Biomedical Engineering Reference
In-Depth Information
humor if they migrate or are transported into the intercellular space. Therefore, aqueous
humor formation is induced by the active transport of sodium ions and the passive move-
ment of water.
The composition of aqueous humor is similar to arterial plasma. However, there is a
significantly higher concentration of ascorbate in the aqueous humor than in the blood
plasma, and there is a marked reduction in proteins and blood cells (no cells should com-
pose aqueous humor). Ascorbate is an ester of vitamin C and is used to maintain the
health of the lens. With a deficiency in ascorbate, it is common for cataract formation and
macular degeneration to proceed. Plasma proteins cannot be filtered by the ciliary bodies,
and therefore, proteins account for approximately 0.02% of the total aqueous humor com-
position (whereas proteins account for nearly 10% of the plasma composition). The major
functions of aqueous humor include maintaining intraocular pressure, providing nutrients
to the cornea and lens (which are avascular), and removing wastes from the cornea and
lens.
10.3 AQUAPORINS
Aquaporins are trans-membrane proteins that regulate the flow of water into and out of
cells. For many years, it was believed that water movement into and out of the cell was
1) not regulated in any manner and 2) was simple diffusion across the cell membrane.
However, the rapid movement of water in aqueous humor formation could not be
described by simple diffusion. In the early 1990s, aquaporins were discovered, and it was
found that they can selectively control water movement into and out of cells. One of the
critical functions of aquaporins is that while they allow the passage of water they prevent
the passage of ions. If aquaporins allowed ions through their channels, all ion concentra-
tion gradients across the cell membrane would approach zero (i.e., all ions would be in
equilibrium) and the cell would not be able to perform many of its critical functions that
depend on the concentration gradient of various molecules. Also, if ions were allowed to
pass through aquaporins, the amount of energy that cells would expend on maintaining
the necessary ion concentration gradient would exceed the amount of energy produced
during cellular respiration.
Aquaporins are found in a high concentration in the epithelial cells that produce aque-
ous humor. These pores allow water molecules through in a single file. To understand
how aquaporins regulate water movement into a single file and prevent the movement of
other ions, it is important to understand the three-dimensional structure of the aquaporin
protein. An aquaporin channel is composed of six trans-membrane
-helices, with both
the amino and carboxyl terminal on the cytoplasmic side of the membrane. Two of the
five loops that connect the six trans-membrane helices are extremely hydrophobic. One of
these loops is on the intracellular side of the membrane and the other is on the extracellu-
lar side of the membrane. The two hydrophobic loops contain a three amino acid
sequence, termed the NPA (Asparagine-Proline-Alanine) motif. The NPA motif folds
back into the channel aquaporin created by the six trans-membrane helices. In three-
dimensional space, the folding back of these two domains resembles an hourglass shape
(or a bottle neck for flow). This hourglass constriction restricts water molecules to a single
α
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