NMR relaxation studies of syneretic response to pressure change in bovine lenses

Purpose. Reversible syncretic response to pressure in bovine and rhesus mon key lenses has been demonstrated previously by invasive techniques, such as differential scanning calorimetry and thermogravimetric analysis. This stu dy is designed to investigate whether such a response could be observed by non-invasive techniques, namely by relaxographic imaging studies, in situ, in the intact, albeit excised lens. Methods. Excised bovine lenses were incubated in media at 37 degreesC in sp ecialized pressure chambers for 24 hrs. Three pressures, 2, 1 and 0.03 atm, were employed. The pressure chambers were placed in the cavity of an NMR m agnet. Seven sections of the lens, under 2 atm pressure, from anterior oute r cortex to posterior outer cortex were imaged and the T-1 (spin-lattice) a nd T-2 (spin-spin) relaxation data on each section were collected. The pres sure was then released and NMR data were collected under 1 atm. Similar arr angement was followed on lenses under initial 0.03 atm pressure. T-1 and T- 2 relaxations were analyzed by fitting pixel intensity to one and two term exponential expressions. Results. Analysis of the time dependence of the T-2 relaxation time indicat ed that the response to a change in pressure is complete within 2 hours. Bo th T-1 and T-2 relaxation times showed minimal values in the nuclear region and maxima at the two outer cortexes. With increasing pressure both relaxa tion times decrease. The effect of pressure on both relaxation times was sm aller in the nucleus and more enhanced at the outer cortexes. The pre-expon ential terms of the fittings of both T-1 and T-2 relaxations indicate the a mount of protons participating in the relaxation. Thus they serve as a popu lation index. The T-2 population index had a maximum in the nucleus and min ima in the two cortexes. The population index of T-1 relaxation exhibited m inimal value in the nucleus and maxima at the two cortexes. The pre-exponen tial term of T-2 relaxation increased with increasing pressure. The preexpo nential term of T-1 relaxation did not show consistent pressure dependence. Conclusions. The positional dependence of T-2 relaxation times as well as t hat of its population index indicated that it represents the behavior of th e bound water in the lens. The positional dependence of T-1 population inde x suggests that this relaxation represents the total water that has a minim al value in the nucleus. Both the relaxation times as well as the populatio n indices indicated that as pressure increases the strength of hydrogen bon ding as well as the amount of bound water increases. This also means that t he free water/bound water ratio decreases with increasing pressure. Thus NM R imaging and relaxation Studies confirm significant syncretic response to applied hydrostatic pressure in bovine lenses.