Previous studies using a variety of experimental techniques such as vibrating probe,
16,52,53 Using chambers,
21 MRI,
20 and computational modeling
54 have all shown that incubating lenses in high extracellular K
+ is an experimental intervention that inhibits
20,21,51 and can actually reverse the circulating fluxes. High extracellular K
+ by depolarizing the negative transmembrane potential gradient
54,55 abolishes the electrochemical gradients for Na
+ and K
+21 to produce a cation-shift phenomenon in which the concentration of Na
+ and K
+ increase and decrease, respectively, in the lens.
56 More recently, it has been shown that incubating lenses in high extracellular K
+ effectively eliminates the hydrostatic pressure gradient in the lens
18 that is proposed to drive water removal from the lens core. Consistent with this finding our previous T1-weighed MRI results
20 showed that incubating lenses in AAH-High K
+ reduces the inherent water gradient in the lens by increasing the water content in the core (
Fig. 4). In this present study T2 mapping of water to protein ratios in lenses incubated in AAH-High K
+ have shown that the observed increase in water content observed with T1 imaging results in changes to lens geometry (
Fig. 1) and GRIN (
Fig. 3) relative to lenses maintained in AAH. As an asymmetrical oblate spheroid the axial diameter of the lens is less than its equatorial diameter, and its anterior and posterior surfaces have different radii of curvature.
35 Incubation in AAH-High K
+ caused a significant increase in only the axial diameter of the lens due predominantly to a decrease in the radii of the anterior surface, which effectively increased the curvature of the anterior surface of the lens (
Fig. 1). Subsequent analysis of the T2 maps (
Fig. 2C) showed a preferential increase in T2 signal in the anterior relative to the posterior pole of the lens, indicating that a local increase in water content is the underlying cause of the increase in the curvature of the anterior surface. Conversion of T2 values to refractive index enabled GRIN profiles to be extracted (
Fig. 3). This analysis revealed that incubating lenses in AAH-High K
+ reduced refractive index values in all regions of the lens producing a homogenous flattening of the GRIN a result that is consistent with the observed increase in water content obtained by T1 measurements. Because AAH-High K
+ is inhibiting the circulating water fluxes into and out of the lens by depolarizing the lens potential we proposed that this homogenous increase in water content is due to the passive diffusion of water into the lens that is driven by osmotic forces that are accentuated by Cl
− diffusion into the lens in response to the loss of a negative membrane potential. This water influx by increasing the curvature of the anterior surface of the lens and globally reducing the GRIN changes the optical properties of the lens causing a small increase in optical power (
Fig. 5A) and a significant reduction in negative spherical aberration (
Fig. 5B) that affects overall vision quality (
Fig. 8).