Effects of Topical K-Strophanthin on Aqueous Humor and Corneal Dynamics

Abstract

Sodium-potassium-activated adenosine triphosphatase (Na/K ATPase) is present in the ciliary epithelium, where it serves a regulatory role in fluid transport and the production of aqueous humor. The present study investigates the ability of topical k-strophanthin, a water-soluble mixture of glycosides, to lower intraocular pressure. K-strophanthin was topically administered to one eye and vehicle to the fellow control eye of rabbits, cats, and human volunteers. Single drop 0.01 M k-strophanthin delivery lowered pressure in rabbits and cats to a maximal decrease of 5-6 mm Hg 3-4 h after drug administration. Higher drug concentrations irritated the eye, whereas lower concentrations had no effect. Pressure reduction in rabbits was associated with decreased aqueous humor formation, and no change in outflow facility and episcleral venous pressure. Drug delivery in humans had a minimal intraocular pressure effect. Slit-lamp and histologic examination of rabbit eyes after chronic drug administration was normal. However, single drop administration of 0.01 M drug was found to significantly increase corneal thickness, the aqueous to cornea fluorescein transfer coefficient, and the corneal endothelial permeability coefficient. Topical k-strophanthin lowers pressure via reduction of aqueous production and most probably by inhibition of the enzyme Na/K ATPase in the ciliary epithelium. The drug's effect on corneal function probably relates to a similar block of this enzyme in the epithelial layers of the cornea, which limits the usefulness of k-strophanthin and other previously studied Na/K ATPase inhibitors as an ocular hypotensive agent.