In vitro hydration kinetics of recent post-mortem tissue versus pre-dried corneal stromal tissue

Swelling pressure and hydration behavior of porcine corneal stroma

PURPOSE

The aim of this study was to characterize swelling pressure-thickness, swelling pressure-hydration and hydration-thickness relations of porcine cornea.

METHODS

Mechanical compression tests and free swelling experiments were performed on porcine cornea. A rheometer (DHR-2, TA Instruments) with a thermally controlled fluid chamber filled with 0.9% NaCl solution was used to measure the equilibrium swelling pressure of (n = 17) corneal stromal specimens. The samples were compressed incrementally and their swelling pressure-thickness relations were obtained. In parallel to this investigation, a transient digital imaging microscope (H800-CL, American Scope Inc.), a USB autofocus camera (UM05, ViTiny), and a precision weighing scale (AGZN100, Torbal) were simultaneously used to measure the weight-thickness relation of (n = 8) corneal specimens. This experimental study gave the thickness-hydration relationship required for expressing swelling pressure measurements as a function of hydration.

RESULTS

At the in vivo 666 ± 68 µm central corneal thickness, an average swelling pressure of 52 ± 13 mmHg and hydration of 3.36 ± 0.25 mg H2O/mg dry tissue were found. The swelling pressure was reported as functions of both tissue thickness and hydration. The average fixed charge density of ρF/F ~ 42.8 mM and dry density of 1.47±0.15 g/cm3 were found. The thickness-hydration relationship was only linear when the tissue thickness was within the range of physiological thickness.

CONCLUSION

Overall, the physiological hydration and swelling pressure of the porcine cornea were within the same range of those reported previously for other mammalian corneas such as steers, rabbits and humans. Nevertheless, the thickness-hydration behavior of the porcine cornea was only similar to that of the human cornea.

Implications of the alpha dispersion for studies on interaction of tobacco smoke--corneal tissue

In this work, we have carried out a dielectric study to determine the effect of tobacco smoke on the rat corneal function. Measurements were performed over the frequency range of 500 Hz-100 kHz in air and at the temperature of 35°C. The frequency dependencies of the loss tangent for both healthy and smoky cornea exhibit two peaks with different width occurring as a narrow at 2 kHz and a broad at around 16 kHz. The distribution parameter α at 2 kHz has a value of about 0.3, which increases to 0.6 at 16 kHz. The magnitude of the permittivity decrement at 2 and 16 kHz is about two and four times higher, respectively, for the smoky cornea than that for the healthy one. These dielectric studies indicate that the present method is useful in detection of the effect of tobacco smoke exposure on the corneal behavior.

Swelling of the collagen-keratocyte matrix of the bovine corneal stroma ex vivo in various solutions and its relationship to tissue thickness

AIM

The mammalian corneal stroma, like some other connective tissues, can absorb fluid, swell and become oedematous. Since studies on the corneal stroma have been carried out with different types of preparations and solutions, inter-study comparisons are very difficult. A study was thus undertaken on a standardised preparation to assess the relative magnitude of this swelling and its relationship to thickness of the preparations.

METHODS

From selected recent post-mortem eyes of adult cattle, stroma preparations were cut from the central part of the cornea. These preparations were immersed in various solutions of known pH and osmolality, and the time-dependent changes in wet mass were assessed over 9 h at 37 degrees C. The relative rates and magnitude of the swelling of the tissue were then compared.

RESULTS

A reference value for stromal swelling was obtained by incubation in a 35 mM bicarbonate-buffered mixed salts solution equilibrated with 5% CO2-air (pH 7.60) where a 3.39-fold increase in wet mass and a 4.58-fold increase in thickness was realised in 9 h, at an initial rate of 76 +/- 3%/h. The swelling was essentially the same in an organic buffer-mixed salt solution (pH 7.5) but progressively greater in phosphate-buffered saline (pH 7.5), a range of phosphate buffers (10-67 mM, pH 7.5), NaCl solutions (0.025-1%) and with gross swelling observed in water (where a 15.9-fold increase in wet mass occurred along with a 25-fold increase in thickness, at an initial rate of 643 +/- 62%/h). Overall, the wet mass changes were strongly related to thickness (P < 0.001).

