RESUMEN
PURPOSE: (1) To update Fatt's mathematical model of the distribution of oxygen tension (pO2) across the cornea and contact lens (CL) to include the recent finding that corneal oxygen consumption increases with the acidification that occurs with CL wear. (2) To estimate the minimum transmissibility (CL Dk/t) to avoid epithelial anoxia or to avoid stromal anoxia. METHODS: A five-layer static and one-dimensional mathematical model of oxygen diffusion through the cornea based on Fatt's models was used. The relationships between acidosis and increased QO2, and acidosis and CL Dk/t were used to estimate corneal QO2 for a given CL Dk/t. RESULTS: (1) Revised model predictions are in agreement with direct tear pO2 measurements beneath CLs in the rabbit. (2) For the human eye, the minimum CL Dk/t for oxygen delivery to the basal epithelial cells was determined to be 23 for the open eye and 89 for the closed eye. To prevent anoxia throughout the entire corneal thickness the Dk/t requirements are 35 for the open eye and 125 for the closed eye. CONCLUSIONS: (1) Model predictions of the oxygen distribution beneath contact lenses are significantly lower than previous models that did not include the effect of acidosis on corneal QO2. (2) Minimum Dk/t values that allow oxygen delivery to the basal epithelium are in agreement with the Dk/t needed to avoid corneal edema.
Asunto(s)
Lentes de Contacto/efectos adversos , Córnea/metabolismo , Enfermedades de la Córnea/prevención & control , Hipoxia/prevención & control , Modelos Biológicos , Oxígeno/metabolismo , Acidosis/metabolismo , Animales , Difusión , Humanos , Consumo de Oxígeno , ConejosRESUMEN
PURPOSE: To determine whether corneal acidosis, which occurs during contact lens wear, alters corneal O2 consumption (QO2) and if so, whether increased ion transport activity could contribute to altered QO2 during acidosis. METHODS: PO2 was measured, using the phosphorescence quenching of Pd-meso-tetra-(4-carboxyphenyl) porphine, in an airtight chamber that held a trephined rabbit cornea. The rate of change in chamber PO2 was used as a measure of QO2. QO2 was measured at pH 7.5 and then at either pH 6.7, 7.1, or 7.3. Measurements of QO2 at pHs 7.5 and 6.7 were repeated in the presence of 0.5 mM amiloride and 0.5 mM ouabain. RESULTS: When pH was changed from 7.5 to 6.7, 7.1, or 7.3, O2 consumption increased by a factor of 1.80+/-0.11 (+/-SE), 1.65+/-0.12, and 1.44+/-0.06, respectively. Amiloride (0.5 mM) and ouabain (0.5 mM) inhibited 50% and 65%, respectively, of the increase in QO2 at pH 6.7. CONCLUSIONS: Corneal acidosis leads to increased QO2 in a dose-dependent manner. The increased QO2 is in part secondary to the activation of pH regulatory mechanisms, including Na+/H+ exchange, which then stimulates Na+/ K+-ATPase activity. These findings indicate that contact lens-induced acidosis can exacerbate corneal hypoxia and related complications.
Asunto(s)
Acidosis/metabolismo , Córnea/metabolismo , Consumo de Oxígeno , Oxígeno/metabolismo , Amilorida/farmacología , Animales , Córnea/efectos de los fármacos , Relación Dosis-Respuesta a Droga , Concentración de Iones de Hidrógeno , Ouabaína/farmacología , Consumo de Oxígeno/efectos de los fármacos , Porfirinas/metabolismo , Conejos , Intercambiadores de Sodio-Hidrógeno/metabolismoRESUMEN
PURPOSE: Tear oxygen tension beneath contact lenses, measured in a previous study, was found to be lower than mathematical model predictions. In this study, the authors developed a phosphorescence-based technique for measuring corneal O2 consumption (QO2) to determine whether errors in previous determinations of QO2 could explain the discrepancy between measured and predicted tear oxygen tension. METHODS: Corneal oxygen consumption is measured using the phosphorescence quenching of Pd-meso-tetra-(4-carboxyphenyl)porphine by oxygen. Oxygen tension of a pH 7.5 Ringer's solution is measured in an airtight chamber that holds an excised rabbit cornea, and QO2 is calculated from the decreased PO2 in the stirred chamber solution. QO2 rates are measured for the whole trephined rabbit cornea with the epithelium, the endothelium, or both, removed, which makes possible the estimation of the consumption rates of the epithelium, stroma, and endothelium. RESULTS: Control experiments indicated that the cornea QO2 was constant for 3 hours and that exposure to 2,4-dinitrophenol increased QO2. Mean QO2 rates (microliters of O2/cm2 per hour) were: whole cornea, QO2 = 7.53; epithelium, QO2 = 3.73; stroma, QO2 = 2.97; and endothelium, QO2 = 0.86. CONCLUSIONS: Phosphorescence-based measurements can be used to determine the QO2 of the component layers of the cornea. Estimates of QO2 in this study were similar to previous measurements, indicating that the discrepancy of tear PO2 measures and model predictions are not a result of errors in the QO2 measurements at pH 7.5.
