Lipopolysaccharide-Induced Functional Alteration of P-glycoprotein in the Ex Vivo Rat Inner Blood–Retinal Barrier

At the inner blood–retinal barrier (BRB), P-glycoprotein (P-gp) contributes to maintaining the homeostasis of substance concentration in the retina by transporting drugs and exogenous toxins from the retina to the circulating blood. Under inflammatory conditions, P-gp activities have been reported to be altered in various tissues. The purpose of this study was to clarify the alterations in P-gp activity at the inner BRB due to lipopolysaccharide (LPS), an inflammatory agent, and the molecular mechanisms of the alterations induced by LPS. Ex vivo P-gp activity was evaluated as luminal accumulation of 7-nitro-2,1,3-benzoxadiazole-cyclosporin A (NBD-CSA), a fluorescent P-gp substrate, in freshly prepared rat retinal capillaries. The luminal NBD-CSA accumulation was significantly decreased in the presence of LPS, indicating that P-gp activity at the inner BRB is reduced by LPS. This LPS-induced attenuation of the luminal NBD-CSA accumulation was abolished by inhibiting toll-like receptor 4 (TLR4), a receptor for LPS. Furthermore, an inhibitor/antagonist of tumor necrosis factor receptor 1, endothelin B receptor, nitric oxide synthase, or protein kinase C (PKC) significantly restored the LPS-induced decrease in the luminal NBD-CSA accumulation. Consequently, it is suggested that the TLR4/PKC pathway is involved in the reduction in P-gp function in the inner BRB by LPS.