Changes in rabbit and cow lens shape and volume upon imposition of anisotonic conditions

In vivo, mammalian lenses have the capacity to effect fully reversible changes in shape, and possibly volume, during the accommodation process. Isolated lenses also change shape by readily swelling or shrinking when placed in anisotonic media. However, the manner by which the lens changes its shape when its volume is changed osmotically is not firmly established. Putatively, the lens could swell or shrink evenly in all directions, or manifest distinctive swelling and/or shrinking patterns when exposed to anisotonic media. The present study measured physical changes in lenses consistent with the latter alternative using methods we developed for determining rapid changes in lens shape and volume. It was found in isolated rabbit and cow lenses that the length of the axis between the anterior and posterior poles (A-P length) primarily increases under hypotonic conditions (-40 to -100 mOsM), with smaller, or no changes, in equatorial diameter (ED). Hypertonic conditions (+50 to +100 mOsM) on rabbit lenses elicited a predominant reduction in ED, while the A-P length was only marginally reduced. Hypertonic solutions of +150 mOsM were required to obtain similar changes in cow lens shape. The ratio of the A-P length to the ED was taken as a measure of "circularity". This ratio increased gradually in rabbit and cow lenses bathed in hypotonic solutions because of the increase in the A-P length. The calculated lens volume increased in tandem with the increase in "circularity". Lens circularity also increased under hypertonic conditions due to the decrease in ED, but this increase in circularity during shrinkage was not as pronounced as that which occurred during swelling. As such, the lens has a tendency upon swelling to change its shape by approaching the structure of a globular spheroid (as occurs during accommodation for near focusing), but lens shrinkage does not result in a flatter lens with a reduced A-P length as occurs during dis-accommodation for distance focusing. Moreover, osmotically evoked shape changes appear irreversible, in contrast to the mechanically elicited shape changes of accommodation.