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

Abstract

Purpose. Reversible syneretic response to pressure in bovine and rhesus monkey lenses has been demonstrated previously by invasive techniques, such as differential scanning calorimetry and thermogravimetric analysis. This study 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°C in specialized 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 magnet. Seven sections of the lens, under 2 atm pressure, from anterior outer cortex to posterior outer cortex were imaged and the T 1 (spin-lattice) and T 2 (spin-spin) relaxation data on each section were collected. The pressure was then released and NMR data were collected under 1 atm. Similar arrangement 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 indicated that the response to a change in pressure is complete within 2 hours. Both 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 relaxation times decrease. The effect of pressure on both relaxation times was smaller in the nucleus and more enhanced at the outer cortexes. The pre-exponential terms of the fittings of both T 1 and T 2 relaxations indicate the amount of protons participating in the relaxation. Thus they serve as a population index. The T 2 population index had a maximum in the nucleus and minima in the two cortexes. The population index of T 1 relaxation exhibited minimal value in the nucleus and maxima at the two cortexes. The pre-exponential term of T 2 relaxation increased with increasing pressure. The pre-exponential term of T 1 relaxation did not show consistent pressure dependence. Conclusions. The positional dependence of T 2 relaxation times as well as that of its population index indicated that it represents the behavior of the bound water in the lens. The positional dependence of T 1 population index suggests that this relaxation represents the total water that has a minimal value in the nucleus. Both the relaxation times as well as the population indices indicated that as pressure increases the strength of hydrogen bonding as well as the amount of bound water increases. This also means that the free water/bound water ratio decreases with increasing pressure. Thus NMR imaging and relaxation studies confirm significant syneretic response to applied hydrostatic pressure in bovine lenses.