CONCLUSIONS

The results confirm that the selection of solution(s) for studies on corneal stromal swelling is critical. The swelling (oedema) is lower in a physiologically-relevant solution (similar to the aqueous humour of the eye). This indicates that the swelling tendency of the corneal stroma has been overestimated in the past, and that a similar discrepancy may also exist for studies on other connective tissues ex vivo when non-physiological experimental solutions are used.

Re-assessment of the potential impact of physiologically relevant pH changes on the hydration properties of the isolated mammalian corneal stroma

The pH sensitivity of the swelling of the mammalian corneal stroma was reinvestigated to assess whether or not there were detectable differences in the hydration properties of this collagen-keratocyte matrix within a physiologically relevant range (as opposed to extremes of acid or alkaline pH) and at a physiologically relevant temperature. From recent post-mortem eyes of adult cows, square (8 x 8 mm) samples of corneal stroma were prepared and incubated in an isotonic, buffered (HEPES etc.), mixed salts solution with added glucose at 37 degrees C. The time-dependent changes in wet mass were assessed over 24 h. The rate and magnitude of stromal swelling were different within the range of pH 6.5-8.5. The wet mass of stromal samples increased almost 2-fold within 1 h, and then at lesser rates to realise 3.25-3.75-fold and 4-5-fold increases in wet mass by 9 h and 24 h respectively. The maximum increases were observed at pH 7.25-7.5, with most of the effect being the result of differences in the initial rate of swelling. The discontinuous swelling and the pH effect on the rates of swelling were also evident when the data were fitted to a previous kinetic model (Elliott et al., J. Physiol. (Lond.) 298 (1980) 453-470). It is concluded that pH changes in the physiological range can have a small but reproducible impact on the swelling kinetics of the isolated mammalian corneal stroma ex vivo.

Assessment of the effects of cetylpyridium chloride on water content of the collagen-keratocyte matrix of the mammalian corneal stroma ex vivo

The effects of cationic surfactants on the time-dependent increases in hydration of the corneal stroma were investigated to assess if the contribution of the proteoglycans could be titrated and how it might relate to the maximum and minimum swelling properties of the corneal stroma. From recent post-mortem eyes from adult sheep, square (8 x 8 mm) samples of corneal stroma were prepared and incubated in isotonic neutral pH mixed salts solution with added glucose, or pure water, at 37 degrees C. The time-dependent changes in wet mass were assessed over 24 h in the absence or presence of 0. 001-2% w/v cetylpyridium chloride (CPC) or benzalkonium chloride (BAC). The rate and magnitude of stromal swelling was reduced in a concentration-dependent fashion by the surfactants. In mixed salts solution, 100% inhibition of swelling could be achieved at 2% CPC and BAC. In pure water, the relative swelling was much more substantial and could only be attenuated by CPC.

Some kinetic properties of lactate dehydrogenase activity in cell extracts from a mammalian (ovine) corneal epithelium

The purpose of the following study was to examine the kinetic properties of lactate dehydrogenase (LDH) in cell extracts of corneal epithelium. The epithelium, from quality- and size-selected ewe corneas, was dispersed in 10 mM Trizma maleate with or without sucrose at pH 7.0 and cell fractions obtained by homogenisation and differential centrifugation for evaluation of LDH at 37 degrees C. Pyruvate-dependent oxidation of NADH was optimal at pH 6.5, while lactate-dependent reduction of NAD was optimal at pH 9.5; the former activity averaged 3 to 3.5 fold higher than the latter. NADPH and NADP were poor cofactors. The apparent Km values for pyruvate and lactate were 99 micromoles and 3.9 millimoles respectively. At pH 6. 5, substrate inhibition was observed with pyruvate in excess of 0.25 mm with 50% activity measured at 1.2 mm. Excess substrate effects were not seen with lactate at pH 9.5, but pyruvate inhibited lactate-dependent reduction of NAD with 50% activity measured at 1.1 mm. Differential centrifugation confirmed that the activity was predominantly localised in the soluble (post mitochondria-lysosome) fraction of the cells.