The effects of digitalis-like compounds on rat lenses

Fundamental to the maintenance of ionic concentration gradients and transparency of the lens is the activity of Na+,K+-adenosine triphosphatase (ATPase) in the epithelial layer. Recent studies have identified endogenous digitalis-like compounds (DLCs) and 19-norbufalin and its peptide derivatives in human cataractous lenses. These compounds inhibit the activity of Na+,K+-ATPase and have been suggested to be involved in cataract formation. The present experiments were designed to test this hypothesis by determining the ability of digitalis and DLCs to induce changes in protein composition and leakage from rat lenses in organ culture.

DLCs were determined in rat lenses using three independent assays: interaction with ouabain antibodies, interaction with bufalin antibodies, and inhibition of [3H]-ouabain binding to red blood cells. Rat lenses were incubated in modified TC-199 medium in 5% CO2 atmosphere at 37 degrees C for the time of the experiment. The onset of cataractogenesis was assessed by measuring protein leakage from lenses and by crystallin composition in the lens and media.

DLCs were present in rat lens with concentrations 7 to 30 times higher in the capsular-epithelial layer than in the lens fibers regions. Ouabain, bufalin, digoxin, and DLC induced dose- and time-dependent leakage of protein from rat lenses. Lenses incubated with these compounds showed alterations in crystallin content consistent with changes that initiate opacity. All the compounds caused a multilayering of epithelial cells in the region surrounding the mitotic area and, at the same time, cell death in the central anterior region.

Digitalis and endogenous DLCs are cataractogenic factors. These results, together with the demonstration of DLCs in the normal lens and their increased levels in human cataractous lenses, strongly suggest their involvement in the molecular mechanisms responsible for cataract formation.