FLUOROPHOTOMETRIC MEASUREMENT OF THE BUFFERING ACTION OF HUMAN TEARS IN-VIVO

Purpose. The buffering action of human tears is thought to be importan t to keep its pH constant. We measured the change in pH in the precorn eal tear film in vivo when the acidic solution is challenged, using a fluorophotometric technique. Methods. Twelve eyes from 6 healthy subje cts were entered in this study. Each subject was pretreated with eithe r one drop of 0.4% oxybuprocaine for once (light anesthesia), three ti mes (deep anesthesia), or none (controls). The measurement was initiat ed by instilling 20 mu l of 0.067 M phosphate buffer at pH 5.5 contain ing 2 mM bis-carboxyethyl-carboxyfluorescein free acid, a pH sensitive dye, into the subject's eye. The pH was determined by the ratio of fl uorescent intensities at two excitation wavelengths (490 and 430 nm). pH recovery time (PHRT) as defined by the time required for pH to reac h 95% of pH at equilibrium was used for the marker of tear buffering a ction, Tear turnover rate was also determined using the fluorescent de cay curve at 430 nm, which was independent of pH, but dependent on dye concentration. Results. Immediately after the instillation, the pH va lue in the tear film was around 6.0-6.5 in all cases. The tear film ra pidly became more alkaline, reaching its normal value in 2.3 +/- 0.5 m in in untreated eyes. The pretreatment with 0.4% oxybuprocaine retarde d the neutralization process. A single regression analysis revealed th at the PHRT had a significant negative correlation with the tear turno ver rate (r = -0.78). Conclusions. Our results suggest that the neutra lization process of tears largely depends on the tear turnover rate. T he buffering action of tears in vivo consists of the tear turnover as well as its chemical buffering capacity.