan tly located on the apical plasma membrane of the conjunctival epithelium. I t may play an important role in restricting the conjunctival absorption of some lipophilic drugs.