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Glucose transporter (GLUT)

Na + /K + -ATPase P-glycoprotein/MRP

to actively drive solutes from one side of the plasma membrane to the other, in which case it is called as a pump. A uniporter transports one solute at a time. A symporter transports the solute and one or more cotransported solutes at the same time in the same direction. An antiporter transports the solute in (or out) and another solute in the opposite direction. Transporter proteins may exist in different isoforms with the same function but with variable affinities, uptake kinetics, and tissue expression profiles. For example, facilitated glucose transport is mediated by a family of glucose transporter (GLUT) proteins, seven isoforms of which are known to date (Thorens, 1996). GLUT1, GLUT2, GLUT3, and GLUT4 are involved in cellular glucose uptake; GLUT1, GLUT3, and GLUT4 are high-affinity glucose transporters, and GLUT2 has a significantly lower affinity. GLUT5 is a high-affinity fructose transporter with poor glucose transport capacity. GLUT7 is closely related to GLUT2, but it is retained in the endoplasmic reticulum and not transported to the plasma membrane. The role of GLUT6 is still unknown. Similarly, various families of transmembrane transporters exist in the mammalian plasma membranes for amino acids and their derivatives (Palacin et al., 1998; Saier, 2000b). Transfer of amino acids across the hydrophobic domain of the plasma membrane is mediated by proteins that recognize, bind, and transport these amino acids, which exhibit broad substrate specificity and stereospecificity. There are three best-known amino acid transport systems present in the plasma membrane of mammalian cells based on the type of amino acid the protein moves and the thermodynamic properties of the transport: (a) zwitterionic amino acid systems (A, ASC, N, BETA, GLY, IMINO, PHE, B0, L), (b) cationic amino acid systems (B0, + ', b + , y + , y + L, b0, + ), and (c) anionic amino acid systems (X~ AG, XC). A detailed description of these transporters is provided elsewhere (Palacin et al., 1998).

III. TRANSPORT BARRIERS IN THE EYE A. Cornea

While primarily being a refractive element of the eye, the cornea acts as a mechanical and chemical barrier to intraocular tissues. The mammalian cornea, with relatively few exceptions, consists of five to six distinct layers, with a total thickness of 300-500 mm (Pepose and Ubels, 1992). These layers include (Fig. 3): corneal epithelium, underlying basement membrane, acel-lular Bowman's layer, corneal stroma, Descemet's membrane, and corneal endothelium. Bowman's layer is absent in the cornea of rabbit, a routinely used animal model in eye research (Fig. 3).

Figure 3 The cornea. Cellular organization of various transport limiting layers.

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