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Abstract: The blood–ocular barrier system is formed by two main barriers: the blood–aqueous barrier and the blood–retinal barrier (BRB). The BRB is particularly tight and restrictive and is a physiological barrier that regulates ion, protein and water flux into and out of the retina. The BRB consists of inner and outer components, the inner BRB being formed of tight junctions between retinal capillary endothelial cells and the outer BRB of tight junctions between retinal pigment epithelial cells.
Leakage can result from disruption of the retinal vascular endothelial cell tight junctions or the breakdown of the tight junctions between retinal pigment epithelial cells (the inner and outer blood-retinal barriers, respectively). Examples include macular edema from diabetic retinopathy, cystoid macular edema, and central serous chorioretinopathy. In addition to abnormalities of the retinal vascular system or pigment epithelium, leakage is observed in a variety of conditions associated with the development of new blood vessels.
OCTN2 was first cloned from human kidney and identified as the transporter responsible for systemic carnitine deficiency . OCTN2 has high degree of sequence homology with OCTN1 . Substrates of OCTN2 include TEA, quinidine, verapamil, pyrilamine, choline, short-chain acyl esters of carnitine, zwitterionic beta-lactam antibiotics, L-lysine and L-methionine [5, 6]. OCTN2 mediates the active absoprtion of L-carnitine in the small intestine and its reabsorption in the proximal tubule. OCTN2 also mediates the uptake of L-carnitine into adipocytes, cardiac myocytes, skeletal muscle cells, neurons, brain, lymphocytes, spermatozoa, and across the blood-retinal barrier.
The retinal pigment epithelium (RPE) is an specialized epithelium lying in the interface between the neural retina and the choriocapillaris where it forms the outer blood-retinal barrier (BRB). The main functions of the RPE are the following: (1) transport of nutrients, ions, and water, (2) absorption of light and protection against photooxidation, (3) reisomerization of all-trans-retinal into 11-cis-retinal, which is crucial for the visual cycle, (4) phagocytosis of shed photoreceptor membranes, and (5) secretion of essential factors for the structural integrity of the retina.