The Effect of Lens Size on the Circulation of Salt and Water through the Lens

Abstract

Purpose: : Mathias et al. (2010, ARVO abstract) presented data showing the mouse lens has a center to surface gradient in intracellular hydrostatic pressure (p_i mmHg), the gradient was inversely proportional to the density of gap junction channels (N_j cm^-2), and directly proportional to the influx of Na⁺ across fiber cell membranes (j_Na moles/cm²/s). Model calculations were consistent with the above data insofar as they suggested the pressure gradient should scale with a²j_Na/N_j, where a (cm) is the lens radius. The purpose of the current study was to determine the effect of a on p_i.

Methods: : All studies were done in freshly dissected intact lenses. A manometer was used to balance pressure inside of a microelectrode with that in the lens. Microelectrodes were used to directly measure intracellular voltage ψ_i and to inject the Na⁺-sensitive dye SBFI, then [Na⁺]_i was determined optically.

Results: : The radii (cm) were: mice (0.11±.005), rats (0.24±.002), rabbits (0.49±.018) and dogs (0.57±.019). In all of these lenses, the center to surface gradient in p_i was 330-0 mmHg. This surprising observation prompted us to measure the intracellular electrochemical gradient driving the Na⁺-flux that is thought to generate the circulation of water. In mouse, rat and dog lenses, the center to surface gradient in ψ_i was -45 to -60 mV. In mouse and rat lenses, the center to surface gradient in [Na⁺]_i was 16-6 mM. If the radial distance from the center (r) was normalized to r/a, the distribution of p_i, ψ_i or [Na⁺]_i data from each species fell on the same curve.

Conclusions: : Gradients driving the lens circulation of salt and water are regulated rather precisely to be independent of lens size. The group of parameters a²j_Na/N_j scale each of the gradients measured, so j_Na and N_j are apparently regulated to render the group independent of a. Our hypothesis on the physiological purpose of the circulation is homeostasis of central fiber cells, but there is no known reason why this would require such precise regulation. Ultimately, the lens has evolved to be a precise optical element, so perhaps the regulation is important for its optical properties.

Keywords: ion transporters • gap junctions/coupling • signal transduction: pharmacology/physiology