Asunto(s)
Córnea/metabolismo , Consumo de Oxígeno , Oxígeno/metabolismo , Animales , Mediciones Luminiscentes , ConejosRESUMEN
PURPOSE: To develop a noninvasive measurement of tear oxygen tension and to determine tear oxygen tension and calculated oxygen flux rates during wear of contact lenses of varying oxygen transmissibility (Dk/L). METHODS: The basis of the tear oxygen tension (PO2) measurement is phosphorescence quenching of Pd-meso-tetra-(4-carboxyphenyl) porphine by oxygen. A fiber-optic bundle, positioned approximately 2 mm from a cuvette or from the corneal surface, delivers an excitation flash (< 4 microseconds, 539 +/- 23 nm) and collects the phosphorescence emission (> 645 nm) of the dye at 1 MHz for 2 ms. Porphine (500 microM) + 1% albumin is instilled directly into the tears of a sedated rabbit, and phosphorescence is measured before and after contact lens insertion. RESULTS: Phosphorescence intensity decay lifetimes are related inversely to PO2 and follow the Stern-Volmer relationship. Calibration control experiments revealed a temperature effect on lifetime of 1% per degree, dye concentration independence over a 100-fold dilution, and no significant pH effect between 6.8 and 7.9. Phosphorescence lifetimes indicated that tear PO2 beneath contact lenses reached steady state within 3 to 8 minutes. Mean steady state tear PO2 (mm Hg +/- SE; N > 7) beneath contact lenses was: polymethylmethacrylate (Dk/L = 0), tear PO2 = 0.58 +/- 0.22; Oxyflow f30 (Dk/L = 15.3), tear PO2 = 27.7 +/- 3.4; Fluorex 700 Dk/L = 30), tear PO2 = 52.5 +/- 3.8; Oxyflow 151 (Dk/L = 51.9), tear PO2 = 78.7 +/- 23.9. Steady state tear PO2 beneath an Oxyflow f30 after application of topical anesthetic and 10 minutes of lens wear was 41 +/- 1.2 mm Hg, significantly higher than without anesthetic. CONCLUSIONS: Phosphorescence-based measurements allow the direct, sensitive, and noninvasive assessment of oxygen availability beneath a contact lens. Oxygen levels beneath permeable contact lenses found by direct measurement are significantly lower than what mathematical models predict. The measured values, however, are close to PO2 estimates using the equivalent oxygen percentage (EOP) technique. Tear PO2 measurements may be used to examine metabolic disturbances as shown by reduced oxygen consumption rates after application of topical anesthetic.
Asunto(s)
Lentes de Contacto , Córnea/fisiología , Oxígeno/metabolismo , Lágrimas/metabolismo , Anestésicos Locales/farmacología , Animales , Córnea/efectos de los fármacos , Epitelio/efectos de los fármacos , Epitelio/fisiología , Hipoxia/metabolismo , Mediciones Luminiscentes , Oxígeno/análisis , Consumo de Oxígeno/fisiología , Porfirinas , ConejosRESUMEN
Dynamic random dot targets consisting of many localized motion vectors have been used to study the pooling of local motion signals into a global motion percept (Williams and Sekuler, 1984). In such displays, the dots are displaced with a constant step size and the direction of motion for each dot is chosen at random from a specified distribution. When the distribution extends over 360 deg, the display consists only of local random motion of individual dots and no coherent motion is reported. However, when the distribution is less than 360 deg (biased), the stimulus appears to flow in a single direction. We examined the effects of reducing the number of directionally selective (DS) cortical neurons on this integration process. Normal cats and cats with severely reduced proportions of DS neurons were trained on 2 direction discrimination tasks. The discrimination of opposite directions was examined while varying either the range of directions of local motion, or the proportion of dots moving with biased distribution. When all dots in the display were directionally biased, cats with reduced numbers of DS neurons performed the task as well as normal cats and humans (threshold range: 280-320 deg). However, when the proportion of biased dots decreased, these animals had severe deficits. Thus, in the absence of noise, even a very small number of DS neurons can perform spatial pooling of local directional signals, and support normal discrimination of opposite directions. However, a full complement of directional detectors appears necessary when the motion signal is masked by noise. The discrimination of small differences in direction revealed far more severe deficits, even when all the dots in the display were directionally biased (no noise).(ABSTRACT TRUNCATED AT 250 WORDS)