Visualizing ocular lens fluid dynamics using MRI: manipulation of steady state water content and water fluxes

Abstract

Vaghefi E, Pontre BP, Jacobs MD, Donaldson PJ. Visualizing ocular lens fluid dynamics using MRI: manipulation of steady state water content and water fluxes. Am J Physiol Regul Integr Comp Physiol 301: R335-R342, 2011. First published May 18, 2011 doi:10.1152/ajpregu.00173.2011.-Studies using various MRI techniques have shown that a water-protein concentration gradient exists in the ocular lens. Because this concentration is higher in the core relative to the lens periphery, a gradient in refractive index is established in the lens. To investigate how the water-protein concentration profile is maintained, bovine lenses were incubated in different solutions, and changes in water-protein concentration ratio monitored using proton density weighted (PD-weighted) imaging in the absence and presence of heavy water (D2O). Lenses incubated in artificial aqueous humor (AAH) maintained the steady state water-protein concentration gradient, but incubating lenses in high extracellular potassium (KCl-AAH) or low temperature (Low T-AAH) caused a collapse of the gradient due to a rise in water content in the core of the lens. To visualize water fluxes, lenses were incubated in D 2O, which acts as a contrast agent. Incubation in KCl-AAH and low T-AAH dramatically slowed the movement of D 2O into the core but did not affect the movement of D 2O into the outer cortex. D2O seemed to preferentially enter the lens cortex at the anterior and posterior poles before moving circumferentially toward the equatorial regions. This directionality of D 2O influx into the lens cortex was abolished by incubating lenses in high KCl-AAH or low T-AAH, and resulted in homogenous influx of D 2O into the outer cortex. Taken together, our results show that the water-protein concentration ratio is actively maintained in the core of the lens and that water fluxes preferentially enter the lens at the poles. homeostasis; D 2O; water-protein gradient OUR VISION IS CRITICALLY DEPENDENT on our ability to focus light onto the retina. This refractive power is the combined result of the optical properties of the cornea and lens. While the cornea contributes most of the eye's refractive power, its focus is fixed (24). Young lenses, however, can adjust their focal power through the process of accommodation to elongate or shorten the focal length. As an optical element, the cornea normally contributes a positive spherical aberration to the light path, which is later corrected by the inherent negative spherical aberration of the ocular lens (2, 34). This negative spherical aberration is the result of a gradient in refractive index (21) that is produced by changes in the water-protein concentration ratio of the lens, such that water content is highest in the outside of the lens and lowest in the core (4). While this water-protein concentration ratio is known to create a smooth refractive index gradient that corrects for spherical aberration (26), how this gradient is maintained is unknown.The existence of a gradient in water content would be expected to generate a...

Active Maintenance of the Gradient of Refractive Index Is Required to Sustain the Optical Properties of the Lens

“…In the above equation, T1 values are extracted by applying a train of TI values (20,50, 80, 100, 150, 200, 250, 300, 350, and 400 ms) to obtain 10 data points which were then fitted with a monoexponential curve using a nonlinear curve fitting routine (EZYFIT, in the public domain, http://www.fast.u-psud.fr/ezyfit/). EZYFIT uses a recursive process and unconstrained nonlinear minimization of the sum of squared residuals, to find the optimum fit.…”

“…In a previous study using T1 weighted imaging we showed that inhibiting the circulation system in the bovine lenses by either incubating them in AAH-High K þ , or low temperature, abolished the inherent water gradient and increased the water content in the lens core. 20 In this study, we have complemented our existing T1 imaging results and introduced T2 imaging to determine whether the observed increase in water content induced by inhibiting the lens microcirculation changes overall lens geometry and the GRIN, both key determinants of the optical properties of the lens. In the current study, in addition to inhibiting the circulation system by incubating lenses in AAH-High K þ to depolarize the lens potential, we have incubated lenses in ouabain to specifically block Na þ pump activity, which is known to drive the microcirculation system.…”

“…Bovine eyes were either imaged intact to extract geometrical information on the tissues that comprise the optical pathway, or were dissected to obtain isolated lenses for organ culture experiments. Dissected lenses were incubated in artificial aqueous humor (AAH; NaCl 125 mM; KC1 4.5 mM; MgCl 2 0.5 mM; CaCl 2 2 mM; NaHCO 3 10 mM; glucose 5 mM; Sucrose 20mM; buffered with 10 mM HEPES to pH 7.1), or either High-K þ (NaCl 25 mM; KC1 100 mM; MgCl 2 0.5 mM; CaCl 2 2 mM; NaHCO3 10 mM; glucose 5 mM; Sucrose 20 mM; buffered with 10 mM HEPES to pH 7.1) to depolarize the lens 20 or AAH plus 0.1 mM ouabain to block Na þ pump activity 21 for 4 hours prior to being placed in the scanner. One bovine eye or two isolated lenses with their anterior surfaces facing up were placed in sample holders, the temperature of which was monitored and controlled at 378C by a MR compatible temperature control unit (SAII, Stony Brook, NY, USA).…”

“…18 Thus, active pumping of Na þ by surface cells is thought to regulate extracellular and intercellular fluid fluxes 17 that deliver nutrients and antioxidants to the lens core, thereby protecting crystallin thiol groups from the oxidation effectively preventing protein aggregation and maintaining crystallin solubility. 19 Recently, using T1 magnetic resonance imaging (MRI) in combination with D 2 O tracer experiments 20 to spatially map lens water content, we have shown that reducing Na þ pump activity or depolarizing lens potential, two perturbations known to inhibit measured circulating ionic currents in the lens, 16 not only inhibited fluid fluxes driven by the circulation system, but also abolished the steady state water gradient known to exist in the lens. These experiments showed for the first time that fluid circulates through the lens and that at steady state these fluxes act to maintain a lower water content in the lens core relative to the cortex.…”

Abnormal High Signal Intensity Discovered in Eye Lenses in Routine Brain MRI; Correlation with Ophthalmologist Examination

“…Also, some other studies demonstrated that MRI could be used as an effective method to diffusivity mapping of the lens (24)(25)(26). On the other hand, the positive use of MRI in real-time measurements of water transport in the lens has been reported by previous studies (27)(28)(29). Sheppard et al showed the successful use of MRI for the measurement of mean crystalline lens thickness during accommodation (13).…”