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IV. NUTRITION

IV. NUTRITION. A. Major nutrients are glucose and oxygen in: tears, limbal blood vessels, aqueous humor. IV. NUTRITION.

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IV. NUTRITION

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  1. IV. NUTRITION • A. Major nutrients are glucose and oxygen in: tears, limbal blood vessels, aqueous humor

  2. IV. NUTRITION • 1. Glucose is not permeable to the superficial epithelium. Glucose can diffuse from limbal vessels, however the rate of consumption is too great for this to provide for any more than just the most peripheral cornea.

  3. IV. NUTRITION • B. Experiments • 1. ingested fluorescein will diffuse out of limbal blood vessels, but never get to the central cornea. • 2. Deep limbal cuts, intended to destroy the limbal blood supply, have no effect on the cornea. • 3. implantation of impermeable plastic discs into the stroma, will eventually lead to degeneration of the epithelium and anterior stroma. CONCLUSION----Glucose must come from the aqueous humor.

  4. IV. NUTRITION • C. Endothelial glucose permeability is 10 times that for diffusion of a molecule its size. Therefore glucose uptake across the endothelium must be facilitated.

  5. IV. NUTRITION • D. Between 65-85% of glucose used by the corneal epithelium is metabolized to lactate even under normal conditions. Much more lactate is produced when the epithelium is made hypoxic, i.e. during contact lens wear.

  6. IV. NUTRITION • E. Oxygen is very permeable to all cell layers. Removing oxygen from the tears by using a nitrogen goggle or tight fitting contact lens leads to corneal edema and clouding. When the eye is closed oxygen will diffuse out of the palpebral capillaries to the tears. Oxygen flux from the aqueous to the cornea is especially important when the eye is closed. Limbal oxygen supply is negligible.

  7. V. STEADY-STATE O2 DISTRIBUTION • A. First, carbon dioxide distribution

  8. V. STEADY-STATE O2 DISTRIBUTION • B. The steady-state O2 curves are derived from a complex, non-linear function. If Q = dj/dx and j = Dk(dp/dx) then: • Q = (d/dx) Dk(dP / dx) = Dk (d^2P / dx^2) • Q = O2 consumption rate • d^2P/dx^2 = concentration gradient for O2 across the tissue • D = diffusion coefficient • k = solubility coefficient

  9. V. STEADY-STATE O2 DISTRIBUTION • Dk = oxygen permeability • Dk/L = oxygen transmissibility of a membrane (e.g., contact lens)

  10. V. STEADY-STATE O2 DISTRIBUTION • C. In the open eye at sea level, pO2 = 155 mm Hg (132 mm Hg in Denver) at the tears and 55 mm Hg in the aqueous humor. • Layer = Q ml O2/ ml x sec % • Epi 26.5 x 10-5 40 • Stroma 2.85 x 10-5 39 • Endo 140 x 10-5 21

  11. V. STEADY-STATE O2 DISTRIBUTION • Oxygen Tension = P = (QX^2 / 2Dk) + BX + C; Master Equation, where B and C are constants of integration and are determined by the boundary conditions.

  12. V. STEADY-STATE O2 DISTRIBUTION • 1. Open Eye Boundary Conditions • at x = 0 (aqueous), P = 55 i.e. C = 55 • at x = L (tears), P = 155 • 155 = (QL^2 / 2Dk) + BL + 55, solve for B: • P = (QX^2/2Dk) + (X/L)[100- (QL^2/2Dk)] + 55 • = Oxygen tension

  13. V. STEADY-STATE O2 DISTRIBUTION • 2. Closed Eye Boundary Conditions • at x = 0, P= 55 • at x = L, P = 55

  14. VI. OXYGEN TENSION UNDER A CONTACT LENS

  15. VI. OXYGEN TENSION UNDER A CONTACT LENS • A.Mathematical determination: Diffusion through the lens (Fick's Law) • 1. J = dm / dt, J is the mass of substance moving/unit time across a boundary or layer, m is mass (grams), t is time or J = D a (dc / dx), where D is the diffusion coefficient (cm^2 / sec), a is the area, dc is the concentration (grams/cc) gradient, x is the thickness of the layer.

  16. VI. OXYGEN TENSION UNDER A CONTACT LENS • 2. j = D (dc / dx), small j is the flux per unit area. • 3. remember that for gases c = kP (Henry's Law of gas partial pressure)(c=dissolved O2 conc., k = O2 solubility, P = O2 tension in solution) therefore, J = dm / dt = Dka (dP / dx), if L is the thickness of a contact lens then • j = Dk (dP / dL) and Dk/L represents the oxygen transmissibility of the lens.

  17. VI. OXYGEN TENSION UNDER A CONTACT LENS • B. Experimental Determination of Oxygen Uptake • Background • 1. We can measure the oxygen tension of a solution with an oxygen sensitive electrode. Over the electrode is placed a thin plastic membrane that protects the electrode & acts as a reservoir for oxygen.

