Lens metabolism
The lens obtains glucose from the aqeous humor and because of the low oxygen concentration in the neighborhood of the lens, 7o% of ATP production is via anaerobic metabolism. Aerobic glycolysis and the Krebs cycle are limited to the epithelium or superficial fibers that still have mitochondria. The thickness of the lens cortex, in which newer fibers are present and which still contains organelles, is approximately 100 microns. ATP activity is higher in the epithelial cells and the newer fibers of the cortex near the equator and is lower near the poles. There is no such activity in the lens nucleus, and the fibers in the nucleus are not capable of protein synthesis.
IONIC CURRENT
An ionic current has been identified flowing out of the lens at the equator and into the lens at the poles. It is likely that ATPase activity contributes to this current because the distribution of ATPase pumps is coincident with this pattern. The Na K/ATPase activity generates an electrochemical gradient with the interior of the lens more negative than its surrounding environment. This circulating ionic flow might help circulate solutes to the deep lens fibers and transport wastes out of the fibers and out of the lens. The fluid would follow the same pathway as the ionic current, facilitating water and metabolite(glucose, ascorbate, and amino acids) movement into the deeper fibers. Water and solutes enter the lens through extracellular spaces at the anterior and posterior nolar regions,cross fiber membranes to the lens interior, and then flow through fiber back to the surface at the equator, matching the distribution of ionic pumps and channels.
REGULATION OF FLUID VOLUME
Cl- flux appears to be the important factor in regulating fluid volume. The Na K+/2Cl- and K+/2Cl- cotransporters and Cl- channels maintain ion concentration gradients at a level that keeps water in equilibrium across the cell and fiber membranes. The membranes of the epithelium and the fibers are highly permeable and aquaporins are numerous, enhancing water movement into and out of the lens.