Pressure-induced syneretic response in rhesus monkey lenses

PURPOSE. To investigate the effect of pressure on the freezable and nonfreezable water content of the lens. METHODS. Excised rhesus monkey lenses in tissue culture media were subjected to three different hydrostatic pressures (2 atm, 1 atm, and 0.03 atm) for 24 hours. Then while still under the experimental pressure, the vessels were cooled in dry ice-acetone until the lenses were frozen. While the lenses were kept frozen, nuclear and cortical samples were dissected, enclosed in a sample pan, and weighed. Differential scanning calorimetry (DSC) measurements were performed between - 30°C and 30°C. Total water content of each lens sample was obtained by thermogravimetric analysis at 105°C. The nonfreezable water content was obtained by subtracting the freezable water content calculated from the DSC data from the total water content. RESULTS. The total water content of the lenses did not change significantly as a function of pressure applied. This was true both for cortical and for nuclear sections. The freezable water content increased as the pressure decreased both in cortex and nucleus. Similarly, the freezable water/nonfreezable water ratio also decreased with increasing pressure. CONCLUSIONS. External hydrostatic pressure would generate an influx of water into the lens. To alleviate this diluting tendency and to prevent turbidity as a result of dilution, the lens must effect an osmotic pressure change equivalent to the applied pressure. Change in the osmotic pressure is caused by changing the activity of the water (i.e., converting free water to bound water). This is a reversible and energetically the least expensive response. The release of bound water from the hydration layers of macromolecules and its conversion to free water in condensed systems are known as syneresis. In the lens decreasing pressures induce syneresis as demonstrated by the increase in freezable water content and the freezable water/nonfreezable water ratio. Such a response may be operative also in accommodating lenses.