  18. VI. OXYGEN TENSION UNDER A CONTACT LENS • The oxygen in the solution equilibrates with the membrane and then diffuses to the electrode to produce a steady current, proportional to the oxygen tension in the solution.

  19. 2. Good for measuring O2 in large volumes where the electrode is not affecting the solution pO2. Want to know the O2 tension under a contact lens. The problem is that the tear volume is similar to the membrane reservoir volume & the tear pO2 cannot be maintained if the electrode is pressed to the eye. If an electrode is pressed against the eye the oxygen in the membrane will diffuse out into the cornea.

  20. 3. The cornea will consume the O2 and since the electrode is blocking O2 coming from the air, the O2 in the membrane will be depleted within a minute or so.

  21. 4. Suppose a contact lens is worn for a time (5 -10 min) so that the oxygen tension of the cornea is reduced. When the lens is removed and the electrode is placed on the eye, the oxygen in the membrane may come out faster than if a lens hadn't been worn since the difference in O2 tension between the membrane and the cornea is greater. This in fact occurs, and the absolute O2 flux from the membrane can be calculated.

  22. VI. OXYGEN TENSION UNDER A CONTACT LENS • C. Experimental Methods • 1. Equivalent Oxygen Percentage (R. Hill) • a. An empirical method for estimating tear pO2 in humans wearing contact lenses. • b. Bathe the cornea of a subject with various levels of oxygen. Whip off the goggle and measure the oxygen uptake rate. Create a calibration curve showing tear pO2 vs. O2 uptake.

  23. VI. OXYGEN TENSION UNDER A CONTACT LENS • 1. Equivalent Oxygen Percentage (R. Hill) • Next place a contact lens on the eye, allow the O2 to equilibrate for about 10 minutes, whip off the lens and measure the O2 uptake, find the equivalent pO2 on the calibration curve.

  24. VI. OXYGEN TENSION UNDER A CONTACT LENS • 2. O2 Sensitive Phosphorescence Dye.

  25. VI. OXYGEN TENSION UNDER A CONTACT LENS • D. Effect of Blinking

  26. VI. OXYGEN TENSION UNDER A CONTACT LENS • E. Hypoxia • 1. When cells are made hypoxic, ATP production from mitochondria slows and glycolysis speeds up to try to maintain the ATP supply. This is called the Pasteur Effect.

  27. VI. OXYGEN TENSION UNDER A CONTACT LENS • As a consequence, lactate and proton production will increase and glucose consumption will increase.

  28. VI. OXYGEN TENSION UNDER A CONTACT LENS • E. Hypoxia • 2. factors limiting the rate of glycolysis under hypoxic conditions; glucose supply, diffusion, glycogen stores • 3. Epithelial cells store glycogen, which will last about 2 hours during severe hypoxia.

  29. VI. OXYGEN TENSION UNDER A CONTACT LENS • F. Corneal Edema • 1. Epithelium • a. Haze seen in area covered by contact lens. • b. Haze is eliminated by removing epithelium or reduced by rolling a Q-tip across the cornea. • c. Epithelial thickness does not change, so water is not added between the layers of epithelium. • d. Haze is seen subjectively as colored fringes around bright lights. This is called Sattler's Veil.

  30. VI. OXYGEN TENSION UNDER A CONTACT LENS • 1. Epithelium • e. Haze is due to light scatter at the level of the basal cells. Photographs of basal layer with the specular microscope reveal a matrix of light scattering sites which act as a diffraction grating. This grating produces a diffraction ring that subtends a half angle of 3 degrees using red light, 632.8 nm. • f. Thus, increase in water between the basal cells.

  31. VI. OXYGEN TENSION UNDER A CONTACT LENS • 2. Stromal edema • a. Cornea will swell, i.e. increase in thickness, during hypoxia or contact lens wear. Possibilities: • i. Hypoxia reduces epi- or endothelial barrier function. • ii. Endothelial pump is slowed directly by hypoxia. • iii. Corneal pH drop slows endothelial pump. • iv. Lactate accumulation causes osmotic swelling. • b. Cornea will swell about 4% during sleep, i.e. eye closure.

  32. VI. OXYGEN TENSION UNDER A CONTACT LENS • 2. Stromal edema • c. Acute hypoxia does not compromise the epithelial barrier, however chronic severe hypoxia will. In addition, chronic mild hypoxia can slow the epithelial mitotic and healing rates. All of these problems increase the chance for corneal infection. The effect of reduced pH on these processes is not known, but it could be a contributing factor.

  33. VI. OXYGEN TENSION UNDER A CONTACT LENS • 3. Endothelial Changes from contact lens wear • a. Blebs are transitory dark spots appearing in the specular reflection of the endothelium. Occur about 30 min. after insertion of a contact lens and last about 30-60 min. Most likely due to pH changes. • b. Polymegathism, increased variation in cell size seen in long-term lens wearers: • coefficient of variation = S.D. of cell size / mean cell size • c. Pleomorphism – increased variation in cell shape

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